X-Ray Astronomy to Resonant Theranostics for Cancer Treatment Sultana N

X-Ray Astronomy to Resonant Theranostics for Cancer Treatment Sultana N. Nahar

Abstract location of the object. Figure 1 is an example of an image of galaxy Centauras A with a black Atomic spectroscopy is fundamental to hole in the center. The falling particles spiral study of astronomical objects for their around the black hole, move faster close to it formation, evolution, composition, physical and release energy in the form of radiation, conditions etc. Information are extracted from mainly X-rays. The highly energetic super hot the spectral analysis of the electromagnetic atoms near the black hole are in a plasma state radiation emitted by these objects. This article and emit bright Kα (1s-2p transitions) X-rays. will focus on the study of X-ray radiation which One main evidence of presence of a black hole is used as a powerful diagnostic for is large amount of high energy X- rays, astrophysical plasmas, particularly those especially hard X-rays (wavelength range less surrounding black holes. There are space than 100 ˚) emission. A black often gives out observatories, such as Chandra, XMM-Newton, jets of particles, formed by the conservation of dedicated in measuring the X-ray emissions in energy and angular momentum of the falling astronomical objects. The knowledge of X- ray particles, on the opposite sides. astronomy is very similar to that used in cancer treatment in medical facilities. Using this connection, we have formulated a new method, Resonant Nano-Plasma Theranostics or RNPT, which gives indication for one most eﬃcient way for destruction of malignant cells.

Introduction

The brightness and width of spectral lines from the plasmas of an astronomical object or environment carry important information on elemental composition, physical and chemical Figure 1: Image of galaxy Centaurus A showing the processes, abundances of various ionic states, presence of a black hole and its jets at the center plasma temperature, density etc. The detection (Observed by X-ray space observatory Chandra). In itself of various broad and narrow band the image: red indicates low-energy X-rays. Toward radiation provide considerable knowledge of the the center, the green represents intermediate- energy object. X-rays, and blue that of the highest energy X-rays. The dark green and blue bands are dust lanes that Astronomical objects can be studied in a absorb X-rays. A jet of a billion solar-masses few ways, such as, imaging, photometry, and extending to 13,000 light years is seen being blasted spectroscopy. Images of an astronomical object out from the black hole. can be formed either by broad-band spectrometer or by superposition of low Photometry gives some more resolution pictures of various emissions. It gives information on the elemental composition that general information on the high-low can be used to categorize the astronomical temperature regions, shapes and sizes, and object, such as, a carbon star which produces dominant carbon lines. Figure 2 shows the 2 photometric image of the supernova remnant element and width of the spectral lines indicate Cassiopia A created from observations by three other eﬀects such plasma broadening due to space observatories, Spitzer, Hubble, and collisions, Stark eﬀects etc. Figure 3 shows the Chandra. well-known Kα (1s-2p) transition array lines of iron near a black hole in Seyfert I galaxy MCG- Various colors indicate production of 6-30-15 6 observed by Advanced Satellite for various types emissions, infrared, visible, and Cosmology and Astrophysics (ASCA) and X-rays from the remnant. They provide Chandra. The maximum energy for a 1s-2p information on activities in various regions of transition in iron is 5.4 keV. However, the large the remnant. Supernova explosions introduce extension of the lines toward low energy means elements heavier than iron or nickel in space. In that the escaped photons have lost energies in a star, nuclear fusions create elements from the black hole. X-rays are powerful diagnostics helium to iron and nickel. In the universe the and there are space observatories dedicated in most abundant element, up to 90% is hydrogen. measuring X- ray emissions. Helium is the next with 7%. The rest of the elements which combined together is called the Radioative Atomic Processes metals, consist mainly of Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, S, Ar, Ca, Ti, Cr, Fe, is about 3%. The existence of heavier elements in our solar system indicates that the system was created from the debris of supernova explosions.

Figure 3: The Kα (1s-2p) transition array lines of iron around 6.4 keV observed by ASCA and Chandra. The asymmetric stretching toward lower energy to about 5 keV indicates presence of a black hole nearby. The Kα photons lose energy due to gravitational potential of the black hole.

Figure 2: Photometric image of supernova remnant Cassiopia A observed by three space observatories, Several atomic parameters relevant to Spitzer shows infrared (red) emission, Hubble shows the atomic processes emitting or absorbing the visible (yellow), and Chandra shows X-ray (green photons are needed to study the astronomical and blue) emissions of the remnants. While the star objects. Interaction between an atomic species elements are from H though Fe, some Ni, the heavy X+z of charge z and a X-ray photon or any elements, beyond Iron, are created through nuclear other photon usually lead to two processes, fusion during supernova explosions and are photo-excitation (inverse is de-excitation) and scattered into interstellar medium. photoionization (inverse is electron-ion recombination). In photoexcitation an electron Spectroscopy of an astronomical object absorbs the photon and jumps to a higher gives the most detailed information on physical excited level, conditions of temperature, density, abundances, etc. and the chemical composition. The brightness of the line indicates abundance of the 3 but remains bound to the ion. The asterisk (*) denotes an excited state. Its strength is (2) determined by the oscillator strength (f). The atomic parameter for de-excitation is the radiative decay rate or Einstein‟s A- coeﬃcient. and relativistic one-body terms are,

In photoionization/ photo-dissociation/ the mass correction photo-electric eﬀect an electron absorbs a photon and exits to continuum or a free state: the Darwin term

and the spin- orbit interaction

This direct ionization gives the background feature of the process. Photoionization often occurs via an intermediate The rest of the terms are weak interaction two- state, the doubly excited autoionizing state, as body terms. shown below. The wave functions and energies of the atomic system are obtained by solving HBPΨ = EΨ. The solutions are bound states with E < 0

and continuum states with E >0. They are used An electron collides with the ion and goes to the to calculate the transition matrix elements with doubly excited autoionizing state. The state photon interaction, < Ψf ||D||Ψi > where the leads either to autoionization (AI) when the dipole operator D = ∑i ri and the sum is over all electron goes free or dielectronic recombination electrons, and the corresponding line stregnth (DR) when a photon is emitted. The inverse of S.. The oscillator strength (fij), radiative decay DR is photoionization. The photoionization rate (Aji), and photoionization cross section (σPI) are then obtained from the line strength as cross section σPI give the measure of ionization probability. (3) Theoretical Treatment

(4) The atomic parameters (f, A, σPI) can be obtained from atomic structure calculations or the R-matrix method. In relativistic Breit-Pauli approximation the Hamiltonian is written as The useful quantity mass attenuation (e.g. [1]) coeﬃcient for a photon absorption by an atomic species with atomic weight WA is same as σPI except by a constant factor,

(5)

(1) where u is 1 amu=1.66054e-24 g.

where HNR is the nonrelativistic Hamiltonian 4

Resonant Nano-Plasma Theranostics for addition to more effective treatment, the Cancer Treatment experiment showed that with gold nanoparticles less intense radiation was needed for the The physics of X-ray spectroscopy for a malignant cell destruction. black hole is very similar to that of X-ray sources in medical facilities, especially for cancer treatment. The main difference is in the atomic species. Medical facilities typically use heavier elements because of their characteristic absorption or emission of high energy X-rays. The absorption or emission occurs largely through the inner shell transitions. These transitions can be used as the source of radiation or electrons productions in biomedical applications. Using these facts we have Figure 4: Cancer treatment with gold nanoparticles developed a new method called Resonant Nano- and X-rays [5]. Plasma Theranostics [2, 3, 4] (RNPT) or in short Top left: radiograph of mouse hind leg before and Resonant Theranostics. Theranostics stands for after injection of gold nanoparticles (1.35g Au/kg) in the two words, therapy and diagnostics the tumor. (imaging). Through RNPT we show how Top right: tumor growth reduction in 30 days with monochromatic X-rays, targeted at resonant only gold, only radiation, and with gold and energies, can be used for most efficient radiation (30 Gy). treatment of cancer with elimination or grossly Bottom right: Mice survival treating with only gold, only radiation, radiation and two concentrations of reducing the harmful side effects due to gold. overexposure in irradiation. The objective has to deal with several factors which are described below. Studies are being carried out with many different nanoparticles as gold is expensive. Cancer Treatment with Nanoparticles There are a few criteria to meet by the nanoparticles. Toxicity is one most important Experiments on mice have shown that a issue to resolve since most of the nanoparticles cancerous tumor can be treated more effectively become toxic in vivo. The other issue is with gold nanoparticles embedded in the tumor radiation absorption. Nanoparticles should be and then irradiated with X-rays rather than X- high-Z heavy particles. These can absorb high ray irradiation alone [5]. Figure 4 has 3 pictures energy X-rays that are non-interactive to from Heinfeld et al [5]. Gold nanoparticles were biogenic elements, such as, H, O, C, K, Fe etc. injected in to the hind leg tumor of the mice (top Gold has been the most studied in nanoparticles left figure). Thirty days experiment shows that as it absorbs high energy X-rays and it is with only gold (no treatment) tumor increased, nontoxic to body cell. The other elements of with X-rays the growth was reduced. However, interest are platinum, iodine, bromine, with gold and X-ray reduced the tumor by 80%. gadolinium etc. In a nanoparticle compound, The lower right figure shows mice survival with only the high Z element is able to interact with these three treatments. The topmost line the radiating high energy X-rays since the other (triangles) showed 85% survival over a year constituent lighter elements of the compound with higher gold concentrations and X-ray will be transparent to the high energy photons. irradiation. In 5

Photoionization and Low Energy Electron They correspond to 1s-np transitions. We Productions concentrate on 1s-2p Kα transitions as they are the strongest ones ([7, 8]). The energy for the During irradiation, the gold 1s-2p transitions varies some with the ionic state nanoparticles absorb X-ray photons and produce of the element and gives a narrow band resonant low energy electrons through photoionization. energy for the element. The strength of the These ejected electrons attach themselves to the process depends on the oscillator strength of the surrounding cells leading to breakdown of the transitions. The Kα transitions introduce DNA. Research investigations have been carried resonances in photoionization cross sections that out in an effort to increase the production of are orders of magnitude higher than that around ejected electrons. The focus has been on the K- the K-edge. Hence, although there is a rise, the shell ionization energy of the atom or ion. It is magnitude of the K-edge rise is considerably known that photoionization rises at the insignificant to low energy cross sections and ionization thresholds of various shells as resonances. These resonances for gold are illustrated in Figure 5. It shows the rises in the illustrated in Figure 5. The figure shows sum of background photoionization cross section (blue all resonances due to 1s-2p transitions in all curve) at M-, L-, and K-shell ionization ionization stages, from F- to H-like gold. The thresholds of gold [6]. The rise at K-edge is purpose of the sum is to show the resultant assumed to produce more electrons through effect of Auger transitions as gold is being photoionization of by absorption of X-rays. ionized through cascading as explained below. However, experiments have not been able to detect any such increment. RNPT predicts that the energy for enhanced electron production will occur due to resonances below the K-shell ionization energy.

Figure 6: i) Auger process, ii) Koster-Kronig cascade, iii) inverse of Auger process for gold

Auger Process and RNPT

K-shell ionization of a high-Z element by a X-ray photon ejects an electron and introduce a hole or vacancy. This leads to Auger

Figure 5: Photoabsorption coefficient κ of gold. The process where an upper L-shell electron drops blue curve represents the background with rises at down to fill the vacancy and emits a photon of M-, L-, K-edges. The high resonances (red) lie below excess energy which in turn can eject another L- the K-edge and peak orders of magnitude higher shell electron. The vacancies in the L-shell will than that at K-edge. be filled out by droppings of M-shell electrons. Such process can lead to Koster-Kronig cascade RNPT prediction is based on the atomic giving out a number of photons and electrons as properties of high-Z elements. These properties the element goes through various ionic states for heavy elements are largely unknown. and the vacancies move to the outermost shell. Through atomic structure calculations, we can The first two diagrams of Figure 6 illustrate the predict resonances and the resonant energies. process in gold for which the energy levels go 6 up to P. The resonances in Figure 5 shows that striking a high-Z target such as tungsten (Z=74), such cascading has the maximum probability at producing characteristic radiation, known as the the resonant energy range and such cascade is bremsstrahlung, as they decelerate as shown in are highly desirable in radiation therapy the inset of Figure 7. The emitted radiation application. ranges all energies from zero to the peak value, denoted as KVp for peak voltage in KeV, of the The 1s-2p transitions can occur for nine machine potential. The typical X-ray sources are ionic states, from hydrogen to fluorine like ions. high energy linear accelerators or LINACs of up The 2p subshell is filled beyond fluorine. The to 10-15 MVp, or lower energy machines such number of 1s-2p transitions in each ionic state is as CAT Scanners of up to 100-250 KVp. different because of different number of 2p electrons. Atomic calculations [7] show that The low energy X-rays interact with there are 2, 2, 6, 2, 14, 35, 35, 14 and 2 biogenic elements and cause considerable transitions in H-, He-, Li-, Be-, B-, C-, N-, O-, damage to cells. Hence the exposure by low and F-like ions respectively. Hence in the event energy radiation is reduced by a filter, typically of breaking of gold to its various the ionic states of aluminum. Figure 8 shows the due to Auger process can have a total of 112 Kα bremsstrahlung (curve with squares) of tungsten transitions for the element. For gold, the (W) with low energy radiation filtered out an Al resonant energy range corresponding to all these filter. The resultant bremsstrahlung is an energy transitions is 67 - 71 keV. The right most distribution starting with low intensity X-rays diagram in Figure 7 shows the inverse of Auger which peaks around 1/3 of the peak voltage of process. By an external X-ray source, the K- the machine. However, the patient is still shell electron can be excited to a vacancy in L- exposed to indiscriminate radiation of relatively shell and thus introducing another hole in K- large energy band where the high energy X-rays shell. This will in turn multiply the electron pass through the body without any significant productions. attenuation. Therefore, for both the low and the high energy X-ray sources it becomes necessary to increase radiation dosage to high levels as well as high energy intense beam in order to penetrate the body tissue to reach the cancer site.

Figure 7: Typical set-up of a X-ray source in a medical facility. Inset: diagram of radiation

X-Ray Sources in Medical Facilities

The X-ray sources used for therapy and diagnostics in medical facilities implement the Roentgen X-ray tube which is still very much Figure 8: X-ray bremsstrahlung radiation by W the same as the original set-up. A beam of (squares) with low energy X-rays are filtered out by electrons is accelerated across the potential an Al filter (dotted curve) giving a resultant difference between the cathode and the anode, radiation distribution with a peak around 35 keV. 7

Producing Monochromatic X-Rays gives more detailed microscopic and accurate information. To avoid the radiation damages, RNPT aims to create monochromatic X- rays targeted to the resonant energy. One particular technique to generate the targeted radiation is based on partial conversion of bremsstahlung radiation into monochromatic Kα energy. We were able to create preliminary monochromatic Kα X-rays from the bremsstrahlung radiation of a X-source at a medical facility by using a zirconium plate. The X- rays were absorbed by Zr for inner K- shell ionization which was followed by radiative Figure 9: Monochromatic X-rays (the high peak) from Kα transitions in Zr created from decays by the upper shell electrons. The bremsstrahlung radiation of a typical X-ray machine fluorescence in Zr produced the monochromatic in a medical facility. X-rays shown in Figure 9 (the high peak Kα Top: measured X-ray emission. emission). It may be assumed, to a good Bottom: filtered over the background noise. approximation that each K-shell ionization leads to the production of a Kα photon. The method should be more effective with high Z-elements, such as for Pt or Au, since the K-fluorescence yield, ωK is close to 1, estimated from the branching ratio:

(6) Figure 10: RNPT: Monochromatic Kα X-rays irradiate the tumor doped with high Z nanoparticles. Auger process leads to cascade and ejections of where Ar is radiative decay rate, Aa is electrons for cell destruction while emitted autoionization rate. It also means that X-ray radiations are detected for imaging and to observe photons, from threshold energy of the filter plate attenuation effect. to the peak voltage of the bremsstrahlung, could be used to produce the monochromatic beam RNPT and Monte Carlo Simulation with high efficiency. The work is under process for an enhanced intensity of the monochromatic Combining all elements together RNPT beam with a high Z plate [9]. scheme can be described in Figure 10. A targeted monochromatic X-ray beam, at The advantage of monochromatic over resonant energy of the nanoparticles embedded bremsstrahlung radiation is that the former can in the tumor, irradiates the tumor. The radiation be controlled and targeted at specific features in is absorbed and Auger process is initiated X-ray photoionization cross sections for followed by enhanced emission of electrons and maximal radiation absorption. Such photons from inner shells. While ejected Auger spectroscopic radiation should be far more electrons destroy the surrounding malignant efficient with reduced exposure. In addition, in cells, attenuation of incident X-rays and emitted contrast to broadband imaging, spectroscopy fluorescent photons for imaging are detected.

With the gold atomic data with three X-rays irradiation. The red curve resonances, we carried out Monte Carlo represents the electrons produced with gold simulations of RNPT using the Geant4 nanoparticles in the tumor while the blue curve computational package [8]. The simulation represents the production without them. The showed that the RNPTT scheme would enhance electron count is over 3 at 68 keV in contrast to the rate of Auger processes via Coster-Kronig 0.03 at 82 keV and about 0.7 at 2 MeV. A and Super-Coster-Kronig branching transitions considerably large number of electrons, by more [1], and result in signiﬁcant number of electron than an order of magnitude, were produced by and photon emission. The geometry of the 68 keV X-rays compared to those by 82 keV experiment is given in Figure 11 where a water and 2 MeV. phantom (15 × 5 × 5 cm) models body tissue (density 1.02 g/cc in contrast to 1.00 g/cc of water) with a tumor of thickness 2 cm at the depth of 10 cm below the skin. The tumor is embedded with gold nanoparticles of thickness 2 cm at 5 mg/ml.

Figure 12: Number of Auger electrons produced with depth following X-ray absorptions: Red curve - tumor embedded with gold nanoparticles at 5 mg/ml

Figure 11: RNPT simulation: in region 100 to 120 mm, Blue curve - only water. (Left) A water phantom with a gold layer models a Top: X-ray at 68 keV - averaged Kα resonant tumor of thickness 2 cm embedded with gold energy, Middle: 82 keV - just above K-edge nanoparticles 10 cm from the skim. ionization energy, Bottom: 2 MeV - high energy X- (Right) Snapshot of 68 keV X-rays transmitting rays commonly used in clinics. through the water where considerable number of the photons are lost by Compton scattering before reaching the tumor.

The simulation experiment was carried out with three different monochromatic X-ray beams, 68 keV (the resonant energy for gold for enhanced photoabsorption as predicted in RNPT), 82 keV (K-shell ionization energy that most experiment have focused on), and 2 MeV (voltage at highest intensity in a 6 MeV LINAC at medical facilities). The simulation results are Figure 13: Dose enhancement factor (DEF) with 3 presented in the following two plots, Figures 12 X-ray beams, resonant 68 keV (red), K-shell and 13. Even with significant loss of photons by ionization energy 82 keV (green), and clinically Compton scatterings, the 68 keV beam reaching common beam of 2 MeV (blue). The resonant beam the tumor had the maximum effects. Figure 12 gives the most effect DEF at the lowest presents counts of electron productions at the concentration of gold nanoparticles [8]. 9

DEF (dose ehnacement factor) of X-ray Acknowledgment absorptions is an important factor in cancer treatment. It is the ratio of the average radiation Partially supported by DOE-NNSA and dose absorbed by the tumor when it is loaded NSF. Computations were carried out at the Ohio with a contrast medium or agent, such as with Supercomputer Center. nanoparticles, to the dose absorbed without that agent. Figure 13 shows DEF of the three X-ray References beams at various gold concentrations from 0 to 200 mg/ml [8]. The curves show that the energy [1]. Anil K. Pradhan and Sultana N. Nahar, deposition is most effective with 68 keV (red) Atomic Astrophysics and Spectroscopy where the deposition reaches its maximum with (Cambridge University Press, 2011) the lowest concentration. With 82 keV (green), [2]. A.K. Pradhan, Y. Yu, S.N. Nahar, E. the DEF increaes with higher concentration Silver, R. Pitzer, The Radiotherapy before becoming a plateau at about 80 mg/ml. Dynamics (XVth Int. Conf. Use of The 2 MeV (blue) beam has the least effect, Comput. in Radiat. Ther.) Vol. 2, 89 (2007) slow increase in DEF with higher [3]. E. Silver, A.K. Pradhan,Y. Yu, RT Image concentrations. The DEFs obtained for the 21, 30 (2008) resonant X-ray beam of 68 keV are one order of [4]. A.K. Pradhan, S.N. Nahar, M. Montenegro magnitude greater than those calculated at lower et al., J. Phys. Chem. A 113, 12356, (2009) concentration. [5]. Hainfeld, D. Slatkin, Smilowitz 2004), Phys. Med. Biol. 2004;49:N309-N315 These results indicate that RNPT offers a [6]. NIST website: highly effective radiation therapy and http://physics.nist.gov/PhysRefData/Xcom diagnostics for cancer. Research is under [7]. S.N. Nahar, A.K. Pradhan, C. Sur, J. Quant. progress for experimental verifications and Spec. Rad. Transfer 109 19512008 clinical tests. [8]. M. Montenegro, S.N. Nahar, A.K. Pradhan et al., J. Phys. Chem. A 113 123642009 [9]. Pradhan et al, 2012 (in preparation)

QCD studies with anti-proton at FAIR: Development of SiPM based Scintillation Detector Bidyut Jyoti Roy

Physics Goals The one of the main topics at PANDA is the study of charmonium (ccbar meson: Understanding the strong nuclear force c=charm quark, cbar=anti-charm quark) in terms of quarks and gluons is one of the spectroscopy and search for gluballs and hybrids outstanding questions in nuclear physics. The in the charmonium mass region. Gluballs are Quantum Chromo dynamics (QCD) is the basic excited state of pure gluons whereas hybrids are theory of strong interactions and the study of resonances consisting of a quark, an antiquark QCD will be one of the highlights of the and excited gluons. These gluballs and hybrids, programme at future international facility FAIR in contrast to normal mesons and other fermion– (Facility for Anti Proton and Ion Research), antifermion systems, can have spin-exotic Darmstadt, Germany. The central part of FAIR, quantum numbers as the gluons carry additional which will be an upgraded facility of the degrees of freedom. Lattice QCD calculations existing GSI accelerator complex, is a predict a whole spectrum of bound gluon states synchrotron complex consisting of two separate and hybrids, whereas experimental information synchrotron accelerator rings of same is scarce. Bound gluonic systems offer a unique circumference and housed in same tunnel. One way to study one of the long standing problems of the major goals of the accelerator complex is in hadron physics- the origin of mass of strongly to provide intense pulsed proton beams at about interacting particles. As we know, the Higgs 29 GeV which will be then used to produce mechanism might be responsible for creation of antiproton beams. The antiproton beams will be mass of elementary particles. But for proton, the accumulated and transferred to the High Energy Higgs mechanism accounts for only a few Storage Ring (HESR) where the antiprotons can percentage of its mass while rest of its mass, as be accelerated between 1 and 15 GeV/c thus the believed to be, created by the strong interaction. maximum centre of mass energy in antiproton - The gluons which are massless carry color proton collision will be approximately 5.5 GeV. charge i.e., charges of strong interaction and A versatile detector PANDA (antiProton thereby interact strongly between themselves. ANnihilation at DArmstadt) will be installed in This mutual gluon attraction may allow the HESR storage ring where internal target formation of meson-like bound states of gluons experiments (with frozen hydrogen target in the even if no quarks are present. Thus gluballs form of pellet as well as nuclear targets) can be acquire mass which arises solely from the strong performed. The collaboration, known as interaction. At PANDA it should be possible to PANDA collaboration, has proposed a rich search all glueballs and hybrids upto mass 5.5 experimental programme [1, 2] that aims study GeV and make a high precision study by of fundamental questions of hadrons and nuclear detecting both hadronic and electromagnetic physics, carry out precision tests of the strong decay modes revealing their true nature. interaction, investigation of in-medium modifications of hadron mass as they interact For the charmonium spectroscopy (both with nuclear matter, spectroscopy of single and hidden and open charm), the goal at PANDA is double hyper-nuclei and study of to make a comprehensive measurement of electromagnetic process to measure nucleon spectroscopy of charmonium systems and hence form factor. to provide a detailed experimental information

11 of the QCD confining forces in the charm region neutron or proton do. As a result it can populate to complement theoretical investigation. It all possible nuclear states which are not should be highlighted that most of the accessible otherwise. Certainly hypernuclei charmonium states have very narrow width and studies will provide a sensitive probe to the low production cross section, only in nuclear structure. It is to mention that JPARC experiments like panda with its phase space accelerator in Japan has a programme on cooled antiproton beam and high luminosity it is hypernuclear studies. However, at JPARC possible to measure their mass, width and a hypernuclei are produced with kaon beams that systematic investigation of decay modes. make certain limitation. The PANDA experiment plans to take a totally different and Investigation of medium modification of unique approach to hypernuclear physics in hadron mass in nuclear matter is another antiproton – p annihilation process thereby interesting topic that has been planned at allowing a rich programme on hypernuclear PANDA. These studies, so far, have focused in physics. More details on the physics the light quark sector eg., pion and kaon mesons programme with PANDA can be found in due to the limitation in available energy. The in- refs.[1, 2]. medium pion mass is observed to shift as compared to its vacuum value and pion-nucleus The SciTil hodoscope detector potential has been deduced from experimental studies of deeply bound pionic atoms at GSI[3]. The PANDA detector is a complex For kaons, repulsive mass shifts for K+ and detector designed to achieve 4π acceptance, attractive mass shifts for negative kaons are high resolution for tracking, particle observed experimentally [4 and references identification, calorimetry, and high rate therein]. At PANDA, the medium modification capabilities (about 2 x107 annihilations / s). The of hadron mass studies can be extended to the details of the PANDA detector is described charm sector (with both hidden and open elsewhere [1, 2]. The Indian group has taken charm). For example, experiments are being responsibility in construction of some planned to study sub-threshold production of D- components of the PANDA detector, namely, Dbar mesons in antiproton – nucleus collision. Luminisity monitor (based on silicon microstrip If the D meson mass gets reduced in the nuclear detector) and silicon photomultiplier (SiPM) environment, the in-medium DDbar threshold based fast scintillation detector (known as SciTil would be lowered resulting in an enhancement hodoscope). The present article reports the of production cross section at sub-threshold design details and initial R&D studies with energies. SciTil/SiPM in which the physics department, AMU, Aligarh, has taken interest to contribute The PANDA collaboration has an in this development together with the Nuclear extensive program on single and double Physics Division, BARC, Mumbai and Gauhati hypernuclear physics. Such studies will provide university, Guwahati from the Indian us information on hyperon-nucleus and collaboration side. hyperon-hyperon interaction, the present knowledge of which is very limited. So far only The detector, SciTil, will serve for few double hypernuclear events have been precision time measurements for triggering and observed. Hypernuclei are nuclear systems in determination of time-of-flight. The detector, in which one or more nucleons are replaced by the present design, consists of about 5700 hyperons. A hyperon in a nucleus is not bound scintillator tiles readout by Silicon by the Pauli Exclusion Principle, unlike a Photomultipliers (SiPM) and will be mounted in

12 the space between the electromagnetic calorimeter (EMC) and DIRC Cherenkov detector (DIRC stands for Detection of Internally Reflected Cherenkov light, it is a Cherenkov detector for particle identification of high energy charged particles like pion, kaon, proton etc.). The concept of SciTil provides the use of minimum material (so that not to deteriorate performance of other surrounding detectors) and a good spatial resolution due to its granularity. The timing detector concept is based on 3 × 3 × 0.5 cm3 scintillator tiles matching the front face of the calorimeter crystals. The scintillation photons produced in the scintillator tiles due to passage of charged Fig. 2: (Left) Design view of a single scintillator tile particles will be collected and readout by with SiPM attached. (Right) a quad SciTil module with planned PCB readout. SiPMs. The number and size of the SiPMs and position of SiPM that will be coupled to the tile R&D studies with Silicon Photomultiplier will be optimized based on a detailed simulation and R&D studies. In addition to timing and Silicon photomultipliers (SiPMs) are position information, the hodoscope will allow very new type of photon counting devices that clean detection of gamma-conversions in front show great promise to be used as detection of the EMC in particular within the region of device in combination with DIRC. In addition, it is best suited for scintillators/Cherenkov radiators. SiPM is discrimination between charged and neutral essentially an avalanche photo-diode operated in particles. The conceptual design [5] of the limited Geiger mode. The SiPM module is a SciTil is shown in the Figs. 1-2. photon counting device capable of low light

level detection. It is essentially an opto-

semiconductor device with excellent photon counting capability and possesses great advantages over the conventional PMTs because EMC(calorimeter) of low voltage operation and insensitivity to magnetic fields. In many of the high energy physics experiments, the photon sensors are required to operate in high magnetic fields precluding the use of conventional PMTs. This SciTil Hodoscope problem can be overcome with the use of SiPMs. SiPM operating in Geiger mode, a very (half shell) large gain (~ 106), magnitude of which is determined by the internal diode capacitance and applied over-bias voltage, comparable to

that of PMTs can be achieved. A SiPM consists Fig.1: A conceptual design of half shell of the SciTIl of matrix of micro cells (known as pixels), 2 is shown along with the EMC crystal. typically between 100 and 10000 per mm . Each micro cell acts as digital device where the output signal is independent of the number of

13 photons absorbed. When all the cells are measured the particle detection efficiency connected in parallel, the SiPM becomes an (PDE) as a function of wavelength of the analog device thereby allowing the number of incident photons. For this, a monochromator incident photons to be counted. Detailed R&D that spans wavelength from 200 to 800 nm was studies with SiPM are needed for the use of this used. Different intensity filters were used for device to PANDA experiment. With this light intensity attenuation. The photo-sensitivity motivation in mind, we have developed a SiPM of different SiPMs was normalized with a PIN test facility and have tested several diode which itself was calibrated by the commercially available SiPM for their producer. The dark count of the MPPCs were performance study[6,7,8,9] and comparison measured at 0.5thr and 1.5thr and found to be in with other photon counting devices. Different agreement with the specifications provided by types of SiPM manufactured by Hamamatsu and the supplier. Fig.3 shows a typical SiMP Zecotek with different sizes of pixels and active spectrum for low intensity laser light, showing area have been tested. A list of the SiPMs used up to eight individual peaks corresponding to in the present study is given in Table 1. Studies different number pixels fired. are also being done with Hamamatsu make MPPC of size 3 x 3 mm2, an array of 2 x 2 with total active area 6 x 6 mm2.

Table 1: SiPM details used in our present study. MPPC1 and MPPC2 are Hamamatsu make while MAPD3N is from Zecotek.

Device Active Pixel Pixel area size ensity (mm2) ( m) (1/mm2) MPPC1 1x1 100 100 MPPC2 1 x 1 25 1600 MAPD3N 3 x 3 7 15000 Fig.3: A typical SiPM spectrum from Hamamatsu MPPC triggered by laser light. Different peaks correspond to different photo peaks, hence, the During the measurement, all devices number of pixels fired. In this case up to eight photon peaks are seen (photograph taken from were mounted in a light tight box and were digital oscilloscope with histogram mode on). illuminated by a pico-second pulsed diode laser (make Pico-Quant) of 660 nm wavelength as In fig.4, we plot the photon detection well as LED with λ = 460 nm. In order to be efficiency (PDE) distribution as a function of able to distinguish between single and multi- the wavelength for Zecotek make MAPD3N photon peaks, the laser intensity was controlled. and Hamamatsu make MPPC. For MAPD3N, The voltage and current on the SiPM were the distribution has been normalized with measured by high precision multimeter and I-V PDE=24.5% at = 450 nm[10] and for MPPC, characteristic of the photo-diode was studied. The preamplifier used was “Photonique SA” PDE=32.4% at = 450 nm[11]. It is to be noted make two varieties: one with high gain(20x – that the present data for MPPC shows much 60x) but relatively slow rise time(~5ns) and the broader distribution extended over larger other one with lower gain(10x – 20x) but wavelength range as compared to the report of having faster rise time (~700ps). We have also Hamamatsu.

simulation studies and prototype development for which are in progress [12].

Summary

The facility for antiproton and ion research (FAIR) will be a unique facility for the investigation of hadron structure and QCD in the charm sector. The Indian hadron physics community has taken strong initiative to be a part of this exiting physics programme and to contribute in the detector construction, simulation & software development and physics case studies. The Aligarh Muslim University has shown interest to be a part of the SiPM / SciTil hodoscope development work. Initial R&D studies with SiPM show a great promise of this device to be used as photon counter and a great potential to replace the conventional PMTs.

Acknowledgements

We thank the whole India-PANDA collaboration team and especially Dr. S.kailas,

BARC and Prof. R.Varma, IIT-Bombay for Fig.4: (Left part) Photon detection efficiency distribution of MAPD3N as a function of wave their effort and continued interest in this work. length normalized with data from ref.[10] at = The work reported here has been carried out in 450 nm. close collaboration with the HAD-1 group, GSI, (Right part) Photon detection efficiency distribution Germany. We thank the GSI group (Prof. of MPPC normalized with data from ref.[11] at = K.Peters, Dr. H.Orth, Dr. C.Schwarz, Dr. Lars 450 nm. Smith and Dr. A.Wilms) for their support and interest. For the SciTil hodoscope, in order to optimize the geometry of SiPM and light References collection efficiency, simulations are being performed with a Monte Carlo simulation 1. PANDA collaboration- physics performance program SLitrani. SLitrani stands for light report, arXiv:0903.3905 and PANDA transmission in anisotropic media, with „S‟ technical progress report, Gold version, having the meaning of super. It is a general 2005. purpose Monte Carlo program that simulates 2. QCD studies with anti-protons at FAIR: light propagation in isotropic media (it can also Indian participation in PANDA, S.Kailas, be used for anisotropic media) and is built upon B.J.Roy, D.Dutta. V.Jha, R.Varma, Current ROOT. The one of the main emphasis of this Science, Vol. 100, No.5 (2011) and hodoscope detector is to provide a very fast references therein. timing (sub nanosecond timing), detailed

3. Geissel H et al, Phys. Rev. Lett. 88, 8. In-beam test of a DIRC Cherenkov radiator 122301(2002), Phys. Lett.B., 549 (2002) 64 with SiPM, B.Kroeck, A.Hayrapetyan, and references therein. K.Foehl, O.Merle, M.Duren, B.J.Roy, 4. Nekipelov M. et al, Phys. Lett. B, 540 K.Peters, DAE Symp. Nucl. Phys. Vol. 54, (2002) 207, Rudy Z. et al, Euro. Phys. J 668 (2009). A15 (2002) 303. 9. Development of SiPM based scintillation 5. Proposal for a Scintillation Tile Hodoscope detector for fast timing application in the for PANDA, K.Goetzen, H.Orth, PANDA experiment, H.Kumawat, B.J.Roy, G.Schepers, C.Schwarz, A.Wilms (private V. Jha, U.K.Pal, A.Chatterjee and S.Kailas, communication). DAE symp. Nucl. Phys, (2011). 6. SiPM as photon counter for Cherenkov 10. D. Renker, PSI (private communication). detectors, B.J.Roy, H.orth, C.Schwarz, 11. N.Anfimov, Dubna (private A.Wilms, K.Peters, DAE Symp. Nucl. Phys. communication). Vol. 54, 666 (2009). 12. Simulations of SiPM based scintillation 7. Study of the spectral sensitivity of G-APDs detector for PANDA, U.K.Pal, B.J.Roy, in the wavelength range from 250 to 800 V.Jha, H.Kumawat, A.Chatterjee and nm, B.J.Roy et al, 12th Vienna Conf. on S.Kailas, DAE symp. Nucl. Phys, (2011). Instrumentation, Feb. 2010, Vienna.

Wong vs. Dynamical Cluster-decay Model for Heavy Ion Reactions Manie Bansal, Sahila Chopra and Raj K. Gupta*

Introduction expression via its -summation, which led them [2] to an -summed extended-Wong model, with Heavy-ion reactions at low-energies -summation effects included explicitly. (<15 MeV/A) give rise to compound nuclear systems that are excited, carry angular Gupta and collaborators [3]-[10] also momentum , and decay by emitting multiple introduced the concept of relative pre-formation light particles (LPs: n, p, α) and γ-rays probability P0 of various decay products, in (constituting the evaporation residue ER), their, so-called, dynamical cluster-decay model fusion-fission ff (consisting of the, so-called, (DCM), in order to be able to study the relative intermediate mass fragments IMFs of masses contributions of various components (ER, ff and 5≤A≤20 and charges 2

The Extended Wong Model and the For making an explicit -summation in Eq. (2) Dynamical Cluster-Decay Model (DCM)   [2], the -dependent barrier quantities VB , RB ,

and ω are needed, given by the -dependent Extended-Wong model: Fusion cross-section interaction potential due to two deformed and oriented nuclei (with orientation angles ζi), lying in two different planes (with azimuthal angle Φ between them), V R V R, A , , T, , V R, Z , , T, , and colliding with Ec.m., defined by Wong [1] in  P i i i C i i i terms of  partial waves, is 2  1 (5) 2 R2  m ax (E , , ) (2 1)P (E , , ) c.m. i 2  c.m. i k  0 where temperature T-dependent Coulomb and with k 2 E / 2 . (2) proximity potentials, VC and VP, for deformed, c.m. oriented co-planar (Φ=00) or non-coplanar 0 (Φ≠0 ) nuclei are from [11] or [12]. βλi, λ=2,3,4 P is the transmission coefficient for are the static quadrupole, octupole and each , calculated in Hill-Wheeler hexadecapole deformations, and T (in MeV) is approximation of an inverted harmonic related to the incoming Ec.m. or the CN * * oscillator to interaction potential excitation energy E as E = Ec.m.+Qin=(1/a) 2 V(R,Ec.m.,ζi,Φ) due to incoming channel, as AT –T, with a=9 or 10, respectively, for intermediate mass or super-heavy nuclei. Qin is the entrance or incident channel Q-value. This is  2 (VB (Ec.m. , i , ) Ec.m. ) referred to as the (-summed) extended-Wong P 1 exp . (3) model.   (Ec.m. , i , )

 max is determined empirically for a best Here, VB and ω are the barrier height fit to the measured cross-section (see Fig. 1 in  and curvature at barrier position RB . Wong [2]). If the -summation procedure fails to carried out the summation in Eq. (2) reproduce data, an empirical “modification of approximately by assuming ω=ω0, the barrier” is introduced either via the barrier  0 2 02  0  , VB VB  ( 1) / 2 RB and RB ≈RB , and height VB , or via the curvature   defined as obtained a formula in terms of the =0 barrier- 0 0   emp based quantities VB , RB and ω0, the barrier VB modified VB VB , emp height, position, and curvature for =0, as or   mod ified    . (6)

Note that the “barrier modification” 02 emp emp R  2 VB [or Δ ], is a fixed quantity, (E , , ) B 0 ln 1 exp E V 0 c.m. i c.m. B (4) independent of , added to (or subtracted from) 2Ec.m.  0 the barrier height V  [or curvature  ] for all B  -values. which on integrating over both ζi and Φ gives the fusion cross-section σ(E ). Eq. (4) is c.m. DCM: Introducing the relative pre- referred to as the =0 barrier-based Wong  formula. formation factor P0 for all decay products Ai, i=1, 2, Eq. (2) gives the DCM expression for

CN decay/ formation cross-section for each V(Ra)=V(Rb)=Qeff=TKE(T), which means that  process (LPs, ff), as V(Ra) acts like an effective Q-value, Qeff(T,),

given by the total kinetic energy TKE(T). max is  m ax  m ax fixed for the light particles cross-section (E , , ) (2 1)P  P c.m. i  2 0  (7)  0 k  0 σER()→0 at =max. For the definition of angles αi in radius vectors Ri, and the orientation ζi, see where P  refers to motion in mass asymmetry Fig. 2 in [3]. Note that the neck-length 0 parameter ∆R also contains the effects of coordinate ε=(A1-A2)/(A1+A2), given as the solution of stationary Schrödinger equation in ε “barrier lowering” in it for each decay channel, defined for each  as the difference between V  at a fixed R=Ra, as B     and V(Ra) , VB -[VB - V(Ra ) ] , the actually  2 1 calculated and the actually used barriers. V (R, ,T) ( ) E ( ) Finally, for qf (≡capture), only the incoming 2 B B  channel plays the role, and hence P0 =1 in (8) DCM.

64 112 176 * 64 112 176 * Ni+ Sn Pt (a) Ni+ Sn Pt (b) where ν=0,1,2,., refers to ground-state (ν=0) 103 and excited-states solutions. Then, the 102 probability 2 2 , with a

P0 (Ai ) R ( (Ai )) B 0 (mb)

(mb) 10

A fus Expt = + Boltzmann-like function 101 Expt. fus ER fission Wong: ( , )-integration ER i 2 2 fission -summed ( V =0) . -3 B | | | | exp( E /T ) opt. 10 DCM ( , ) -summed ( V =0)

2i i  B Cross-section 0 Cross-setion ER Wong: -integrated ( =00) i 0 fission 10 -summed ( V =0) qf B -summed ( V =0) fission+qf  B In Eq. (8), the fragmentation potential V(ε) at 10-6 150 160 170 180 190 200 150 160 170 180 190 200 fixed R=R is E (MeV) E (MeV) a c.m. c.m.

2 Fig.1: (a) Calculated DCM σER, σfiss, σqf [9]; (b) VR ,T [VLDM (T) U(T)] VP VC V , i 1 extended-Wong model σfus[10], compared with (9) experimental data. with VLDM and δU as the liquid drop and shell effect energies. Calculations and Results

The penetrability P in Eq. (7) of DCM is the We illustrate the distinguishing features of the  two model (extended-Wong and DCM) for the WKB integral, 64 112 176 * reaction Ni+ Sn→ Pt decaying via ER and

Rb fission processes [14]. Fig. 1(a) gives the DCM 2 1/ 2 P exp 2 V(R,T) Q dR , results, showing (via P0 calculations) that ER  eff Ra and fission are the only viable decays, and the (10) calculations fit the data nicely for co-planar (Φ=00) nuclei, predicting a small qf component solved analytically [13], whose first turning at the highest one above-barrier energy [9], point Ra=R1(α1, T)+R2(α2, T)+∆R(T) is defined which might become zero for, not yet studied, through a neck-length parameter ∆R for the best Φ≠00 case. Note, however, that “barrier fit to measured cross-section for each process modification” at sub-barrier energies is in-built and the second turning point Rb satisfy in DCM [9].

3. R.K. Gupta, S.K. Arun, R. Kumar, and Fig. 1(b) gives the extended-Wong model fitted Niyti, Intern. Rev. Phys. (I.RE.PHY.) 2, 369 σfus, for ζ alone and (ζ, Φ) integrations [10], (2008). requiring „barrier modification” for the best fit 4. B.B. Singh, M.K. Sharma, and R.K. Gupta, at sub-barrier energies in both cases. Phys. Rev. C 77, 054613 (2008). 5. R. Kumar and R.K. Gupta, Phys. Rev. C 79, Conclusion 034602 (2009). 6. S.K. Arun, R. Kumar, and R.K. Gupta, J. We have shown that the DCM is able to Phys. G: Nucl. Part. Phys. 36, 085105 describe the detailed compound nucleus decay, (2009). like the individual decay processes, together 7. R.K. Gupta, S.K. Arun, R. Kumar, and M. with their fragment masses, whereas the Bansal, Nucl. Phys. A 834, 176c (2010). extended-Wong model gives simply the (total) 8. R.K. Gupta, Lecture Notes in Physics, 818, fusion cross-section alone. The non-coplanar “Clusters in Nuclei”, ed. C. Beck (Springer- degree of freedom Φ Verlag 2010) Vol. 1, p. 223. is introduced and the quasi-fission (qf) 9. M.K. Sharma, et al. J. Phys. G: Nucl. Part. component, if any, in σfiss at above-barrier Phys. 38, 055104 (2011); AIP Conference energies, and barrier-modification for σER at Proceedings 1265, edited by R. Alcaron, et below-barrier energies are studied. al., (2010) p. 37. ACKNOWLEDGEMENTS 10. R.K. Gupta and M. Bansal, Intern. Rev. Work supported by the Department of Science Phys. (I.RE.PHY.) 5, 74 (2011). and Technology (DST), Govt. of India. 11. R.K. Gupta, N. Singh, and M. Manhas, Phys. Rev. C 70, 034608 (2004). References 12. M. Manhas and R.K. Gupta, Phys. Rev. C 72, 024606 (2005). 1. C.Y. Wong, Phys. Rev. Lett. 31, 766 (1973). 13. S.S. Malik and R.K. Gupta, Phys. Rev. C 39, 2. R. Kumar, M. Bansal, S.K. Arun, and R.K. 1992 (1989). Gupta, Phys. Rev. C 80, 034618 (2009). 14. W.S. Freeman, et al. Phys. Rev. Lett. 50, 1563 (1983); K T. Lesko, et al. Phys. Rev. C 34, 2155 (1986).

Experimental Reactor Physics and its role in the Nuclear Reactor Design Rajeev Kumar

Introduction systems. Research and development to design an Advanced Heavy Water Reactor (AHWR) The title of the articles sounds somewhat based on thorium fuel cycle is going on in similar to Experimental Nuclear Physics, but Bhabha Atomic Research Center (BARC), that is not the case, in fact it is very much Mumbai, India. Various experiments are being different except one part that is nuclear data carried out in test critical facility to validate the which will be described later in the article. design parameters of AHWR. Basically Reactor Physics is a field which deals with the neutron transport in the reactor core. Experimental Reactor Physics This is further divided into two branches viz., (i) the steady state neutron transport and (ii) the Let us first understand in brief what is neutron kinetics. Theoretical models are used to Reactor Physics all about? The core of a nuclear study the neutron transport in the core. Various reactor consists of fuel viz. Uranium, moderator computer codes based on deterministic as well (to slow down and thermalise the neutrons in as stochastic principles are available. thermal reactor) and coolant to remove the heat produced in the fuel. The fuel channels in the Though, there is enough experience in core are arranged in a symmetric and periodic construction and operation of nuclear reactors manner. The geometry of this arrangement can worldwide, yet when we design new type of be square or triangular/hexagonal depending on nuclear reactor system, it is very important to the desired core feature. Reactor physics deals validate its design in the domain of physics as with the neutron transport through the materials well as in engineering. As far as physics design present in the core in steady state as well as in is concerned, it progresses in two steps, the first transients with respect to time. There are is that one has to model the reactor core several quantities which are required to be theoretically and calculations are done to get the defined to study a reactor core; some very basic various design parameters using the available quantities are discussed here. 1) Neutron flux: computer codes and then all these calculations Number of neutrons traveling through unit area are checked/benchmarked against the in unit time is called neutron flux and denoted experiments carried out in a test reactor facility. by Ф (neutrons per cm2 per second). Neutron This is where experimental reactor physics distribution in energy (energy spectrum) and comes into picture. Various reactor physics neutron distribution in space are important experiments are carried out in the test facility to quantities which depend on the core condition. produce the experimental data which are One thing should be noted here that in spite of compared with the calculated values. The the fact that neutron are fermions, Maxwell comparison between the measured and distribution is followed in reactor physics. The calculated values gives the feedback which reason is that for typical neutron flux 1012 helps in designing the new reactor system. neutron per cm2 per second, the neutron density Presently, there is worldwide interest in is orders of magnitude less than the same inside designing the new type of nuclear system like a nucleus where it behaves like fermions. In Gen IV Reactors, accelerator driven sub critical other words the average distance between the reactor etc. Experimental reactor physics is very neutrons in a reactor is much greater than their much required to validate the design of such de Broglie wavelength (there is no overlapping

21 of wave functions), therefore quantum effect has further two branches one is in core does not come in to the picture. 2) Neutron measurement and the other it out core multiplication factor (k): Nuclear fission is measurements which is generally carried out in chain reactions which multiply the number of radiation shield design. neutrons; this neutron multiplying property is defined by multiplication factor (k) which In-core measurement is being discussed depends on the core configuration and here in brief. The method used in the in-core conditions. It is a ratio of number of neutrons in measurement is activation technique. It is a well a particular generation to the same in preceding established technique and used in various other generation. For self sustaining controlled chain fields also. The neutron detectors used are called reaction the value of k should be equal to unity activation detectors, they are basically small and it is called critical condition. Otherwise if size foils (2 mm x 2mm) of various materials the value of k is less or greater than 1, the like Gold, Copper, Manganese, Iron , Nickel system is called subcritical or supercritical and Titanium etc. The basic measurable quantity respectively. 3) Worth of control system: It is in the neutron environment is the reaction rate related to the properties of the neutron i.e., the number of reactions that take place per absorbing materials (required for controlling the unit nuclei per unit time defined as; chain reaction) 4) Burn up: It deals with the composition of fuel which changes as fuel burns Reaction rate= (E) (E) dE to produce the energy. A number of fission products which keeps on accumulating in the Here, σ(E) is the activation cross section for a fuel, absorb the neutron in a non fission reaction given type of reaction (fission, absorption or and hence affect the neutron multiplication scattering etc) and Ф(E) is neutron flux. Both factor. 5) Nuclear data: It is basically the cross these quantities are function of energy. The sections for several reactions which may take cross-section σ(E) values are known from place when neutrons interact with materials as it nuclear data and hence above mentioned moves in the core. integral can be unfolded to get the neutron flux. Other quantities important to characterize a Nuclear data is very important input to reactor core also can be derived from the the reactor physics calculations. The cross reaction rate and can be used to validate the sections of a given reaction and nuclei are calculated values. measured in various laboratories worldwide with different accuracy and precision. Now let us see how do the reaction rates Sometimes it happens that the cross section for a are measured using activation technique? When particular isotope in high energy (few hundred a foil like Gold (197Au) is exposed in a neutron MeV) range has relatively high experimental environment, it absorbs a neutron ((n,γ) errors. All these facts cause the uncertainty in reaction) and formation of 198Au takes place the reactor physics calculation, since it uses the which is radioactive isotope of gold and nuclear data as one of its input. Apart from undergoes beta decay (Half life=2.7 days) to others, error in nuclear data is a major source of produce 198Hg* having an exited nuclear state. uncertainty in calculated design parameter of a The Excited 198Hg* nucleus comes down to its reactor core. The only way to account these ground state by emitting characteristic gamma uncertainties is to carry out the experiments in rays (411 keV). In a simple language we can say test reactor facility and compare the that neutron flux information is stored in the measurements and calculations. This is where gold foil in the form of induced gamma activity. experimental reactor physics comes into play. It The reaction can be written as equation (1).

is an offline measurement technique and there is no interference of gamma rays which is a matter of concern in other methods. This was all about static neutron flux measurement. For kinetic measurement other technique whose response is The gamma activity of 198Hg* is counted by high a function of time like reactor period purity Germanium detector (HPGe) and the measurement is used. gamma activity can be related to neutron reaction rate written in equation (2). Role of Experimental Reactor Physics

Today, there is an increasing demand of (E) (E) dE = A/ (N 1 2 3G ( ) (1-exp (- t )) exp.(- T )) (2) electricity in the society. Per capita electricity consumption also is an index of development in Where, the country. Considering the facts like green A = Counted gamma activity using HPGe house effect and global warming, nuclear energy is the better option. But at the same time N = total number of target atoms = isotopic abundance there are some problems with issues like nuclear 1 waste management globally and availability of = yield of gamma ray 2 fissile material in Indian context. Research and = photo-peak counting efficiency of HPGe 3 development are going on to take care of these T = time elapsed between end of irradiation and issues where scientist and engineers are working start of counting on alternative fuel cycle and new reactor types, t = duration of irradiation for example Accelerator Driven Subcritical = Decay constant for radioactive isotope System (ADSS) is one of them. The role of G ( ) = self shielding factor of activation foil, experiment is discussed here briefly. In ADSS, a is no of mean free path. subcritical reactor core (K<1) is coupled with (E) = Activation cross-section at energy E accelerator based spallation neutron source. (E) = Neutron flux at energy E. Spallation is nuclear reaction in which few GeV proton bean impinges on high Z material target This was a typical example to show, how and produces lot of neutrons which will help to an activation detector works. However, Gold, sustain the chain reaction in subcritical system. Copper, Manganese are thermal activation The advantage of ADSS is that being a detectors since they have good activation cross subcritical system, safety related issues are section in the thermal energy region (0.025 eV matter of low concern relative to existing to 1 eV). For fast energy region, detectors like critical reactors. The problem of waste Titanium, Nickel and Iron are used, these are management is also supposed to be taken care of called threshold detectors because they have by ADSS. finite cross section only after certain energy for 54 54 example Iron, Fe (n, p) Mn has about 3 MeV ADSS is different from the existing of threshold energy. critical system in many aspects for example fuel type and neutron energy region. The available The merit of activation detector is that nuclear cross-sections of some important they are very small in size hence flux isotopes in high energy region need more perturbation inside the core is negligible and at accurate measurement to minimize the the same time they can be placed any where uncertainties in the calculations. Subcritical inside the core; for example flux inside the fuel reactor physics is different from physics of pin also can be measured using these detector. It

23 critical system. Hence, new computational tools new computational tool can be benchmarked are being developed. Considering all these facts, against the measurements. Experimental reactor it is very important to carry out the experiments physics can help in performing this task and and generate the data so that uncertainties in the hence it always plays an important role in theoretical modeling can be accounted for and designing the new reactor system.

Proton Computed Tomography (pCT) for Cancer Therapy Dr. Naimuddin

Introduction History of proton (hadron) therapy

Proton therapy is a rapidly expanding The use of energetic protons for form of cancer treatment. Because protons have therapeutic means was first proposed by Robert a finite range in matter, this treatment modality Wilson in 1946[1], while working on the design allows for a greater degree of conformality than of the Harvard Cyclotron Laboratory. Wilson conventional external beam X-ray therapy. To proposed that proton beams would be maximize the inherent advantages of proton advantageous for the treatment of deep-seated therapy, the range of protons inside the patient tumours because of the favorable relationship must be able to be predicted with millimetre between deposited dose and depth in the resolution. In current clinical practice, proton absorbing material. This characteristic of heavy therapy treatment plans are made with pre- charged particles is known as the Bragg peak, a treatment X-ray CT scans of the patient. To phenomenon whereby the maximum dose along convert the X-ray CT Hounsfield units to proton a trajectory is deposited in a peak-like relative stopping powers, which are required by distribution toward the end of the range. the treatment planning software, an empirically derived calibration function is used, which is Computed tomography with heavy specific to each X-ray CT machine. However, charged particles, and protons in particular, was because of the different dependence on Z and first proposed as a possible alternative to X-ray the Z/A ratio of X-ray attenuation and proton CT by Allan Cormack [2, 3]. Cormack proposed energy loss, the relationship between Hounsfield that the variable density of matter with constant units and relative stopping powers is not unique. chemical composition could be determined by This conversion process leads to range measuring the energy loss of charged particles uncertainties at treatment time. A preferable in the matter. Cormack suggested that the values scenario is one in which the relative stopping reconstructed with charged particle CT would power of each patient is reconstructed directly. be an important tool in the treatment planning This is the goal of proton computed tomography process of heavy charged particle therapy, (pCT). Figure 1 shows a typical computed where the depth of the Bragg peak must be tomography scanner. determined with a high degree of accuracy.

Though the proton CT was first proposed in 60s but a clinical system is yet to be realized. Difficulties experienced in previous projects included long acquisition times and sub-standard spatial resolution relative to X-ray CT. The current pCT development project makes use of advances in high energy physics detector technology and focuses on generating pCT specific image reconstruction algorithms to counteract the aforementioned issues.

Mimicking the progression of X-ray imaging, the initial heavy charged particle Figure 1: Computed tomography scanner

25 imaging studies were of radiographic nature (2D that reconstruction artifacts were present at projections). Studies showed that by using a boundaries between substances of differing stack of parallel sided aluminium plates of a density. It was suggested that this was the result thickness just less than the proton range, of a differing degree of multiple Coulomb radiographs of much greater contrast could be scattering (MCS) in the two regions. recorded with protons than that obtained with X-rays [4]. Subsequently, it was demonstrated Later, Hanson and colleagues achieved that the high contrast images obtained by proton the first 2D tomographical images generated radiography provided improved imaging of low with protons [8]. Hanson‟s system consisted of a contrast lesions in human specimens over hyper-pure germanium detector (HPGe) and a conventional X-ray techniques [5]. The high multiwire proportional chamber (MWPC) to contrast obtained in this energy-loss form of measure the residual energy and exit position of radiography is a consequence of the sharpness each exiting proton, respectively. Collimated of the Bragg peak. Figure 2 shows the 192 and 240 MeV proton beams from the Los comparison of energy (dose) deposited by x rays Alamos Meson Physics Facility were used to and protons in matter (body). scan 20 and 30 cm cylindrical phantoms submerged in a water bath, respectively. The A tomographical reconstruction with phantoms contained cylindrical inserts of heavy charged particles was firs carried out by varying density to quantify contrast resolution. Goitein in 1972 [6]. He employed projection The scanning was achieved by lateral translation data measured by Lyman with alpha particles to of the phantom across the proton pencil beam demonstrate the utility of his least-squares and phantom rotation following full translation. reconstruction algorithm. Later, in studies by It was found that the pCT scanner could deliver Crowe and colleagues at LBL, it was shown that 9 times less average dose than the contemporary alpha particle CT had a dose advantage over X- X-ray CT EMI scanner for a given density ray CT in human head reconstructions. In this resolution. The disadvantage of pCT was found system, stacks of scintillating plates were used in the lack of spatial resolution. Even with the to determine the residual energy of the alpha use of the position sensitive single particle particles. tracking MWPC to counteract the effects of MCS, a decrease by a factor of 2−2.5 in It was not until 1976 that a prototype comparison to the images generated by the EMI proton computed tomography (pCT) system was scanner was quoted. Hanson pointed to the constructed by Cormack and Koehler and tested possibility of using curved trajectories as at the Harvard Cyclotron Laboratory [7]. Their opposed to straight lines in the reconstruction to system was based on scanning a radially improve spatial resolution. symmetric Lucite phantom, containing sugar solution and polystyrene inserts, with a Hanson and colleagues also went on to collimated 158 MeV proton beam. The detector carry out pCT scans of biological specimens system consisted of a scintillator crystal with a magnetically scanned pencil beam and mounted on a photomultiplier to measure the the same detector system as described above [9]. energy of protons after traversing the phantom. By rebinning individual proton exit positions The radial density profile was reconstructed and using an iterative peak fitting procedure to with Cormack‟s line integral theory [2, 3]. With assign a mean residual range to each spatial bin, this simple system, it was shown that density 2D pCT images of a human heart and brain differences of 0.5% could easily be were generated with the filtered backprojection distinguished. However, the authors also noted reconstruction algorithm. A comparison with

26 two contemporary X-ray CT scanners Current status of proton (hadron) therapy reconfirmed the previous findings that, although a superior dose-density resolution relationship Treatment of patients with protons was could be achieved with pCT, spatial resolution first carried out in 1955 at the Lawrence was degraded. Berkeley Laboratory (LBL), California. In 1961 the Harvard Cyclotron Laboratory and The development of pCT experienced a Massachusetts General Hospital began hiatus following the work of Hanson and collaboration in the pursuit of proton therapy. colleagues. It was not until the expansion of Over the next 41 years, this program refined and proton therapy in the 1990‟s that renewed expanded the founding techniques, in particular interest was placed in proton imaging. by introducing Bragg peak treatments. While Schneider and colleagues at the Paul Scherrer proton treatments at LBL used high energy Institute, Switzerland used a proton radiography “shoot through” style proton treatments, the apparatus to examine the accuracy of proton advantage of stopping the proton beam at the therapy range prediction with X-ray CT distal edge of the tumour was first realised by methods [10, 11, 12]. By comparing X-ray CT the Boston collaboration. During this period predicted residual proton ranges with those patients were also treated at other research measured by a proton radiography system, they facilities in the U.S.A, Sweden and Russia. showed that standard treatment planning However, it was not until 1990 that the first procedures in proton therapy could result in hospital based proton treatment facility was range uncertainties of up to 3% of the proton installed at Loma Linda University Medical range [13]. Center (LLUMC), California. Since then, an ever expanding number of hospital based In 2000, Zygmanski and colleagues facilities have been established. There are presented results from a cone-beam pCT system currently 28 proton therapy centers operating with the goal of applying the stopping power throughout the world; 12 in Europe, the UK and tomographs to proton therapy treatment Russia, 8 in North America, 7 in Asia, and 1 in planning. The residual energy detector system South Africa. In addition there are 21 facilities consisted of a solid state intensifying screen in the planning stage or under construction. By viewed by a cooled CCD camera. This planar the end of 2008, over 60,000 patients had detector configuration allowed for fast received proton therapy. The primary factors acquisition and reconstruction of 3D proton RSP limiting even further expansion of proton maps. therapy are construction and running costs, and the size of the accelerators and beam delivery The Feldkamp-Davis-Kress (FDK) [14] systems required to treat with up to 250 MeV cone-beam reconstruction algorithm was used to protons in a rotating gantry. Unfortunately there obtain the CT voxel data representing proton is not a single hadron therapy center in India at stopping powers. It was found that the pCT the moment. Nevertheless, India is seriously reconstructed values were closer to real considering building one proton therapy center phantom stopping powers than the values at the Variable Energy Cyclotron Center calculated with X-ray CT followed by (VECC) at Kolkata during the next plan. Tata conversion. However, due to the significant lack Memorial Hospital at Mumbai is also working of spatial resolution and large degree of noise, on a plan to install one hadron therapy machine. this concept was not pursued further.

How it works In the active scanning approach, the energy of the proton beam is altered at the level To cover an entire tumour volume with a of the accelerator. The pencil beam is then uniform dose, the range of protons must be magnetically guided to cover the target volume modulated and the initial pencil beam must be laterally and vertically. Although active spread laterally. This 3D modulation can be scanning systems are somewhat more of a achieved by passive scattering or active technical challenge, a number of advantages scanning means. In a passive scattering system over passive scattering systems exist. A the energy of the proton beam emerging from significant factor is that the proton beam the accelerator is modulated by a rotating wheel traverses fewer components in the beam line, of varying thickness. These modulator wheels resulting in more efficient treatments and a can be designed in such a way to produce a reduction of unwanted neutron dose to the plateau high dose region over a specified range. patient. Also, truly 3D intensity modulated This dose distribution is known as a spread out proton therapy (IMPT) becomes possible. Here, Bragg peak (SOBP) as shown in Figure 2. the intensities of individual pencil beams are Between the nozzle exit and the patient surface, optimized, resulting in highly conformal dose a field-specific collimator is used to shape the distributions. However, more research and field laterally to conform to the target volume development is required before intensity and a range compensator is used to correct for modulated proton therapy becomes common patient surface irregularities, density place. heterogeneities in the beam path, and changes in the shape of the distal target volume surface. To capitalize on the fundamental The lateral spread of the beam is usually properties of proton beams, the distribution of achieved with a double scattering system in proton stopping powers within the body must be which the initial pencil beam is scattered to a well known prior to treatment. Currently, this Gaussian distribution and then flattened with a information is gained from pre-treatment X-ray second compensating scatterer. Passive computed tomography (CT) scans of the patient. scattering systems are the most common method Such a system reconstructs the distribution of used in current clinical practice. photon relative linear attenuation coefficients, values known as Hounsfield units. To obtain the proton relative stopping power (RSP) map, the reconstructed Hounsfield units are converted with an empirically derived calibration curve. The machine dependent calibration function is calculated by taking an X-ray CT scan of a phantom containing inserts of known RSP values. However, because of the different dependence on Z and the Z/A ratio by photon and proton interactions, the relationship between Hounsfield unit and proton RSP is not unique. Figure 2: A comparison of energy deposited by X This conversion process can lead to range ray and protons as a function of depth. X ray deposit uncertainties of up to 3% of the proton range, energy in small packets all along their path through resulting in a possible under-dose to the tumour tissue, whereas proton deposit much of their energy volume or an over-dose to the surrounding at the end of their path (called “Bragg Peak”) and healthy tissue. While more accurate conversion deposit less energy along the way. methods have been proposed, a favorable

28 solution is to measure and reconstruct the proton mathematics, computer science, medical RSP distribution directly. This is the primary science, accelerator physics and high energy goal of proton computed tomography (pCT). physics.

What are we doing

We have started construction of a clinically usable pCT detector system for imaging head-size phantoms in a single gantry rotation of 3600. Irradiation times less than 6 minutes are required which in turn require high data acquisition (DAQ) rates for proton tracking and energy loss measurements. We have proposed to use scintillation fiber technology with sub-millimeter resolution to replace the Silicon strip tracking system that is presently being used in prototypes, for example one at

Loma Linda University Medical Center Figure 3: Conceptual design of proton computed (LLUMC) in California, USA. A new proton tomography system. Two 2D sensitive proton range detector with stacked scintillator plates tracking modules are positioned pre- and post- will replace the segmented CsI calorimeter used patient. A segmented crystal calorimeter records to enhance data rate capability as shown in residual energy. Figure 3. Second, develop image reconstruction software capable of reconstructing images of Conclusions head phantoms in less than 10 minutes from approximately 108 proton histories recorded by Proton therapy is becoming one of the the pCT detector. most effective radiation therapy for cancer treatment. Because of the finite range of protons Development of the proton computed in matter, this treatment modality allows for a tomography is multidisciplinary in nature and greater degree of conformality than the hence expertises of many fields are required. conventional x-ray therapy. But, in order to With the advances in accelerator physics it has maximize the inherent advantages of proton become feasible nowadays to obtain high therapy, the range of protons inside the patient intensity proton beams for cancer therapy at must be able to be predicted with millimetre relatively lower cost. The development of high resolution. In current clinical practice, proton resolution and precise detectors for the detection therapy treatment plans are made with pre of fundamental particles has made it possible to treatment X-ray CT scans of the patient. The use develop the detector system for the proton of X-ray CT images for proton treatment tomography without much problem. Also, the planning ignores fundamental differences in use of software to simulate the detector and then physical interaction processes between photons to reconstruct the real image of the scan is and protons and is therefore inherently humongous. Since this technology is finally inaccurate. X-ray radiographs depict only going to be used by the radiation oncologists so skeletal structures; they do not show the tumour their participation is also important. So we can itself. In view of this, imaging the patient say that in true sense this work is directly with proton CT by measuring the multidisciplinary in nature which includes

29 energy loss of high-energy protons that traverse tomography," IEEE Transactions on the patient is highly desirable. Nuclear Science, 25, 657−660 (1978). [9]. K.M. Hanson, J.N. Bradbury, R.A. Acknowledgement Koeppe, R.J. Macek, D.R. Machen, R. Morgado, M.A. Paciotti, S.A. Sandford, Much of the introductory sections of this and V.W. Steward, "Proton computed article has been adopted from Scott Nocholas tomography of human specimens," Penfold Ph.D. Thesis, who worked on the Physics in Medicine and Biology, 27, development of second generation of pCT. 25−36 (1982). [10]. U. Schneider and E. Pedroni, "Proton [1]. R.R. Wilson, "Radiological use of fast radiography as a tool for quality control in protons," Radiology, 47, 487−491 (1946). proton therapy," Medical Physics, 22, [2]. A.M. Cormack, "Representation of a 353−363 (1995). function by its line integrals, with some [11]. P. Pemler, J. Besserer, J. de Boer, M. radiological applications," Journal of Dellert, C. Gahn, M. Moosburger, U. Applied Physics, 34, 2722−2727 (1963). Schneider, E. Pedroni, and H. StÃ¤uble, [3]. A.M. Cormack, "Representation of a "A detector system for proton radiography function by its line integrals, with some on the gantry of the Paul Scherrer radiological applications. II," Journal of Institute," Nuclear Instruments and Applied Physics, 35, 2908−2913 (1964). Methods in Physics Research A, 432, [4]. A.M. Koehler, "Proton radiography," 483−495 (1999). Science, 160, 303−304 (1968). [12]. U. Schneider, J. Besserer, P. Pemler, M. [5]. V.W. Steward and A.M. Koehler, "Proton Dellert, M. Moosburger, E. Pedroni, and beam radiography in tumor detection," B. Kaser- Hotz, "First proton radiography Science, 179, 913−914 (1973). of an animal patient," Medical Physics, 31, [6]. M. Goitein, "Three-dimensional density 1046−1051 (2004). reconstruction from a series of two- [13]. B. Schaffner and E. Pedroni, "The dimensional projections," Nuclear precision of proton range calculations in Instrumentation and Methods, 101, proton radiotherapy treatment planning: 509−518 (1972). experimental verification of the relation [7]. A.M. Cormack and A.M. Koehler, between CT-HU and proton stopping "Quantitative proton tomography: power," Physics in Medicine and Biology, Preliminary experiments," Physics in 43, 1579–1592 (1998). Medicine and Biology, 4, 560−569 (1976). [14]. L.A. Feldkamp, L.C. Davis, and J.W. [8]. K. M. Hanson, J.N. Bradbury, T.M. Kress, "Practical cone-beam algorithm," Cannon, R.L. Hutson, D.B. Laubacher, R. Journal of the Optical Society of America Macek, M.A. Paciotti, and C.A. Taylor, A, 6, 612–619 (1984). "The application of protons to computed

Muon and neutron induced background in low-level Gamma-ray spectrometry A. J. Khan*

Low-background γ spectrometry using The low-background systems are located at high purity germanium (HPGe) detectors is an ground level or at shallow underground important tool for basic and applied projects locations. In those systems, active shields are involving γ-ray measurements and assessments. necessary. There are significant variations in the Low-background γ spectrometry has been detector + shielding + location configurations. originally driven by neutrino physics at deep Although the physical principles of γ underground research laboratories; however, background are understood, it takes a emphasis has been given recently to considerable effort to construct a functional environmental measurements. Latest reviews low-background γ spectrometer, frequently can be found in Refs. [1,2,3].The principal resulting in new information. At Wadsworth source of the background in γ spectrometry Center, Albany, NY, we have been performing consists of the terrestrial γ radiation, which can low-background γ spectrometry for two be considerably reduced by passive shielding. decades, which often involved long-term However, highly penetrating cosmic-ray muons planning and undergoing upgrades. The applied enters the passive shielding at ground-level uses of γ spectrometry at Wadsworth Center locations and produce a characteristic involve monitoring of environment, food, air, continuum with the maximum around 200 keV water, surveillance of nuclear facilities, as well and a pronounced annihilation peak in the γ-ray as health physics and homeland security spectrum [4]. Cosmic-ray muons can only be applications. Low-background γ spectrometry is rejected with the active shielding. Cosmic-ray applicable to very low activity matrices, such as muons are also subjected to muon capture in the water or chemically separated samples. Specific Ge-detector crystal and lead shielding resulting projects include mandated analysis of 226, 228Ra in the formation of highly exciting nuclei which in drinking water and monitoring of 134,137Cs at emit neutrons [5]. These neutrons either scatter nuclear facilities. We also actively participate in inelastically or cause thermal capture as well as routine radiological exercises to be prepared for thermal and fast activation in Ge, Pb, and any nuclear emergency. In the present work we materials surrounding the detector, leading to describe significant reduction of the γ-ray many γ rays in the background [6,7]. Another background at Wadsworth Center. It was source of background is radon and daughters achieved by muon rejection with plastic [8]. Further background is caused by β-particle scintillators as well as by employing an ultra- bremsstrahlung from the decay of 210Pb/210Bi pure shielding lead, outlined in the subsequent present in the lead shielding [9]. Finally, sections. Due to the exceptionally efficient background arises from radioactive impurities in muon rejection, some of the most intense peaks the detector materials and shielding. One of the in the residual γ spectrum are those from the most detailed backgrounds in γ spectrometry neutron interactions. Therefore, this work was measured in Ref. [10]. Usually, a complements previous investigations on the distinction is made between ultra-low cosmic-neutron interactions. Future planned background and low-background. Ultra-low upgrades are also described. background pertains to the systems located in deep underground locations, where muons and neutrons are either considerably reduced or absent, making active shields often unnecessary.

Experimental Canberra) which processed the energy pulses from the Ge detector. We currently use a 132% efficient (2.584 kg) HPGe detector (Model GC13021 by Results and discussion Canberra Inc., Meriden, CT, USA) in an ultra- low background U-type cryostat configuration The effect of Alpha-lo lead insert was (Model 7915-30ULB by Canberra). The investigated by measuring the total spectrometer detector is inserted into a 3-layer lead shield. background with and without the insert. In this The original shield consisted of two layers: an measurement, muon shield was not used. A 12 outer 3''-thick layer made of Boliden grade lead % reduction of the background was observed (20 Bq kg–1 210Pb content) and an inner 3''-thick with the insert. This appears to be a modest layer grade lead (< 3 Bq kg–1 210Pb, < 1 mBq reduction. However, if it was measured with the kg–1 40K). This shield was custom designed by muon shield on, it would be a major reduction in Canberra (Model 777S). We have recently background (see below). Since the average added an Alpha-lo grade 2 cm-thick lead insert. environmental γ-ray energy is below 2 MeV and This ultrapure lead has α flux of 7×10–4 cph cm– the average energy loss by the muon in the 2 and 0.01 ppm of K impurity. Ge detector plastic scintillator is about 10 MeV, γ rays and chamber is purged with the boil-off nitrogen muons are, in principle, well separated in the from the Ge detector Dewar in order to remove plastic scintillator by the pulse height. However, radon and daughters. The lead shield is if the plastic scintillator is not shielded, there is surrounded from the top and the sides by a 6- a considerable counting rate from environmental panel 2''-thick plastic scintillator muon shield γs in the scintillator resulting in blurred (one side is split into two panels for cryostat separation. Vetoing Ge detector in such case insertion). The muon shield is made of BC-408 leads to a very high dead time and the muon grade plastic scintillator. Each scintillator panel shield is less effective in lowering the has two embedded hemispherical background. At Wadsworth Center location, photomultiplier tubes which allowed for a plastic scintillators are shielded from much of compact and convenient configuration. The the environmental radiation by positioning them spectrometer is placed in a 6''-thick wall steel inside the steel room. Therefore, the total plastic room made of pre-World War II steel, which is scintillator signal, without the need of γ/muon located under a 47-story building providing 33 separation, can be used to veto Ge detector, and m of water equivalent (mwe) shielding from this is a key element in excellent rejection of cosmic rays in the vertical direction. However, cosmic-ray muons from the background (see since the laboratory is located at the ground below). We performed the measurements of γ level, it is much less shielded against cosmic intensity loss in the Ge spectrum due to rays streaming from the sides (Fig. 1). There are scintillator vetoing, using common radioactive several known electronics setup methods for standards. We observed a 3.8 % loss of γ rejection of muons event-by-event [11]. In the intensity at 60 keV, 2.3 % loss at 1173 keV, and preliminary tests described here we chose the a 4.3 % loss at 1332 keV. The background γ-ray signals from the plastic-scintillator energy spectra obtained under varied shielding photomultiplier tubes were matched using high- configurations are depicted in Fig. 3. The voltage bias. The signals were then mixed and spectrum inside the lead shield (second one the sum was preamplified, amplified, and from the bottom) is primarily from muons discriminated. The resultant logical TTL gate of interacting with the Ge crystal and lead. It 50 μs duration was used to prompt veto the agrees very well with the spectrum calculated in Digital Signal Processor (Model 9660 by Ref. [4]. By applying muon-shield veto, that

32 background is reduced by a factor of 17. among ground-level laboratories. Additional Overall, we have achieved background advantages of our facility are easy access and a reduction by a factor of 9436 relative to the location in the vicinity of several large ambient. Our integrated background rate in the γ metropolitan population centers. They enhance energy range of 50-2700 keV was measured as our role as a reference laboratory and for 2.3 counts per min, corresponding to 15 counts emergency response. The γ-ray spectrometer at ks–1 kg–1 Ge. This compares well with the IAEA Wadsworth center, Albany, NY is currently MEL Monaco laboratory, which achieved the undergoing further upgrade. It will consist of a value of 10, the lowest among ground-level or new 130 % Ge detector in an XtRa shallow-underground located laboratories configuration with a carbon composite window [12].The lowest background γ-ray spectrum for detection of low-energy γ rays from bulk from Fig. 3 is replotted in Fig. 4. The samples [3]. Ultrapure materials for cryostat corresponding resolved γ peaks and they construction will be used to reduce natural assignments are listed in Table 1. The most radioactivity component. Bottom plastic intense peaks are results of scattering, capture, scintillator will be added to prevent muon leaks and activation of Ge by neutrons from muon through the corners of the scintillation panels capture. Due to excellent muon-background and to veto possible scattered muons. A reduction, this work offers one of the best cryogenic radon trap is being constructed to measurements of cosmic-neutron interactions in reduce radon and daughters background [8]. Ge. Many of the Ge metastable states have life- Additional measures will be attempted in to times much longer than the duration of the veto possibly further reduce the background by gate (50μs) and, therefore cannot be rejected in inserting low-Z fillers in the vicinity of the the present configuration. However, several detector, and by electronic tuning. prompt peaks are also present, indicating that the electronic system is not fully optimized. References There are many natural-radioactivity peaks, most likely from the materials used in the 1. M. Laubenstein, M. Hult, J. Gasparro, D. construction of the cryostat. The combined γ- Arnold, S. Neumaier, G. Heusser, M. peak intensity is 22 % of the total background Köhler, P. Povinec, J.-L. Reyss, M. intensity. Also a reasonably 511-keV peak is Schwaiger, P. Theodórsson, Appl. Radiat. still present. One can conclude that still Isot. 61 (2004) 167. unrejected muons are recorded by the Ge 2. M. Hult, Metrologia 44 (2007) S87. detector. This offers some more room for further 3. M. Köhler, D. Degering, M. Laubenstein, reduction of background. P. Quirin, M.-O. Lampert, M. Hult, D. Arnold, S. Neumaier, J.-L. Reyss, Appl. Conclusions Radiat. Isot. 67 (2009) 736. 4. P. Vojtyla, Nucl. Instr. Meth. Phys. Res. B We have constructed a low-background 100 (1995) 87. γ-ray spectroscopy facility. An excellent 5. S. Charalambus, Nucl. Phys. A 166 (1971) cosmic-ray muon background rejection has been 145. achieved due to, in part, positioning of the 6. G. Heusser, Nucl. Instr. Meth. Phys. Res. A plastic scintillators inside a shielded steel room. 369 (1996) 539. Therefore, this work offers one of the best 7. J.-H. Chao, Appl. Radiat. Isot. 44 (1993) measurements of muon and cosmic-neutron 605. interactions in Ge. We have achieved 8. M. Wójcik, G. Zuzel, Nucl. Instr. Meth. background level which is among the lowest Phys. Res. A 539 (2005) 427.

9. P. Vojtyla, Nucl. Instr. Meth. Phys. Res. B 11. S. Hurtado, M. García-León, R. García- 117 (1996) 189. Tenorio, Appl. Radiat. Isot. 64 (2006) 10. P. Bossew, Appl. Radiat. Isot. 62 (2005) 1006. 635. 12. P.P. Povinec, J.-F. Comanducci, I. Levy- Palomo, Appl. Radiat. Isot. 61 (2004) 85.

Dinosaurs versus Dark Matter Syed Afsar Abbas

Not too long ago the Hollywood movie much so that only some primitive form of "Jurassic Park III" was running to packed mammals such as rodent could survive and that houses all over the world. This was a sequel of too by scurrying into tiny holes while these the earlier movies the "Jurassic Park" and "The gigantic creatures walked the Earth as they felt Lost World". The main reason for the popularity like. Recently it has also been becoming clear of these movies is the gigantic and ferocious that some of the species of the dinosaurs were creatures shown therein. These creatures are developing into intelligent animals. For example called dinosaurs. The movies were Velocitiraptor, the swift dinosaurs that one saw fictionalization of a hypothetical scientific in the "Jurassic Park" were also highly technique of gene manipulation which brought intelligent. Had the dinosaurs not become these dinosaurs to life in the modern context. extinct then the history of the Earth would have been quite different today. In that case, it may In reality dinosaurs which could be as not be too bizarre to visualize that today an tall as a several stories high building, ruled the intelligent dinosaur would have been writing an Earth for about 150 Million years. Note that in article for the Physics Bulletin and other contrast the mankind at best is perhaps only intelligent dinosaurs would have been reading it. about 1 Million years old. And let us not forget It would be anyones guess as to how they would that already the mankind has created a situation be looking like! Well, anyway, it was mainly that has the potential of destroying the whole due to the extinction of the dinosaurs that the world. Viewed this way, mankind should be mammals were allowed to proliferate, expand perceived as the species that has failed the and evolve so that at some stage an intelligent Universe and the extensive span of the Homo Sapiens species came into existence. dinosaurs species would attest to their having been highly successful creatures of the Now let us go to another mystery of the Universe. Universe. The famous science fiction novel, “The Invisible Man " by H G Wells starts with As such the dinosaurs should have the arrival of a stranger in a quiet English continued to dominate for much longer but then village. He was wrapped up from head to foot. something happened and some 65 Million years Bizarre incidents start occurring in the village. ago the reign of the dinosaurs came to an almost Ultimately the villagers broke into his room and abrupt end. That is to say that all the dinosaurs demanded that he show his face. With laughter became extinct. Today one of the outstanding he unwound the bandages. Once the clothes mysteries of science is what caused the were gone, there was nothing there. They could extinction of the dinosaurs. see right through him. Obviously there was a person there but that his body was invisible. The extinction of dinosaurs is significant for the mankind in another way. Mankind never It is amazing that currently in co-existed with the dinosaurs. While the astrophysics it is turning out that there is an dinosaurs were roaming the Earth as its supreme analogous problem of the missing Universe. As master, the species of mammals were just sophisticated instruments have allowed us to evolving. However the domination of the peel off the 'bandages' of the universe we have dinosaurs was such that it would not allow any become aware of the fact that much of the other species, such as mammals to grow. So Universe is invisible. This means that just as in

35 the above story the body of the universe is there, of matter which is not visible to us, the so called but it cannot be detected directly by any Dark Matter. Careful calculations reveal that instruments available to the scientists. surprisingly as much as ninety percent of the total matter in the universe is made up of the The first one to point out this was Fritz Dark Matter. Zwicky of USA in the 1930's. But most of the scientists rejected this proposal.However further Note that there is another mystery of the observations in the recent years have vindicated Universe which we are not tackling here and Zwicky. It is now generally accepted by that is that of the Dark Energy. This is quite scientists that the visible matter in the universe different from Dark Matter. At present we constitutes only a miniscule amount and that the believe that of all that is there in our Universe bulk of 'all that is out there' in not visible to us. Dark Energy constitutes about seventy percent, Amazingly almost 90 percent of matter in our Dark Matter twenty five percent and visible own galaxy may be invisible. The matter which matter only five percent. However one should is invisible to us is called the 'Dark Matter' by not confuse the numbers, locally as far as our the scientists. We have very little idea as to what own galaxy is concerned plus other galaxies, the it is. The situation is similar as to the existence data calls for ninety percent of Dark Matter and the identity of the invisible man. constitution.

The main evidence for the existence of The only thing that we know about the Dark Matter comes from the gravitational pull Dark Matter is that it is out there and that it is of one object upon the other. We can determine the predominant form of matter in the universe. its mass using simple physics. For example, the We know practically nothing else about it. One knowledge of the distance of moon from Earth would definitely like to know the identity of this and it's speed enables us to calculate the mysterious Dark Matter. The only way that one gravitational pull needed to keep its orbit. This can identify an object unambiguously is to catch is all that is needed to calculate the weight of the it. Hence all over the world a sort of race is on Earth. Similarly we can determine the weight of to catch a Dark Matter particle and some twenty the sun by studying the Earth's motion around it. detectors at present are active.

We can use the same methodology to In trying to unravel the mystery of the study our galaxy - the Milky Way. It consists of Dark Matter, theoretically too there have been about 100,000,000,000 stars with our own sun umpteen number of suggestions ranging from being a typical star. The force of gravity holds brilliant to bizarre (of course what is brilliant for these objects together. The Milky Way is one may be bizarre to another one) have been known to rotate around its centre. By studying presented. The number of suggestions abound. the rotational motion of stars lying in the outer One hears terms of the kind MACHOS, periphery of the system we can obtain the total WIMPS, strangelets, rungions etc. It would be mass of the galaxy. But now one encounters a good if a Dark Matter particle is found through conundrum. One finds that the gravitational direct detection. However, one may ask if there force needed to account for the observed are any indirect signatures of dark matter. This velocities is about ten times larger than what is the way that Samar Abbas and myself while would result from all the visible stars and all the at the Institute of Physics, Bhubaneswar had gases alone. The same puzzling situation is approached the problem in 1998. We found that found to exist in other galaxies also. Hence one amazingly this way we were able to explain the is forced to conclude that there is a huge amount mystery of the extinction of dinosaurs.

Dark Matter is viewed as occupying a pollution. It is estimated that the Deccan Traps spherical halo surrounding the galaxy. The earth ejected more than 10 million cubic kilometres of is constantly passing through the dark halo of basalt, 200 billion tonnes of hydrochloric acid, 1 the Milky Way. As it does so, it will gradually trillion tonnes of sulphuric acid and 20,000 capture Dark Matter particles by elastic tonnes of fine dust. The environmental impact scattering, gravitational attraction and orbital of such an event would be severely debilitating: capture. The rate at which the Earth shall the quantities of pollutants and dust belched into accumulate Dark Matter can be worked out the atmosphere would be akin to a full-scale mathematically. This captured Dark Matter nuclear winter, leading to global climate gradually drifts towards the centre of the earth. deterioration. Here the number of Dark Matter particles continues to increase as this is the stablest least- Indeed, the resultant ecological energy configuration. Once a critical density is catastrophe would involve the extinction of reached, however, the Dark Matter particles much life. And as per our suggestion this is start to annihilate with one another. This leads exactly what happened 65 Million years ago to to generation of huge amount of heat in the core wipe out dinosaurs. Another popular suggestion of the Earth. This excess heat is 1000 - 100,000 for the extinction of the dinosaurs is due to an times the current heat output of the earth. This asteroid/meteor impact 65 Million years ago. heat is produced in the core of the Earth. What And although an impact crater of similar age has exactly would be the geological effects of such been discovered in the Yucatan peninsula of an event? Mexico, the Deccan flood basalt volcanic episode also coincides with this event. Much These large quantities of heat would controversy surrounds the true cause of the gradually escape from the core, melting its way catastrophe, but some investigators believe that to the surface in the form of a superplumes. the effects of the impact were limited to North Geothermodynamic theory indicates that America, while the Deccan flood basalt superplume formation is one of the most volcanoes were responsible for the extinction efficient methods by which the Earth's core can worldwide. rapidly shed excess heat. In addition, the heat is likely to destablize the core, leading to The significance of what we have geomagnetic reversals, which also display a discussed here lies in the fact that it connects positive correlation with volcanism. On the two of the most puzzling and outstanding surface this would lead to explosive silicic mysteries of modern science viz, the extinction volcanism, followed by large-scale basalt of dinosaurs and the enigmatic Dark Matter of volcanism. These truly cataclysmic events lead cosmology. Dark Matter was the cause that led to the formation of the massive flood basalt to the extinction of dinosaurs. And as dinosaurs volcanic provinces such as the Deccan Traps of departure in a way opened the way for the Central India which has been dated at 65 arrival of mankind, we have reasons to be Million years ago. grateful to our mysterious friend, the Dark Matter. Such enormous volcanic provinces were associated with significant atmospheric

The Story of Heat and Engines W. Haider

It was earlier assumed that the The steam engine is essentially a relationship of energy and the consequent work machine to convert energy of some kind of fuel done holds only if friction is absent or (eg. Oil, coal, nuclear energy of uranium) into negligible. If frictional forces are present, work heat energy which is then used to do some done in moving objects warms up the objects mechanical work. Other devices have been offering friction. As a result that much less work constructed now but steam engine is still a good is available to increase the potential/kinetic model to understand the process od energy energy of the objects. Thus it seems possible to conversion. modify the laws of conservation of mechanical energy to include the effect of frictional forces. Since ancient times it has been known that heat can be used to produce steam which As an example consider the case of a can be used to do mechanical work. The early book that has been given a push such that it toys (eg. Aeolipile by Heron at Alexandria in slides on a horizontal table. If the surface is 100 A.D.) worked on Newton‟s third law even slightly rough the book would stop moving enunciated much later. However this was only a after a while and its kinetic energy (K.E.) would toy and not until the late eighteenth century disappear. Further as its Kinetic Energy commercially useful steam engines were disappears there is no increase in its potential invented. energy. Thus it appears that the mechanical energy is not conserved. However a close We now say that steam engines use heat examination reveals that as the moving book energy to do mechanical work. However many stops the book as well as the surface of the table inventers in the eighteenth and even nineteenth are warmer than before. Thus the disappearance century thought of heat as some kind of thin of kinetic energy of the book is accompanied by liquid. Further steam engines were first appearance of heat. This suggests that the developed by men more concerned with making kinetic energy of book was converted into heat money as well as effectiveness of safety of and that heat is a form of energy just like mines. It was later that the involvement of Kinetic Energy or Potential Energy. That heat is knowledgeable people with curiosity to a form of energy was gradually accepted by the understand the working of engines led to middle of nineteenth century. In this small discoveries in physics. article I would try to bring out this story which was understood as a result of practical The first commercial steam engine was knowledge gained in developing the steam invented by an englishman Thomas Savery engine. (1650-1715) to pump water out of the deep coal mines. The serious drawback of the Savery‟s Two hundred years ago most work was engine was the use of very high pressure steam done manually. During 1700 A.D. as mines which led to serious accidents. This defect was were dug deeper in search of coal the problem removed by another englishman Thomas of pumping water out from the flooded mines Newcomen (1663-1729) and his engine could had to be solved. The steam engines were do other work also. However in both these developed initially to solve this problem. engines several valves had to be manually closed and opened at proper times but later a method was invented to do this automatically

38 using the rhythm and some energy of moving small step was crucial in enhancing the parts of the engine to control various sequences efficiency of the engine by more than twice. By of its operation. This idea is called the feedback. saving of fuel, James Watt was able to make a fortune by selling his engine to coal mine The engine developed by Newcomen owners. The money charged by Watt was was widely used in U.K. and other European dependent on the power of the engine. Power is countries throughout the 1700‟s. It used large now defined as the rate of work and now amount of coal to produce little work. However appropriately called as Watt. the engine was used due to the urgent demand of machines to pump out water from the mines. 1 Watt = 1 Joule/sec.

James Watt and the Industrial Revolution As engine with a separate condenser by Watt, much superior to Newcomen‟s engine, led A much better steam engine was to its application for many kinds of jobs eg. invented by James Watt whose father was a running of machines in factories, railway carpenter constructing equipment for ship engines, steam boats etc. It stimulated a big builders. Due to poor health James Watt could boost to the growth of industry in Europe and not continue his education and worked at his USA and a big economic and social father‟s shop. He went to London to learn transformation of Western civilization: instrument making trade. After getting some Industrial Revolution. It resulted in raising the training he come back to Scotland in 1757 and living standards of people. However, not all was appointed as instrument maker at Glasgow effects of industrialization were positive. The University. factory structure provided a large number of greedy employers to exploit workers. They During the winter break of 176-64 Watt made huge profits while keeping the workers was given the job of repairing Newcomen‟s and their families on the verge of starvation. engine for demonstration purpose at the The worst excesses had to be eliminated through University Lectures. While doing this he learnt reform in the early nineteenth century in about the amount of steam required to run the England through new laws to reform the adhoc engine. After doing several test runs he realized system. that the major problem in the wastage of energy was due to the temperature of cylinder walls. He Nowadays the steam engines are no realized that most of the heat energy was being longer widely used a source of power to run used up in heating the cylinder which was industry or transportation. However, steam is cooled down again to condense the steam. He used still as a major source of power. The soon realized that this wastage of energy could invention of steam turbine by Charles Parson in be rectified. He modified the engine such that 1884 has replaced all older kinds of steam when the steam had done its work of pushing engines. They are also used at most electric the piston up it was taken to a separate chamber power stations running the wheel of modern to be condensed. This helped keep the cylinder civilization. Even in nuclear reactors the nuclear hot and condenser cool all the time, thus fission energy is used to produce steam to drive avoiding the loss of steam energy in heating up turbines and electric generators. the cylinder again and again. The basic principle of Parsons Turbine is It seems that Watt‟s invention of a that a jet of high pressure steam hits the blades separate condenser is a small step. However this of a rotor, driving it at high speed.

The Joule’s Experiment famous experiment even now done in laboratories was using the potential energies of The question disturbing to some people falling weights to turn paddles in a container of was “what happens to heat as it does work water. The friction between the paddles and through a steam engine?” water creates heat to raise the temperature of water. The answer given by scientists in the early nineteenth century was that the amount of The results of his experiment published in heat remains constant while it does work when 1849 are: it passes from a high temperature to a lower temperature. Heat was regarded as some thin 1. The amount of heat produced is always substance called “Caloric”, whose amount in proportional to the amount of mechanical Universe is conserved. It was thought that heat energy spent. is like water which can be made to do some 2. The amount of heat required by 1 pound of work as it falls from height. However, some water to raise its temperature by 1˚ F was intelligent people disagreed with this created by a fall of 772 lb of weight by 1 explanation and thought that heat is a form of foot. energy. James Prescott Joule conducted a series of experiment in the 1840‟s to clearly show that The first result shows that heat is a form heat is just another form of energy. Through of energy while the second result gives the exact verifiable experiments he showed that same relationship between a unit of mechanical expenditure of mechanical energy resulted in a energy and heat energy. In the MKS system we fixed amount of heat energy where there was no express this by saying the unit of heat; a heat. Kilocalorie equals 4184 Joules of mechanical energy. In one experiment he used falling weight to drive an electric generator. The current The mechanical equivalent of heat generated was used to heat a wire immersed in makes it possible to define heat engines in terms water. The rise in temperature of water can be of efficiency. Efficiency is defined as the used to calculate the heat energy produced by fraction/percentage of input energy appearing as falling weights. In another experiment he useful output work. The law of conservation of compressed a given volume of gas in a bottle energy implies that the best engine would be immersed in water. One can easily calculate 100% efficient, i.e., when all the heat energy is amount of work spent to compress the gas and converted into work. However, it turns out that the heat produced by noting down the rise in there are theoretical limits on efficiency. temperature of water. However, the most

An elementary note on the nuclear forces B. P. Singh* and R. Prasad

In the year 1911, Rutherford established that 9. Mirror nuclei (nuclei having same A but 3 every atom has a nucleus at its centre. Later on interchanged values of N and Z like 1H & 3 it was well established that the atomic nucleus 2He ;etc.) show similarities in their nuclear consists of neutrons and protons. The nucleus of structure. A the atom X is denoted as ZX , where A is called the atomic mass number (total number of In a nucleus the nucleons are held together nucleons), Z the atomic number (number of in a small volume of, generally, spherical shape protons) and (A- Z) = N, is the number of (radius ≈10-12 cm or 10 fm) by means of neutrons in the nucleus. Some of the important strongly attractive forces called nuclear forces. facts about stable nuclides are; The nuclear forces are very strong comes from the fact that protons which are positively 1. There are about 300 stable nuclides and charged are held together inside the nucleus (by many more unstable (radioactive) nuclides. the attractive nuclear forces) overcoming the 2. All the positive charge and more than 99.9% large repulsive Coulomb forces. mass of the atom is contained inside the nucleus of the atom. First indication about the existence of 3. Most of the nuclides are spherical in shape. nuclear forces was obtained from the hyperfine 4. A few nuclei are non spherical in shape and structure in the atomic spectra. Further evidence are called deformed nuclei. The deformation about the presence of strongly attractive nuclear from spherical shape, which is either prolate forces was gathered from the anomalous (cigar like) or oblate (disc like) is measured scattering of high energy alpha particles by in terms of the quadrupole moment Q, which atomic nuclei. Since, the target nuclei and the has positive value for the former and the incident alpha particles are both positively negative value for the later. charged, a beam of high energy alpha particles 5. All the nuclides behave like a spinning top. undergo Coulomb scattering (also called A quantum number S or J is assigned to the Rutherford scattering). Since, Coulomb force is nucleus to represent its spin. For some well known the scattering cross-sections for nuclei it may be zero. Rutherford scattering can be easily calculated 6. Many nuclei behave like a tiny bar magnet. for all values of impact parameters. Magnetic dipole moment µ is associated Experimentally, it has been observed that the with a nucleus to represent this behaviour. measured scattering cross-sections for small 7. The largest number ( 165) of stable nuclides impact parameters deviate by a large amount is of those which have even Z and even N; from the values calculated assuming Coulomb even-odd (65) or odd-even (60) are next in interaction. This happens because the nucleons abundance. Odd-odd stable nuclides are only inside the incident α-particle start feeling the a few (4). presence of the nuclear field of the target 8. Nuclei having N and/or Z equal to 2, 8, 20, nucleus at small impact parameters (i.e., when 28, 80 and 126 show special stability. They they pass through closer distance to the target are either very strongly bound or have many nucleus). stable isotopes. These nuclides are called magic nuclei and their corresponding As is well known there are FOUR basic nucleon number as magic number. forces or interactions in nature viz., Strong

41 nuclear, Weak nuclear, Electromagnetic and volume indicates that forces responsible for Gravitational. Just to give a feel of the relative holding the nucleons together are really very strengths of various interactions they are strong. compared below in TABLE 1. According to the modern theories, every field/interaction is brought about by the quanta of field, which mediates the interaction. The mediating quanta for each interaction are also indicated in this table. Photon, the quanta of electromagnetic field, W-boson the quanta for weak interaction and π-meson the quanta of strong nuclear field have already been discovered. Search for graviton, the assumed quanta of gravitational field is still on. Each of these fields has a specific range. The range of gravitational and electromagnetic fields is infinite. However, the Fig.1: A typical nuclear charge density distribution range of strong and weak fields is very small as a function of distance from the centre of a typically of the orders of a few Fermi. nucleus.

TABLE 1 For stable nuclei it has been Interaction Strength Quanta experimentally found that the radius R of a Strong 1 -meson nucleus having A number of nucleons is related -2 to the atomic mass number A by the relation; Electromagnetic 10 Photon R = r A1/3 Weak (beta 10-12 W-boson 0 Where, r is a constant. As such, the volume V decay) 0 of a nucleus of A nucleons, assuming it to be Gravitational 10-39 Graviton spherical is; 3 3 In order to appreciate the strength of nuclear V= R = π ro A = K A. forces, let us look at the size of the nucleus. As Here, K is a constant. Further, the mass M of a a matter of fact every nucleus has a distribution nucleus having A nucleons is proportional to the of positive charge inside it. Experiments carried number of nucleons A, i.e., out by Hofstadter have revealed that the charge M = K1 A density of the positive charge inside the nucleus Here, K1 being another constant. varies from the centre of the nucleus as shown in Fig.1. The distance from the centre at which From the above two equations one can the charge density falls to half of the central observe that the density ρ (M/V = K1/K= value is generally defined as the nuclear charge constant) of the nucleus is constant. The fact radius or simply the nuclear radius R. In that the density of nuclear matter is constant is general, the radii of all stable nuclides have reflected in another important property of been found to be of the order of Fermi (10-15 m). nuclear forces called the saturation property, The fact that nucleons are bound in such a small and will be discussed later.

Another classical force is Gravitational What holds the nucleus together? force. Gravitational force also has infinite range. In the past quarter century physicists The attractive force that binds the progressively growing galaxy together is the gravitational have devoted a huge amount of force. It is the force that not only holds us to the experimentation and mental labour to Earth but also reaches out across the vastness of this problem – probably more man- intergalactic space. The binding energy due to hours than have been given to any gravitational force between two nucleons at a other scientific question in the history of distance 2x10-15 m inside the nucleus is only mankind. 10-37 MeV. From the above, it is clear that ----- Hans A. Bethe nuclear forces which are non-central having short range and provides very strong binding to the nucleons are entirely different from the infinite range central classical forces that can Another measure of the strength of the provide very weak binding. It may therefore be nuclear force is the binding energy with which concluded that classical forces are not adequate the constituent particles are held together. to explain the strong binding of nucleons inside Nuclear forces are very strong. This is also the nucleus. evident from the large binding energy of nucleons inside the nuclei i.e., 8 MeV per Further, we know from our nucleon. For comparison, the average binding undergraduate classes that, in order to confine a energy of electrons in the atoms is eV to keV nucleon within a nucleus of the radius of a few only. Moreover, there is a very big difference Fermi, its de-Broglie wavelength must be between the binding of nucleons in a nucleus correspondingly small. The de-Broglie and the binding of electrons in an atom. In an wavelength λ is related to the momentum p (= atom the positively charged nucleus at the mv) of the particle by the relation: centre provides the Coulomb field which λ = = extends up to infinity with decreasing strength. Negatively charged electrons in an atom are But the kinetic energy E= ) held by this central Coulomb field at different distances from the centre. As such, the valence So, its kinetic energy must be around; electrons which are farthest from the centre are )= ) held with smaller binding energy (~eV), while inner electrons have binding energies few keV. If one calculates this value of kinetic In the case of nucleus, there is no central force. energy, taking λ equal to the radius of the nucleus and „m’ equal to the mass of the The nucleons inside the nucleus interact with -27 each other through strong nuclear force and this nucleon (~10 Kg), it comes out to be ≈20 mutual interaction provides the binding. Later, it MeV or so. It means that a nucleon inside the will be shown that the range of nuclear force nucleus must be moving with a kinetic energy i.e., the distance up to which they affect the 20 MeV or more. In order to confine this high other nucleons is very small, only a few Fermi kinetic energy nucleon inside the nucleus, the ( 2x10-15 m). As such, it is evident that the nuclear force must provide a potential well. The nature and strength of nuclear forces are very depth of this potential well must be equal or much different from the Coulomb force. larger than the sum of the kinetic energy ( 20 MeV) and the binding energy ( 8 MeV) Therefore, to confine a nucleon of this much

43 energy, it requires a large average potential well the neutrons and protons undergo spin motion of depth ≈30 MeV. around their axis giving rise to the spin of ½ħ. Spinning positive and negative charges give rise to current elements in nucleon, which in turn may generate intrinsic magnetic dipole moment. The intrinsic magnetic moment of the proton µp, is taken as positive, while the intrinsic magnetic moment µn of the neutron is negative because of the negative charge towards its surface (see Fig.3). Nucleons inside the nucleus may interact with each other through their intrinsic magnetic moments. The magnetic potential energy due to intrinsic magnetic moments µn and µp of 3 Fig.2: A typical nuclear potential well for a depth ≈ neutrons and protons is ≈µn x µp /r . At the 30 MeV separation r≈2x10-13 cm, this value comes out to be ≈0.03MeV. The sign of the magnetic force It is now well established that both i.e., whether it is attractive or repulsive will proton and neutron are not point particles and depend on the relative orientation of the spins of consist of charge distributions. The typical neutron and proton. It will be of opposite sign charge density distributions of proton and for parallel and of negative sign for anti-parallel neutron are shown in Fig.3. spins. However, experiments have indicated that nuclear forces are attractive for both the parallel as well as anti-parallel spin orientations. Hence, the nuclear forces cannot be solely of the magnetic origin. We may, therefore, conclude that the electrostatic (Coulomb), gravitational and magnetic forces are qualitatively and quantitatively inadequate to act as anything more than very minor perturbations on the specifically strong nuclear forces.

Some of the important properties of nuclear forces studied on the basis of various observations and empirical facts are; (a) short range (b) state or spin dependence (c) charge Fig.3: Charge density distributions of nucleons symmetry & charge independence (d) saturation

(e) tensor nature (f) many body forces etc. In the It may be observed that a neutron, following an attempt has been made to explain though overall electrically neutral, has a positive these properties in terms of simple explanations charge distribution around its centre and a and analogies. distribution of negative charge towards the surface. However, the total amounts of positive As has already been indicated, that and negative charges inside the neutron are nucleons inside the nucleus are in constant equal. As such, for an external observer the motion having large kinetic energies. Nucleons, neutron appears electrically neutral. Although, similar to the electrons in the atom, move in spin is an abstract quantum mechanical concept definite orbits having different values of relative but for simplicity it may be assumed that both

44 orbital angular momentum ℓ. Apart from that nucleons also have intrinsic angular momentum called spin S (= ). The total angular momentum of a system of two nucleons having relative orbital angular momentum ℓ is given by the quantum mechanical sum ℓ ± s1 ± s2. In the ℓ =0 state (called the S-state) the total angular momentum of a two-body system may be zero (singlet state, anti-parallel spins) or 1 (triplet Fig.5: A typical classical analogue of the spin dependence of forces. state, parallel spins). The Pauli‟s exclusion principle does not allow di-proton or di-neutron In 1939, it was found that the deuteron as stable systems. However, a neutron and a has a finite non-zero quadrupole moment with a proton do make a stable system H2, called 1 value of Q=0.003 barns. This value of deuteron. The binding energy of the deuteron is quadrupole moment (QM) indicates that about 2.2 MeV. deuteron is not a spherical structure. The

positive value of QM indicates prolate Spin dependence of nuclear forces may deformation with spin axis being the symmetry be explained with the help of example of the axis. It indicates that deuteron looks like (Fig.6) simplest bound system, the deuteron. the picture on the left than on the right. Experimentally, it is known that spin quantum number of deuteron J=1. Now; J= ℓ (relative motion) ±sn ± sp (Quantum mechanical sum)

If we assume that in its ground state deuteron has ℓ =0 or in s-state. The values of intrinsic spins sn and sp each are equal to ½. Therefore;

J= 0 ± 1/2 ± 1/2 Fig.6: Parallel spins of neutron and proton J=0 (singlet state) or 1 (triplet state) indicated in two different orientations. J=0 may be obtained if we assume the spins of neutron and proton inside a deuteron to be anti- It means that probability of finding a parallel, while J=1 is obtained if we assume neutron and a proton side by side is less than the spins to be parallel. probability of finding them on top of each other. Positive QM of the deuteron indicates that there Had the nuclear forces been spin is a part of the nuclear force which depends on independent, then the possibility of finding the spin and spatial position of the nucleons at deuteron with J=1 and J=0 should have been the same time. In other words, it means that equal. However, experimentally J=0 (bound force between a neutron and a proton in state) of deuteron is not found. This indicates deuteron also depends on the angle between that somehow, nuclear forces prefer parallel spin orientations and the radius vector. (See spins rather than anti-parallel spins (in s-state) figure below). This part of the nuclear force is or in other words, nuclear forces are stronger called tensor force. when the spins are parallel. An analogous classical force occurs between two bar magnets Fnp=f( 1, 2); as shown in Fig.5.

Thus, observed value of J=1 for deuteron may be obtained with two possible values of ℓ=0 (s-state) and ℓ=2 (d-state). The nuclear force which can mix different ℓ-states is called the tensor force. The first evidence of the non central nature or the tensor nature of nuclear forces came from the experimental information Experimentally, it has been found that of the quadrupole moment of deuteron. The the J of deuteron is 1 and it has even parity. tensor force in the deuteron is attractive in the Using the quantum mechanical expression for cigar shaped configuration and repulsive in the the addition of angular momentum; disc shaped one. As has already been said the J= ℓ ± sn ± sp. deuteron in ground state is not a pure s-state but is a mixture of s-state and d-state. Calculations To obtain the value of J=1, the possible have shown that, when about 96% of the s-state options are (i) the ℓ =0 and the spins are parallel is taken along with about 4% of the d-state, it (b) ℓ =1 and parallel spins (iii) ℓ =1 and anti- reproduces the correct value of the quadrupole parallel spins and (iv) ℓ =2 and parallel spins. moment Q, as well as the magnetic moment µ Further, the parity of a system depends on the for the deuterons. It may however, be pointed value of the orbital angular momentum ℓ out that the role of tensor forces in nuclei is still through the following relation; a somewhat open problem. It certainly plays an Parity = (-1)ℓ. important role at large inter particle distance.

The positive parity (+1) for deuteron The most elementary force is a two body may be obtained for two different values of force. For simplicity let us assume that the ℓ(=0 and 2). So the deuteron may not be a pure nuclear forces are two body forces between the s-state or in other words the deuteron may be a pairs of nucleons. These forces may be of three mixture of various ℓ-states including ℓ=o. Since, types (i) Fpp (ii) Fpn and (iii) Fnn, respectively parity of deuteron is positive, so the possible ℓ between pairs of protons, between proton & values which may yield the experimental value neutron and between the pairs of neutrons. of J=1, may be 0 and 2. These nuclear forces do not include the gravitational force and Coulomb force between nucleons. As has already been pointed out the strong nuclear force Fpp between two protons (p-p) represents the attractive force between two protons and does not include their purely classical Coulomb force Fpp(Coul). Moreover, Fpp (Nucl)>>Fpp(Coul). This follows from the fact that protons (charge density) inside nuclei are uniformly distributed throughout the nuclear volume. This is possible only if Fpp (Nucl) is much stronger than Fpp (Coul), otherwise the protons would have been concentrated at the Fig.7: Prolate and oblate nuclei, with spins pointing surface of the nuclei. in the z-direction. The nuclei are assumed to be axially symmetric; z is the symmetry axis. The nuclear forces Fpp, Fpn and Fnn are all, basically, attractive. Existence of neutrons

46 and protons in a bound state (in the form of to have equal number of neutrons and protons nuclei) is itself an evidence of this. Further, i.e., N=Z. However, as Z increases the these nuclear forces have short range. If it were disruptive forces due to Coulomb repulsion not so, all the nucleons in different nuclei will would prohibit the formation of stable nuclides, coalesce into one big nucleus and everything if some extra attractive forces were not brought will probably turn into a huge nucleus like a into nuclear structure. These extra attractive super-neutron star, and there will be not be a forces are provided by neutrons whose number variety of elements that are found in nature. N exceeds Z, i.e., N>Z, by a larger and larger amount, if Z increases further. The charge symmetry of nuclear force means that Fpp = Fnn, while charge independence means that Fpp= Fnn= Fpn. These properties of nuclear forces are derived from the binding energies and the structure of excited states of mirror nuclei. As an example 3 3 let us consider the mirror pair 1H and 2He . The 3 binding energy for nucleus 1H which has one proton and two neutrons will come from 2-pairs of Fpn and one Fnn. Similarly, the binding 3 energy of 2He (two protons and one neutron) will come from 2-pairs of Fpn and one pair of Fpp minus the Coulomb energy of the repulsion Fig.8: A typical representation of neutron number of two protons. Since, the number of pn pairs in verses proton number in the stable nuclides. the two nuclei is equal, any difference in their binding energies will arise due to the difference In Fig.8, a plot of neutron number N, vs in the values of binding provided by Fnn and proton number Z, for stable nuclides is shown. Fpp and Coulomb energy of the excess proton The deviations from the N=Z line indicates the 3 neutron excess i.e., (N-Z). In Fig.9, this neutron in 2He . Experimentally determined difference in the binding energy of the two nuclides is 0.76 excess (N-Z) is plotted as a function of atomic MeV, which is just equal to the Coulomb energy mass number „A’. 3 of the excess proton in 2He . It means that, Fpp is equal to Fnn. Detailed studies of the energy spectra of mirror nuclei shows that Fnn=Fpp=Fpn, proving the charge independence of two body nuclear forces. The fact that Fpp ≈ Fnn is also indicated by the fact that the pairing energy for protons and for neutrons is almost same.

A look at the chart of stable nuclides shows that light stable nuclides prefer to have equal number of neutrons and protons. However in heavy nuclides the number of neutrons N is always more than the number Z of the protons. Fig.9: The excess neutron number (N-Z) as a Looking to the neutron excess (N-Z) in nuclides, function of mass number A for the stable nuclides (Fig. from R.D. Evans). it may be pointed out that only light nuclei tend

As can be seen from Fig.9, a good fit to Experimentally, this square law is not the (N-Z) plot is obtained from a simple followed. Experimental binding energy per relationship (N-Z) ≈ Constant x A5/3. This nucleon is shown in Fig. 10. As may be equation is of interest here, because it contains observed from this figure, leaving the region of important information about the nuclear forces. very light nuclei, the average binding energy per To understand this, let us consider the Coulomb nucleon is nearly constant. It means that the disruptive energy Ec of a charge „Ze‟ distributed total binding energy of the nucleus is, therefore, throughout a volume of radius R. It is proportional to A and not to A2. The proportional to (Ze)2/R. Further, it has already approximate constancy of B/A over most of the been shown that the radius of the nucleus R α range of the mass region is indicative of the 1/3 A . As such Ec becomes proportional to saturation property of the nuclear forces. It (Ze)2/A1/3. To a first approximation, it may be means that inside the nucleus each nucleon assumed that Z is proportional atomic mass attracts only a few nucleons around it and not all number A or Z α A. Coulomb repulsive energy the other nucleons in the nucleus. 2 1/3 Ec would then be proportional to ≈ A /A ≈ A5/3. From the forgoing discussion, it follows This is analogous to the chemical that the major role of the neutrons excess (N-Z) binding energy, between the atoms in a liquid, is to neutralise the Coulomb repulsive energy, which is known to be proportional to the total which also varies with A in the same fashion as number of atoms. For example, in a drop of the neutron excess (N-Z). water there is a strong homo-polar binding Another important property of nuclear forces is between individual pairs of hydrogen atoms saturation that leads to the exchange forces. To with the formation of H2 molecule. A third understand this, let us consider a nucleus having hydrogen atom is not nearly so strongly A nucleons. If each nucleon exerts the same attracted and hence the H2 molecule is said to be attractive force on all other nucleons then there saturated. The total binding energy of a drop of would be A(A-1)/2 attractive bonds. If so, for liquid is approximately equal to the combined A>>1, the binding energy would increase as energies of the individual pairs of hydrogen A2. atoms i.e., proportional to the total number of atoms present. The successful mathematical representation of the homo-polar bonding is obtained using the concept of exchange forces, which physically corresponds to a continued process of exchanging the electrons of one atom with the other atom in the molecule. The concept of exchange forces has been adopted in nuclear physics principally because such methods are known to give saturation.

Fig.10: A typical binding energy per nucleon curve as a function of atomic mass number. Fig.11: A typical representation of exchange of - meson between nucleons.

The particle which is exchanged Majorana forces: These are the forces in which between two nucleons is –meson or pion. A there is exchange of position coordinates but not typical representation of this exchange is shown of spin. These can be visualised physically in in Fig.11. Symbolically, the exchange force terms of exchange of -mesons. These are between a proton and a neutron can be described attractive for relative angular momentum ℓ= 0, as; 2, 4... while repulsive for odd relative angular p + n → (n’ + +) +n → n’ + (π++ p) momentum. → n’+ p’ Bartlett Forces: In these forces there is Here, the initial proton has become a exchange of spin coordinates. neutron by losing a positive pion, which then joins the original neutron and converts it into Thus, using the concept of exchange proton. The original protons and neutrons have forces saturation of nuclear forces has been now changed their coordinates. Positive, achieved. negative and neutral pions are involved in the exchange process according to; Although, the assumed two body forces n + p → (p’ + -) +p → p’ + (π- + n) to describe strong nuclear interaction have been →p’ + n’ able to explain some basic properties of nuclei, n + n → (n’ + o) + n → n’ + (π0+ n) →n’ + particularly the low lying states, binding n’ energies, spins etc., of light nuclei but they are p + p → (p’ + o) + p → p’ + (π0+ p) not adequate to explain the properties and → p’ + p’ structure of heavier nuclei. It is not unexpected also, as two body force is the most elementary The exchange character of nuclear forces force, which may hold in a system of few is a purely quantum feature. Two nucleons, nucleons. A real nucleus is a many body system when interacting with each other, can exchange and it appears that more complicated force that their position, spin, and iso-spin, or a may describe the simultaneous interactions combination of these. There are basically three between many particles is required. From the types of exchange forces which have been point of view of calculations it will be extremely studied extensively. These are briefly described involved to solve such simultaneous equations. below; Recently, some attempts have been made to invoke three body forces to reproduce the 3 Heisenberg forces: These forces brings about properties of 1H nucleus. the exchange of both the position and spin coordinate of two interacting nucleons. The Researches during the last two decades Heisenberg forces are attractive for triplet have indicated that both neutrons and protons interaction while repulsive for singlet are not so much elementary as had been interaction. The total nuclear forces can not be thought. Nucleons are themselves made up of of Heisenberg type, as the most stable system is some more elementary particles called Quarks. alpha particle (binding energy 28.3 MeV) not As a matter of fact it is now established that not the deuteron (binding energy 2.2 MeV). only nucleons but all other nuclear particles like Therefore, all the nucleons inside an alpha π-mesons etc., are made up of different types of particle should have same spin orientations, but quarks. These quarks are very strongly bound in it is not so. the nucleons and may be released from the nucleons only at very high energy interactions.

TABLE 2 Availability of high energy and heavy-ion Quark structure of some particles accelerators have given a big boost to the S.No. Particles Quark structure research in this direction. Quarks have non- 1. Proton uud integer charge. There are several types of quarks 2. Neutron udd specified by their quantum numbers denoted by 3. π+ u „up‟; ‟down,; „colour‟, „charm‟ etc. In table 2, 4. π0 u quark structure of some particles is given. The 5. π− d quark field is assumed to be mediated by gluons, the quanta of the field.

Unified View of Physics Abbas Ali

ABSTRACT audience disappears in the middle of the lecture. Since I would like to present the very best to my Twentieth century saw the coming of younger brothers and sisters I shall not aim for age and maturing of Physics just as nineteenth that ideal. Your expertize should disappear century saw maturing of Chemistry. What was, much before the middle of my talk. I hope you by the end of twentieth century, the current view can hang on to my presentation till the end and I of Physics? In the beginning of twentieth urge you to strive for that. On my part I shall century it was realized that there are four endeavour my best to make the things easy. We different types of fundamental forces and shall begin with electricity and magnetism. interactions, namely, gravitational, Gauss' law for electrostatics, corresponding law electromagnetic, weak and strong nuclear for magnetism, Faraday's law of interactions. By the end of twentieth century the electromagnetic induction and the Ampere's last three were nearly effectively described by circuital law give you four mathematical unified theories, called the GUTs or grand equations of electricity and magnetism. You unified theories. Of course some problems still either have studied them or you shall encounter remain there but these are not so serious. Then them in your graduate course which has any there are proposals for giving a unified emphasis on Physics. These equations are description of all four fundamental interactions. written in the integral form to begin with and An overview of this saga is the subject matter of then we write them in differential form for the this article. The description is at popular level rest of the time. but some physics background is still required. These are as follows: Key Words: Fundamental Force, Gravitational, Weak, Strong, Electromagnetic (i) Gauss' law for electrostatics Interactions, Special and General Theories of (1) Relativity, Quantum Mechanics, Quantum Field E = , 0 Theory, Quantum Electrodynamics, Quantum (ii) Gauss' law for magnetostatics Chromodynamics,Glashow-Salam-Weinbeg B = 0, (2) Model, Grand Unified Theories, Supersymmetry, Supergravity, Superstring (iii) Faraday's Law for electromagnetic Theory. Induction B E = (3) The Physics You Already Know t (iv) Ampere's law B = J (4) Dear Students, 0

Today I shall make a long scientific Here Faraday's law says that changing journey with you. It is part of the occupation of magnetic field gives electric field. In Ampere's a scientist to explain his findings to the public. It law we get magnetic field from electricity. Here is not easy. And it is not going to be easy even if we already get a hint of the type of things we the audience is scientifically literate like the are going to encounter - for general public gathering today. Ideally the organization of the electricity is different and magnetism is material should be such that expertize of the

51 different. For us they are very closely related to An single set of equations is sufficient to cover each other. three completely different fields of Physics. These are electricity, magnetism and optics and in fact much more. Can we do more? Can we Maxwell's Electromagnetic Theory combine more things together to get a unified description? Answer turns out to be Maxwell modified these equations to the overwhelmingly yes. following form: Special Theory of Relativity

(5) E = , Let us take the Lorentz force equation. 0

B = 0, (6) F = q[E v B]. (9) B E = (7) t Now if your friend is moving in a train E and there is magnetic field around you shall say B = 0 J 0 0 . (8) t that a force will act on him if he has a charge on him. Since he is stationary with respect to How do they differ from the earlier himself he has zero velocity and he will say that form? As you see these differ from earlier no force is acting on him. Only one of you can equations only in the last term in the last be correct. Who is correct? It turns out that this equation - that is all. Now these are called problem can not be solved unless we do what Maxwell's equations. None of them is from Einstein did. This is to introduce Special Theory Maxwell but we still use his name because of of Relativity. Introduction of special theory of his introduction of a term in the Ampere's relativity is also like unification. This time circuital law. The new term added by Maxwell mechanics is being unified with electromagnetic is called the displacement current. This has the theory. For sake of completeness here are the important implication. As a result of this two postulates of special theory of relativity. (1) addition we realize that changing electric field Laws of Physics are the same in all inertial will give magnetic field. This can happen even frames. (2) The maximum possible speed for a in vacuum. Thus in vacuum a changing electric signal is the speed of light in vacuum and it is field will give magnetic field and a changing the same for all inertial observers. A frame or a magnetic field will give electric field and so on. reference frame is just a coordinate system Thus they will sustain each other. We call such except that now time is also plotted as an a situation an electromagnetic waves. Even light additional, that is, fourth axis. In case anybody is an electromagnetic wave. A closer look at has still missed - we have reconciled mechanics these results tell us that we have unified several with electricity and magnetism! It is not much things together. Electricity, magnetism and different from unification. light. In fact the whole electromagnetic spectrum is included here. We have the usual Non-commercial Break VIBGYOR spectrum of visible light. Before that we have the radio waves, microwaves and The geographical region to which we the infra red radiation. After the visible belong is somehow deprived of some finer spectrum we have the ultraviolet waves, X-rays things of life. Poet Surendra Sharma said that in and the -rays. All of them are described by the name of culture we have only agriculture. In Maxwell's equations! This is an exciting thing. this region the development of literature,

52 including poetry and prose, and other cultural nuclei are quite different. In fact the rules that artifacts of a mature and grown up societies are are applicable to small systems start looking like missing near completely. It was my personal the usual rules when we look at the larger and feeling that we could not talk about any freedom larger systems. These rules, the laws of Physics fighter, a writer or any other figure of national for very small systems, constitute the real laws importance. To give you an idea of the of Physics and we call the collective knowledge discrepancy I'd like to give you an example. The as Quantum Mechanics. Among the most national language of India is defined as the common results of quantum mechanics is the Hindi spoken between Delhi and Meerut. That is fact that small systems are described by an exactly where we live! Now if that is the equation called Schrodinger Equation. It looks national language then why are the local words like this: of this area completely missing from standard Hindi vocabulary. Just tell me the local word for 2 (x,t)  2 cow dung cake and the corresponding word i = (x,t) V (x,t) (x,t). (10) t 2m used in Hindi. Our cultural refinement stops at Ragini, Kabaddi and Volleyball tournaments. I Here (x,t) is called the wave function suppose we should be paying some attention to of the system that we are trying to describe, for this aspect of life. In this regard it gives me example, an atom, a molecule or a nucleus. further pleasure that we are talking about those Many interesting things are discovered here things here that other much more developed about the world that are not apparent to us in the societies talk about. common life. Only few of them can be

mentioned because we have to cover a long list Quantum Mechanics of developments. For example energy of most of

the physical systems can not be continuous. This More than a century ago we were getting is what we discover when we study quantum such results in Physics which could not be mechanics. In many cases the energy is explained by the laws known at that time. For quantized - it can take only discrete values. This example if we use electromagnetic theory then is the case, for example, energy of electron in we find that any accelerated charged particle hydrogen atom. Then there is another result. In will radiate electromagnetic waves and hence hydrogen atom or otherwise if we take an will loose its energy. Now electrons in atoms electron then we can not measure both its move in circular or elliptical orbits and in both position and momentum with complete of these cases electrons are accelerated and accuracy. If we measure the position very hence they should loose energy. If that is the precisely then we have very little knowledge of case then slowly the electrons will move its momentum and other way round. This is towards the nucleus and fall into it and any and called Heisenberg's uncertainty principle. every atom will be destroyed. People thought Mathematically we write it as that all atoms will destroy themselves but it will take some time. After calculations it was realized that atoms should destroy themselves in 1 x px . (11) the fraction of a second. But that is not 2 happening. So what is happening? Around 80 to 90 years ago it was realized that the rules that Here x is the uncertainty is the we were using to draw above conclusions are measurement of the x -coordinate of the particle applicable to large systems only and the rules and px is the uncertainty in the corresponding for small systems like atoms, molecules and component of the momentum. Finally I'll

53 mention another puzzling result of quantum on the ground. From these things we realize that mechanics. At small scales, when we are there are at least two different types of forces studying small systems, we realize that particles and hence interactions. Number one - the might behave like waves and waves might electromagnetic force that we have discussed behave like particles. This is called wave earlier. The magnet is only one manifestation of particle duality. So is an electron a particle or a that. Then there is gravitational force that is wave? Experimentally in some cases it behaves very well known to us. Are there other forces like a particle. In some cases it behaves like a also? Answer is yes because we already know wave. Same is true for electromagnetic waves. about radio activity - accidentally discovered by These are waves as we know but in photo- Henry Becquerel in 1896. We know radio electric effect these waves really behave like a activity has some thing to do with the nucleus of particle that we call photon. And that is the the atom. It turns out that inside the nucleus reality as we see at small scales. Depending there are two more types of forces sitting - weak upon the situation a physical system will behave and strong. Our radioactivity, nuclear power, like a particle or a wave. atom bomb, hydrogen bomb and nuclear medicine are possible because of them only. Quantum Field Theory Thus we have fore fundamental forces or interactions in nature: (i) Gravitational (ii) Now since the theme of the talk is Electromagnetic (iii) Weak and (iv) Strong. Out unification we shall again ask the related of these the gravitational one is the weakest. question - can we combine special theory of Scientist : Gravitational force is the weakest. relativity and quantum mechanics together? A TV Host : Have you ever fallen from the second simple mathematical combination in this case floor? This seems like a puzzle. Basically this is leads us to a theory which we call relativistic still true because most of the matter is quantum mechanics. A little bit of further electrically neutral, like atoms, so we do not see investigation tells us that this can not be the the electromagnetic force in the same powerful complete story and something more is required. form as gravitational one except in case of This lead us to the overarching framework, a thunders and power lines of thousands of volts. generalization of relativistic quantum Finally weak and strong interactions are mechanics, called quantum field theory. It observed inside the nucleus only and hence we differs from relativistic quantum mechanics, do not feel them that much. We get some idea of among other things, that there can be creation the strength of the nuclear forces from the fact and destruction of particles. This creation and that radioactivity can not be stopped by means destruction of particles is not unfamiliar to us. of chemical or physical processes. When an electron in an excited atom jumps to a lower level of energy it emits a photon. Before Quantum Electrodynamics emission the photon did not exist. This is the quantum field theory that Four Fundamental Forces and Interactions describes electromagnetic interactions. remember that quantum field theory is a We shall use the words forces and generalization of a mixture of special relativity interactions interchangeably. So how many and quantum mechanics. This is amongst most different types of fundamental forces, and hence successful theories of physics. When you do interactions, are there? When we bring a magnet your practical in the laboratory you start getting near a brass object nothing happens. But if we your errors at the first decimal place. In case of leave a brass object from our hand then it drops Quantum Electrodynamics, or QED, theoretical

54 and experimental results match with each other things that result as an accumulation of the upto nine decimal places. smaller things. This is called reductionism. From the uncertainty principle quoted above we Glashow-Salam-Weinberg Model can argue that we need higher and higher energy to study smaller and smaller systems. This is Now we have interactions and their because fundamental theories. Like QED is the fundamental theory of electromagnetic  p . (12) interactions. So what will be the quantum field x 2 x theory that describes weak interactions? In 1967 Abdus Salam and Steven Weinberg proposed a The conclusion is that to study particle theory that is called Quantum Flavourdynamics physics, that is smallest things called elementary (QFD) or Glashow-Salam-Weinberg theory of particles, we need highest energies. electroweak interactions. This is a theory that Consequently the insider name for particle not only describes weak interactions but it has physics is High Energy Physics. We have eaten up the earlier theory called QED! This is already encountered two of the main things in our finest example of unification in Physics. this field - QED and QFD or Glashow-Salam- Rather than giving a theory of weak interactions Weinberg Model. they gave a theory that combines and unifies two interactions together. For this work they got Quantum Chromodynamics the Nobel Prize for Physics in 1979. Quantum Chromodynamics, or QCD, is Branches of Physics the quantum field theory that describes strong interactions that we encounter deep inside the The physics of materials is called solid nucleus or in particle physics. I do not know state physics or the condensed matter physics. whether Lagrangian and Hamiltonian dynamics This is the subject that has interesting are taught to you but those are the beginning phenomena like superconductivity. If we take an things if we want mathematical formulation of atom or a molecule only and then we study them any field theory. Anyway by now I can only then we are in atomic and molecular physics. recite the names because any inclusion of There is another name for it - it is called technical details is sure to give you headache. spectroscopy. This is the subject that has interesting phenomena like lasers. If we go Standard Model of Particle Physics further deep into the atom we, of course, find the nucleus there. Study of the nucleus is the QCD taken together with QFD is called nuclear physics. We have already mentioned the the Standard Model of Particle Physics. interesting things in this case - atom and hydrogen bomb, nuclear power plants and Grand Unified Theories (GUTs) nuclear medicine. The nucleus has the neutron and protons. In fact it is easy to find other The way Abdus Salam and Steven animals also there - like pions. If we want to Weinberg combined the weak and study these things then we are in particle electromagnetic interactions together is it physics. You must have noticed that we are possible to combine all three of them together - trying to study smaller and smaller things. weak, strong, and electromagnetic? Basically we are hoping that by understanding Mathematically people have tried this also and the smaller things we can understand the larger these are called the grand unified theories or the

GUTs. Two most famous models in this class Supersymmetric transformations are the are called the Georgi-Glashow model and the transformations that change bosons to fermions Pati-Salam model. These models demand that and fermions to bosons. the proton should decay after some time - This is a mathematical framework - a theory. though that time is long. Some experiments were done including the one in Kolar God Science is empirical. This means a Mines in Karnataka many years back and there theory is right if it confirms with experiments was no indication of proton decay. So at this otherwise wrong. moment we do not know whether nature really likes to combine the three interactions together. CERN and LHC

Supersymmetry So far we have not got any experimental evidence that nature is supersymmetric. But this Though we do not know about grand is one on the actively investigated theory at unification - whether nature uses it or not but LHC (Large Hadron Collider) ant CERN scientists have gone a few steps further. To talk (European Center for Nuclear Research) in about those directions in which physicists have Geneva, Switzerland. More than ten billion exercised their energies we have to talk about dollars have been spent on the machine where bosons and fermions. Bosons are named after these experiments to verify supersymmetry and our S.N.Bose and Fermions after Enrico Fermi. to find a particle called the Higgs particle are Bosons have either zero or integral spin in units going on. Ten billion dollars will be equal of  - the Planck constant. Fermions have half nearly 500 billion rupees. This is 500 Arab integral spins in the same units. Photon is a rupees. Governments of many countries in the boson and electron is a fermion. This spin, world have contributed this money including though like angular momentum, is an intrinsic India, the US and many European countries. A quantum mechanical property and we can not research group at this laboratory can have give a common life comparison of this. (As a thousands of scientists and there names appear side remark we shall add that spin, mostly of in a research paper! One of the instruments electrons, is responsible for all of the magnetic there, the ATLAS detector, is five story high phenomena - para, ferro, ferri, diamagnetism in and the main machine is a circular metro like matter.) Fermions obey the Pauli exclusion tube with a perimeter of 27 km. principle - two fermions can not have the same quantum numbers. Two or more bosons can Einstein's General Theory of Relativity have same quantum numbers and that is why we get the things like lasers. Clearly fermions and Out of the four fundamental interactions bosons are quite different from each other. Can we have not talked about gravitation. But you we combine fermions and bosons together into a already know about it. It is described by unified description? Newton's law of gravitation! Most of the things related to gravitation can be described by Is there some possibility that will take us Newton's law. from fermions to bosons and vice versa? That is precisely supersymmetry. m m F = G 1 2 . (13) 2 r We say that a theory has supersymmetry if that theory is invariant, that is it does not In spite of the above fact Einstein change, under supersymmetric transformations. proposed a new theory of gravity called the

General Theory of Relativity in 1915 - just ten just very small strings. In fact all of them are the years after his theory of special relativity. There same string but it is vibrating with different was no experimental requirement for this theory frequencies and hence we see different particles. at the time of its proposal but Einstein said that This theory combines, unifies and incorporates this can be verified at the time of an eclipse by all of the diverse theories that we have observing whether the sun bends the light enumerated in this long winding discussion - coming from distant stars. This was and in fact more. experimentally observed in 1919 and thus this theory was verified to be true. Newton's law of Apology gravitation comes out of this theory. This lecture was supposed to be about Supergravity Theory string theory. And what we have ended up with? The moment the name of superstring thoery We have talked about supersymmetry came up the lecture is over. Rather than ending and we have talked about general theory of the lecture should have begun here. My relativity. apologies for the disappointment. I am sure you understand that if I would have talked about the What will happen if we combine two of technicalities of string theory then it would have them together? been of very little use. And thank you very much for your patient hearing of this Well people have already done that. In presentation. fact when we try to play with supersymmetry, when we try to make it local, we automatically References get general theory of relativity! This new thing is called supergravity theory. There are many books at various levels on these topic in the market but the best thing is Superstring Theory the Internet where incredible amount of informations is available for free. Can there be something beyond supergravity also. The answer is yes. There is a (This article is based on a talk delivered to theory called superstring theory or simply the undergraduate students at Mihir Bhoj P.G. string theory that incorporated supergravity College, Dadri, Gautam Budha Nagar, U.P..) inside it and many other things. According to superstring theory all the particles of nature are

Design and Fabrication of Bulk Heterojunction Solar Cells, utilizing graphene and transparent conducting oxides based electrodes Firoz Alam

Organic/Polymer solar cells have match the organic cell‟s flexibility, transparency attracted attention in recent years, because of its and low cost. advantage of being inexpensive, light weight, flexible, large area feasibility and continuous The standard material used so far for these manufacturing ability. Organic photovoltaic electrodes is Indium tin oxide (ITO). ITO has solar cells (OPVs) are based on layers of been widely used in optoelectronic devices as solution processed bulk-heterojunctions (BHJs) transparent conducting electrodes. However, of semiconducting polymers, sandwiched ITO adds to the cost because of the limited between two electrodes. At present, these solar availability of Indium on earth, furthermore the cells have shown power conversion efficiency issues like Indium ion diffusion into organic of ~10%, which needs to be scaled up to be layers, and mechanical brittleness of ITO makes commercially feasible. it less ideally suitable for OPV device. Graphene, because of it‟s high electrical The major future challenges in development of conductivity, flexibility and transparency, is Organic solar cells are: being envisioned as a promising material for the 1. Significant improvement in efficiency of replacement of ITO. However, various OPV cells. processing difficulties lies ahead to use atomic 2. Replacement of transparent ITO electrodes. layer graphene as an electrode material in 3. Stability and Reliability. Organic solar cells. 4. Development of low cost processing methodology. Graphene

A target improvement of >15% is to be Graphene is a 2-D carbon material with achieved to make OPV cells commercially single atomic layer thickness made of sp2 feasible. To overcome above challenges, carbon atoms and has a large specific area with extensive research in the area of material high mobility of 10, 000 cm2V-1s-1, and tunable development, optimization of device band gap. Graphene is also a promising parameters, modification in device architecture electron acceptor in photovoltaic devices. needs to be done. Graphene, has been the focus of much research since its isolation because of the unique A promising approach for making solar cells transport properties. Because of graphene‟s high that are inexpensive, lightweight and flexible is optical transmittance and conductivity it is also to use organic (that is, carbon-containing) being considered as a transparent conductive compounds instead of expensive, highly purified electrode. In comparison to traditional silicon. But one stubborn problem has slowed transparent conductive electrodes such as the development of such cells. Researchers have indium tin oxide (ITO), graphene films have a hard time coming up with appropriate high mechanical strength, are flexible, and are materials for the electrodes to carry the current chemically stable. Production of large-area and to and from the cells. Specifically, it has been high-quality graphene films is necessary for hard to make electrodes using materials that can device applications such as Organic field effect

58 transistors (OFETs), Organic solar cells (OSCs) coatings on windows. Fluorine-doped tin oxide and Organic light emitting devices (OLEDs). (SnO2:F) is becoming more popular in research The biggest problem with getting graphene to based sensitized photovoltaics. Although the work as an electrode for organic solar cells has conductivity performance can be slighter than been getting the material to adhere to the panel. ITO, (SnO2:F) is generally less expensive in Graphene repels water, so typical procedures for material cost and manufacturing. producing an electrode on the surface by depositing the material from a solution won‟t Conjugated Polymer work. Organic polymer having extended - However, the flexibility and light weight conjugated system are special class material of organic solar cells with graphene electrodes which on doping with specific charge carrier could open up a variety of different applications behaves like a semiconductor and this novel that would not be possible with today‟s characteristic make them synthetic metal. The conventional silicon based solar panels. Organic electronic conductivity of these conducting solar cells, being transparent could be applied polymers can be varied from 10-10 - 105 S/cm. directly to windows without blocking the view, chemically doping and undoping can reversibly and also could be applied to irregular wall or control the electrical properties of the polymer rooftop surfaces. In addition, they could be and that is the reason of extensive use of these stacked on top of other solar panels, increasing polymers as sensors. the amount of power generated from a given area. And they could even be folded or rolled up Importance of the present Work for easy transportation. Graphene is transparent, so that Transparent Conducting Oxides electrodes made from it can be applied to the transparent organic solar cells without blocking The transparent conductive oxide any of the incoming light. The proposed work (TCO), such as indium-tin-oxide (ITO), Al- will be focussed on researching unique doped zinc oxide (AZO) and Zn-doped indium methodology to prepare high quality single layer oxide (IZO) have attracted much interest in the graphene sheets. These graphene sheets will be application of optoelectronic devices such as processed further and employed in device solar cells and liquid crystal displays due to fabrication as a substitute for ITO, thus their high conductivity and high transparence in providing us a cheaper, flexible and efficient visible region. device. Prototype development of graphene based organic solar cell module will also be ITO (Indium Tin Oxide, 80-90% Indian attempted. This work could also lead to flexible oxide with a minor amount of tin oxide has been organic light emitting devices (OLEDs) and very popular for LCD flat panel displays. They organic thin film transistors (OTFTs). generally have a slightly yellowed appearance and have also been used as infrared-reflecting

Quantum Computer: A Big Technological Challenge Azharuddin

Introduction machine will be far more efficient at factoring a big number or searching database compared to There are many complex problems such its classical counterparts. Quantum computers as finding prime factor of a large number, promise to exceed the computational efficiency searching an entry in a big unsorted database in of ordinary classical computers because real time, molecular simulation etc. that are quantum computing algorithms allow to thought to be intractable and cannot be solved perform tasks in fewer steps. and simulated even on the latest supercomputer. These problems are not practically feasible What is a Quantum Computer? because of the speed limitation and lack of good efficiency of algorithms of today‟s classical A quantum computer resembles a computers. Downsizing the size of the classical one. It processes quantum information. electronic components, e.g., transistors, on ICs Quantum information can be implemented by we can enhance the speed of classical silicon utilising any property (of course one must have computers. But how much can we reduce the enough external control on that property to be size? Is there any lower limit of this? Yes, there utilised) of a particle or an atom or a molecule is a lower limit. If we keep on reducing the size, or a cluster of atoms or molecules. For example, we shall reach the barrier of quantum world spin of a particle can be treated as to hold below which the things are no more classical. quantum information. Similarly, magnetic One can be fool enough to fabricate an IC moments of molecules, polarizations of photons, having component size lying in the range of energy states of ions etc. can also be the other quantum world and to apply classical laws to its candidates. A quantum computer makes direct circuitry operation. use of quantum mechanical phenomena, such as superposition and entanglement, to perform Is the practical limitation of classical operations on quantum data. computer‟s speed weakening our interests in research in those fields where problem A quantum computer can process vast complexity is increasing day by day? In fact, amounts of information in a very little span of when a usual trend in science and technology time. This weird potential is very useful for reaches its limitation, a new trend comes into cryptography, searching data, factoring large existence for the continuation of the numbers quickly and the simulation of quantum- developments and advancements of scientific mechanical systems as these processes demand researches and practical implementation of very large amount of data processing. theories. Then, one can guess that there would be a nice gift of science and technology which is The reversible feature of a quantum able to overcome all the limitations of classical computer makes it capable of functioning with computation. Yes, there is something which is no net energy consumption (theoretically). To being expected to solve all the practically elaborate quantum reversibility, it drives itself intractable problems in real time. This forward in infinitesimal (reversible) steps. In “something” is “quantum computer”. contrast to “run” terminology of classical computer programs, quantum computer A quantum computer is a strange programs "evolve" during the processing of exploitation of quantum mechanics. Such a quantum data. Incidentally, reversibility also

60 means that the inputs of a quantum computer they acted like a single one that could occupy can be regenerated from the outputs, provided two different energy states. the program should run backwards. In 2010, researchers at the California Quantum Computing Candidates Institute of Technology (Caltech) have demonstrated quantum entanglement for a Massachusetts Institute of Technology, quantum state stored in four spatially separate Oxford University, IBM and Los Alamos atomic memories. National Laboratory are the most successful in development of quantum computers. In 2011, scientists from Oxford University have made a significant step towards Nuclear Magnetic Resonance (NMR) an ultrafast quantum computer by successfully technology is the most popular today, because generating 10 billion qubits of quantum of some successful experiments. MIT and Los entanglement in silicon for the first time. Alamos National Laboratory have constructed a simple quantum computer using NMR People have also succeeded to run technology. Some other designs are based on Shor‟s algorithm on a silicon-based quantum ion trap and quantum electrodynamics (QED). computing chip, based on quantum optics. Scientists are utilizing many quantum properties of materials for the implementation of quantum Qubit: Unit of Quantum Information computers. There are a number of quantum computing candidates. A partial list is given below:

Nuclear magnetic resonance on molecules in solution (liquid NMR) Trapped ion quantum computer Superconductor-based quantum computers Spin-based quantum computer Electrons on helium quantum computers Topological quantum computer Quantum dot (a single electron trapped inside a cage of atoms) Optical lattices Solid state NMR Kane quantum computers Molecular magnet Fullerene-based ESR quantum computer Optic-based quantum computers (Quantum

optics) A quantum system having two distinct Transistor-based quantum computer quantum states (e.g., spin-up and spin- down states of a particle) can help implement 0 and 1 In 2009, researchers at Yale University qubit states. 0 corresponds to one quantum state made the first simple solid-state quantum and 1 corresponds to another quantum state of processor. The two-qubit superconducting chip the quantum system. For example, if |1> was able to run elementary algorithms. Each represents 1 state and |0> represents 0 state, then qubit consisted of a billion aluminum atoms but

61 pure qubit state would be a linear combination Speed and Power of a Quantum Computer of these two states:

|ψ> = α |1> +β |0> where |α|2 is the probability of outcome |1> and |β|2 is the probability of outcome |0> and |α|2 + |β|2 =1. Now depending upon the values of α and β the qubit value of the quantum system can be either 1 or 0 or any quantum superposition of them. Similarly, a string of qubits can be in any superposition of its bit-string configurations. This quantum superposition property greatly enriches the kind of information that can be represented by qubits.

A quantum computer maintains a set of qubits and operates by manipulating those qubits, i.e. by transporting these qubits from memory to quantum logic gates and back. A Chuang stated, “A supercomputer needs pair of qubits can have four discrete about a month to find a phone number from the combinations i.e., 00, 01, 10, 11. If a datum is database consisting of world's phone books, represented by two qubits, then it can have any whereas a quantum computer is able to solve discrete state of the above four, or a state of this task in 27 minutes”. quantum superposition of these four discrete states. In general a quantum computer with n Quantum computer with 500 qubits qubits can be in an arbitrary superposition of up (assuming that each qubit has only two 500 to 2n different states simultaneously (this distinguishable states) gives 2 independent compares to a classical computer that can only superposition states as shown in fig (2). Each be in one of these 2n states at any one time). A superposition state is a string of five hundred 1‟s 500 quantum computer operates by manipulating and 0‟s. Such computer can operate on 2 those qubits with a fixed sequence of quantum states simultaneously. logic gates. This sequence of gates is called a quantum algorithm. During the measurement of quantum data, the string of five hundred qubits collapses 500 Is there any restriction of choosing into a single superposition state out of 2 quantum properties for the implementation of possible states giving a single answer. To praise qubits? In fact any system possessing an the potential of such a quantum computer, it can 150 observable quantity A which is conserved under defeat a classical one with approximately 10 time evolution and such that A has at least two processors. discrete and sufficiently spaced consecutive eigenvalues, is a suitable candidate for According to Moore's Law, the number implementing a qubit. of transistors of a microprocessor continues to double in every 18 months. This law predicts a classical computer in year 2020 with 40 GHz CPU speed and 160 GB RAM. An analogue of Moor‟s law for quantum computers states that

62 the number of quantum bits would be double in though it does not sound like the opposite ends every 18 months. But adding just one qubit is of the universe, is very significant to the scale to quite enough to double the speed. build quantum computer components.

Quantum Entanglement Quantum mechanically, information can be sent from one particle to another faster than Quantum entanglement is a typical the speed of light. Although this sounds property of a quantum system. Components of a incredible it has been proven. If we have a quantum system are linked together through system of two particles, let‟s say electrons, that quantum entanglement. Classical physics are entangled and we change the spin of one of undoubtedly fails to explain this strange them, the other will automatically change its quantum mechanical feature. Description of one own spin at a speed faster than light even object among many entangled objects requires though they are kept at opposite ends of the full mention of its counterparts. Even though the universe. This property, if fully controlled, objects are spatially separated this requirement would make quantum computers run at infinite is essential. Quantum entanglement helps speed. entangled objects share information among them. This interconnection is purely a quantum Quantum Decoherence phenomenon and it cannot be explained by classical physics. Quantum system being extremely sensitive to interaction with the surroundings, a Spin property of a system of two quantum computer is very difficult to maintain entangled particles can be correlated. Both the its quantum state during probing the system particles can have the same spin or the opposite from the outside. Any interaction (or spins. Measurement of one‟s spin gives the spin measurement) leads to a collapse of the wave of the other. Distance between the two particles function of the quantum system. This wave is irrelevant. function collapse is called decoherence. Quantum decoherence is one of the major In pair production, the system of two obstacles in the quantum computer particles produced in the process can also be implementation. It is extremely difficult to termed as entangled system. One particle has isolate a quantum system from the environment. many physical properties such as charge, spin The larger the number of qubits the harder is it etc. opposite to the other one. So, knowledge of to maintain the coherence. one‟s properties tells the other‟s. Decoherence arises when a system A quantum computer would have to interacts with its environment in a entangle the qubits over a significant distance. A thermodynamically irreversible way. One can classical computer uses electrical signal to consider quantum decoherence as the loss of transmit information from one device to information from a system into the environment. another. Analogously, a quantum computer exploits entanglement property of the qubits for In order to make quantum computers information transmission. A credit goes to powerful, many operations must be performed Andrew Berkley and colleagues for their before quantum coherence is lost. It can be successful implementation of entanglement impossible to construct a quantum computer that between two qubits inside a silicon chip over a will make calculations before decoherence time distance of 0.7 millimetres. This distance, (about 10-27 seconds!). But if one makes a

63 quantum computer, where the number of errors bit is flipped, one gets a state, for example 011, is low enough, then it is possible to use an error- which can be corrected to its original state 111. correcting code for preventing data losses even when qubits in the computer decohere. Quantum Software

Quantum Logic Gates and Circuits A quantum software or program is a particular quantum state of a system of thousand Ordinary computers deal with data or million or billion qubits fabricated on a processing on electronic circuits mainly quantum chip that enables a quantum computer containing logic gates such as AND, OR, NOT to perform a specific task. Such quantum etc. Likewise, in a quantum computer, software is very difficult to prepare specially for information is passed through quantum circuits non-professional users. A user can acquire the containing quantum gates. A quantum gate is a quantum software from a vendor rather than unitary operator U. prepare it himself. Quantum software is a consumable product and it is damaged after a Not all the classical logic gates are single use. So, if a user wants to perform the reversible. For example, if the output of AND is same job many times, then he will have to 0, there are three possible values for the input purchase the same quantum software each time. i.e., 00, 01, and 10. All quantum gates, on the This can be a misfortune for users but a good other hand, are reversible. Reversibility means fortune for the vendors. Thus we can foresee a inputs can be restored from outputs. flourishing of a quantum software industry. A company can design valuable quantum software A quantum logic gate must perform its and use a special-purpose device to make and operation faster than decoherence time. store multiple copies of it. A user can download Otherwise, quantum information may get altered the quantum software for a fee from the due to decoherence and there will be errors in company through quantum internet. the output of the logic gate. Quantum Algorithms Quantum Error Correction A quantum computer performs a job A quantum computer will certainly following a sequence of some pre-defined steps. interact with its surroundings, resulting in This sequence of steps is called an algorithm. decoherence and hence quantum information Superiority of a quantum computer over a stored in the device will decay. Quantum gates classical computer comes due to a perfect are unitary transformations and cannot be quantum algorithm that exploits quantum implemented with perfect accuracy. A small parallelism. Without a proper quantum imperfection in the gates will cause a serious algorithm a quantum computer will not failure in the computation. Any effective necessarily outperform classical computer at all scheme to prevent errors in a quantum computer computational tasks. A quantum algorithm is must protect against small unitary errors in a very difficult to formulate. Among many, Shor‟s quantum gates, as well as against decoherence. and Grover‟s algorithms are the most significant. These algorithms are extremely There are a lot of error-correcting codes. efficient to reduce computational complexity The simplest one is called repetition code. 0 is and enable a quantum computer to defeat a encoded as 000 and 1 as 111. Then if only one conventional computer by a significant margin. For example, Shor's algorithm allows extremely

64 quick factoring of big numbers. A classical Ultra-secure and super-dense computer can be estimated at taking millions of communications: years to factor a 1000 digit number, where as a quantum computer would take only few It is possible to transmit information without minutes. On the other hand, Grover‟s algorithm a signal path by using quantum teleportation. empowers a quantum computer to search an There is no way to interrupt the path and extract unsorted database incredibly faster than a information. Ultra-secure communication is also conventional computer. Normally a classical possible by super-dense information coding. computer would take N/2 number of searches to Qubits can be used to allow more information to find a specific entry in a database with N be communicated per bit than the same number entries. Grover's algorithm makes it possible to of classical bits. perform the same search in √N searches. Molecular simulations: Difficulties in the implementation of Quantum Computers A classical computer cannot simulate quantum effects without slowing down Quantum decoherence is the major exponentially. A quantum computer, on the obstacle in a process of producing of a quantum other hand, can do the same job in real time. computer. If decoherence problem cannot be Molecular simulations of chemical interactions solved, a quantum computer will be no better will allow chemists and pharmacists to learn than a silicon one. more about how their products interact with each other and with a person‟s metabolism or A practical working quantum computer disease. requires a well-defined array of qubits for stable memory, feasible state preparation for the initial True randomness: state, long decoherence time, a universal set of gate operations and capability for single- Classical computers can generate only quantum measurement. Not all these pseudo-random. Pseudo-random generators requirements are currently unconditionally cannot simulate natural random processes achievable. accurately for some applications, and cannot reproduce certain random effects. Quantum Future benefits of quantum computers computers can generate true random numbers. This potential can play a significant part of Shor‟s algorithm and cryptography: applications on statistical approaches of some natural phenomena, for simulations, code Factoring is one of the most important making, randomized algorithms for problems problems in cryptography. For instance, the solving, stock market predictions etc. security of RSA (electronic banking security system) - public key cryptography - depends on Conclusion factoring and it is a big problem. However, breaking any kind of current encryption that It is obvious that making a practical quantum takes almost centuries on existing classical computer is still far in the future. In practice, computers, may just take a few moments on quantum hardware is in its infancy. quantum computer. Programming style for a quantum computer will also be quite different. Development of quantum computer needs a lot of money. Even the best

65 scientists can‟t answer a lot of questions about Aaronson, S. The Limits of Quantum quantum physics. Building a practical quantum Computers (20 08 SCIENTIFIC computer is just a matter of time. Perhaps AMERICAN, INC.) someday, as suggested by Daniel Gottesman of Chirolli, L and Burkard, G. Decoherence in Microsoft Research and Isaac Chuang of the Solid State Qubits (Advances in Physics IBM Almaden Research Center, the capabilities Vol. 57, No. 3, May-June 2008, 225-285) of quantum computers will be significantly Boghosian, B. M. Simulating Quantum extended by „quantum software‟ that can be Mechanics on a Quantum Computer prepared offline and shipped to users over the (arXiv:quant-ph/9701019v2 8 Mar 1997) „quantum Internet‟. Realization of a working Kane, B. E. A silicon-based nuclear spin quantum computer will be one of the biggest quantum computer (NATURE |VOL 393 | steps in science and will undoubtedly 14 MAY 1998) revolutionize the practical computing world. Hughes, R. J. Cryptography, Quantum Computation and Trapped Ions Further Reading P. Walther, K. J. Resch, T. Rudolph, E. Schenck, H. Weinfurter, V. Vedral, M. Preskill, J. Quantum Computation (lecture Aspelmeyer & A. Zeilinger. Experimental notes) one-way quantum computing (NATURE (www.theory.caltech.edu/people/preskill/ |VOL 434 | 10 MARCH 2005) ph219/#lecture) DiVincenzo, D. P. Quantum Gates and Preskill, J. Quantum Computing: Pro and Circuits (arXiv:quant-ph/9705009v1 7 May Con (arXiv:quant-ph/9705032v3 26 Aug 1997) 1997) Knill, E. Quantum computing Vedral, V. Quantifying entanglement (NATURE|VOL 463 | PAGE 441-443 | 28 inmacroscopic systems (NATURE|Vol January 2010) 453|19 June 2008) Preskill, J. Plug-in quantum software (NATURE|VOL 402 | 25 NOVEMBER 1999)

An Introduction to Complex Spectra Swapnil and Tauheed Ahmad

Introduction Recent progress in the study of complex spectra Complex spectra are not just those that are particularly rich in lines or that call for The Relativistic Hartree-Fock especially intense intellectual efforts to unravel calculations are not very easy and even doing them. The feature of the complex spectra is the single configuration calculation doesn‟t help. appearance of multiplets or groups of lines with Large numbers of interacting configurations, characteristic spacings and strengths. For heavy perturb the position of the levels significantly. atoms, the multiplets often overlap one another, This was one of the main reasons for complex and the resulting morass of lines may be spectra being left behind. With the availability responsible for the common misconception that of the fast computers and various computer the word „complex‟ refers to the intricacy of the codes like those of R.D. Cowan[1] ,C.F. spectrum. From a theoretical standpoint, the Fischer[2], MCDF [3,4 ] and the recent version lines of a complex spectrum correspond to of the MCDF program GRASP [5] etc and high transitions between comparatively low-lying resolution spectrographs, it is now possible to configurations that each comprises several compute the structure of the complex system active (i.e. valence) electrons. The reality of and can record high quality dense spectra. The atoms as they exist in discharges or in plasmas next problem lies ahead is the technicalities attracts the attention of the experimentalist. For involved to handle such complex spectra. A the theorist, there is the challenge of studying a comparison of such calculations with the particular aspect of the many-body problem. parameters that emerge from the least-squares Two-electron configurations can give rise to fitting of the atomic data yields vital complex spectra, but our interest here is limited information on the validity of the Hartree-Fock to those features that are susceptible of method and its various elaborations. generalization to many-electron configurations. The most difficult thing in studying complex It would be appropriate to introduce spectra is the reliable predictions of the terms some basic terms which are often used to and levels. describe the spectrum. The multiplicity (M) of the terms is defines as 2S+1. This can explain that for a single electron in an atom with total spin S =1/2, M = 2S+1 = 2; means doublet term system will be formed while for two electron system with total spin S=1 & 0, multiplicity can be 1 and 3 that will give singlet and triplet terms and the same way, a three and four electron system will produce doublet and quartet, triplet and quintet respectively. This is thus obvious that as the number of active electrons increases in the open shell, the term system becomes more Iodine spectra and more complex. For instance the number of levels observed as under:

Configuration Number of The Hamiiltonian for an atom can be written in energy levels general

2 6 10 2 1 z z z 1 closed shell (s , p , d etc) 1 H 2 2 2 e2 K e2 K L e2 H H H H ... 2m j 2M k r r r SS SO hfs etc one electron (p,d,f,g,h, etc) 2 j 1 k K K J Kj K L KL i j i ij Two d electrons (d2) 9 Three- d electrons (d3) 19 Schrodinger‟s wave equation cannot be solved Four d electrons (d4) 34 for such a complicated Hamiltonian. Therefore, Five pd electrons (d4p) 180 certain approximations are used to solve them like Central Field approximation. The best Thus, it is clear that a system starts wavefunction could be estimated through self getting complex when there are three or more consistent field method. To obey the Pauli‟s active electrons involved. A large number of exclusion principle, this wavefunction for N levels will be involved in the emission of a large electrons can be represented in the form of a number of spectral lines. The only large number determinant which satisfies the condition of of lines does not necessarily make system that anti-symmetric wavefuntion. This could be difficult to study but the interactions among the solved in parts only and these various parts are k levels make things worse. The one electron and named as Slater energy parameters (E av, F , δnl, k k even two electron systems could be spotted with G and R et) as explained under: high accuracy with isoelectronic extrapolations and the levels can be found easily. This was the reason that such systems have been studied long Para- Name Description Scaling back. However, for interacting systems these meter of the techniques fail and they could be only predicted parameter through sophisticated multi configuration E av Average represents the 100 % of interaction Hartree-Fock calculations including configur average energy of the HFR relativistic effects. The detailed background ation the configuration about the theoretical approach can be found energy or centre of some elsewhere [6,7]. Only a brief account of gravity of the methodology can be mentioned here to give an configuration idea about the approaches for complex spectra. concerned. k F Slater represents that 85% of Normally, one electron wave function is direct part of the HFR written as radial electrostatic (ml | ml| ) integral energy, which U r, , ( 1) 2 depends on the nlml 4 orientation of the ml (2l 1)(l | ml | )! ℓ vectors and is Rnl r Pl cos exp iml (l | ml | )! responsible for the separation of For N electron atoms the wave function the terms with can be constructed by taking the product of different L values wave function of its entire electron as: but same S value in LS coupling, for instance the (r1, 1, 1...... rN , N , N ) Un l ml (r1, 1, 1)...... Un l ml (rN, N, N ) 1 1 1 N N N separation of the terms like 3P, 3D

,3F etc. structure of the atoms and incorporating the Gk Slater Gk gives the 80% of some of his own suggestions to solve these exchang energies due to HFR complicated equations. Cowan‟s approach was essentially to solve the Slater parameters (Eav, e radial the exchange k k k integral forces, which F , δnl, G and R ). He ran four sets of depend on the programs as named by himself follow the spin orientations. sequence as under: They cause the splitting of terms with same L but different total spin S. δnℓ Spin δnℓ causes the 100- orbit splitting of the 110% interacti terms with same HFR on L and S values integral i.e. it is responsible for the fine structure splitting. Rk Configu Rk parameters are 80 -85% ration the configuration HFR

interacti interaction on integrals. With above-mentioned parameter‟s integral scaling, the initial calculations are performed in HFR (Relativistic Hartree-Fock) mode. If all the Ab Initio Calculations possibly interacting configurations are included in both even and odd parity systems, the In the complex spectra, the levels of one predicted energy levels should be quite close configuration is perturbing the levels of other (500- 1000 cm-1). However, this shift is very configuration provided they should belong to critical to establish. In some cases, the the same parity configurations and have same J experimentally observed levels are found below value. Thus energy of these levels shifted predicted levels and in some cases above the because of perturbation from calculated results. predicted one. To overcome this problem configuration interaction (CI) code is used. CI effect tends to The Spectrum Analysis be largest between configurations when centre of gravity energies Eav are not greatly different Once above requirements are complete, and/or for cases in which the coulomb matrix the identification of the lines may start. This is element are large in magnitude. Large values of K the most important part of the work to pick up R tend to occur particularly when the two the first right line with right ionization configurations belong to the same complex. characteristic. The first identification determines the shift from the predictions therefore; it must Dr. R.D. Cowan [1] of Loss Alamos be the right choice. Once the relative shift from Scientific Laboratory developed a computer the calculated value is known, the establishment code involving the basic theory behind the of the further energy levels rather becomes a bit

69 easier. When sufficient number of levels (more the spectrograms. Charging potential of the than 50% of the total number) usually based on source is varied as well as series inductance coil the better predicted transition probabilities are is introduced in the discharge circuit for established, least squares fitted (LSF) successive exposure of the recordings. High parametric calculations are performed ignoring inductance coil eliminates lines of higher the unknown levels. The program adjusts the ionization but the lines from lower ionization do energy parameters according to the appear on that track. This is very effective experimentally observed levels and hence method of ionization separation. calculating the unknown levels more precisely. This way more unknown levels are established. Finally these LSF parameters are used to recalculate the transition probabilities to locate 1.1 the levels based on the weaker transitions. 5s5p3 Those levels, which are permitted by electric 2 dipole selection rules to give only single 1.0 5s 5p5d transitions, are established only after the

1 2 completion of the analysis with the help of lines 0.9 G 5s 5p6s of right character which are left over in the F2 5s25p5d required region of wavelength. LSF/HFR 2 3 0.8 F 5s5p 3 2 G 5s 5p5d Running least squares fitted calculations G1 5s25p5d also require due attention to monitor the 0.7 1 variation of the energy parameters. A fit is only G 5s5p3 I IV acceptable if the scaling factor is in accordance Xe V Cs VI Ba VII as mentioned earlier, and the standard deviation fig: The scaling factors of FK,GKand parameters of 5s5p3,5s25p5d and 5s25p6s of the fit is less than 250 cm-1. A beautiful configuration of I IV to Ba VII sequence. example of the analysis based on this approach can be seen in Ba VII [8]. In complex spectra, sometime the purity of the leading components Finally a close comparison on the for some levels has been observed as low as variation of the energy parameters along the 13%. Naming a level in these circumstances is isoelectronic sequence is made as see in above meaningless. Therefore, they are designated by figure and if all looks satisfactory, the analysis their energy and J values along with their is accepted. percentage composition but in many other cases still possible designation can be assigned when References level‟s purity is more than 50%. 1. R.D. Cowan, J. Opt. Soc. Am, 58, 808 Since the entire analysis is based on the (1968). correctness of the ionization assignment, 2. C.F. Fisher “The Hartree-Fock method for extreme care has to be taken in deciding the atoms” J. Willey and sons (1977). ionization of each individual line. There are 3. I.P.Grant, B.J.Mckenzie, P.H.Norrington, many cases where single transition is used to D.F Myers and N.C. Pyqer.Comput. Phys. establish the energy level; the ionization Commun 21, 207 (1980). assignment in that case needs to be 100% 4. K.G.Dyal, I.P. Grant, C.I. Johnson, F.A. accurate. This task is achieved by varying Parpiya and E.H. Plummer. Comput Phys. experimental condition during the recording of Commun 55, 425 (1989).

5. N.C. Pyper and I.P. Grant J.Chem Soc. book Co. 2nd Edition (1960) University Faraday Trans 274, 1885 (1978). Press (1953. 6. E.U. Condon and H.G. Shortley. „The theory 8. A.Tauheed, Y.N. Joshi Phys. Scr. 47, 555 of Atomic Spectra” Cambridge (1993). 7. J.C. Slater “Quantum Theory of Atomic Structure”vol. I and vol. II; Mc Graw- Hill

Energy Perspective of Physics Abbas Ali and Shafeeq Rahman Thottoli

In this article we are giving an overview 1 volt = 1 joule/Coulomb (J/C) electron of energy prespective of different branches of charge(e) physics. We will first briefly explain the 1electron charge = 1.602176565 10 19 electromagnetic spectrum and its relevances in coulomb different areas of physical science. Then we will 1 eV = 1.602176565 10 19 joule describe energies involved in thermal physics, solid state physics, spectroscopy, nuclear Micro waves are in the wave length physics, particle physics and string theory. range from mm to meter. In terms of energy the There is a nice linear organization of these corresponding range is from μeV (micro different fields in increasing order of energy. electron volts)to meV. These two are used for communication. Introduction Infrared radiation has the wavelength All of us are familiar with the range microns to mm. The corresponding electromagnetic spectrum. In the increasing energy range is meV to eV. These energies are order of energy and decreasing order of relevant for thermal phenomenum. Thus these wavelength it is organised as follows: radiations are called thermal radiation also. Energies involved in electronics related physics Radio Waves as well as condensed matter physics also lie Micro Waves around this range. Infrared Radiation Visible Light Visible spectrum of light extends from Ultraviolet Radiation 4500A0 to 7500A0 . Corresponding energy X – Rays range is few electron volts. This is the range of Gamma Rays wavelengths are relevant for optics and spectroscopic studies. The energies corresponding to these parts of electromagnetic spectum are relevant Ultraviolet spectrum extends from for different braches of physics. For example 100A0 to 4000 A0. Corresponding energy range radio waves are used for communication and spreads from a few electron vols to 124eV. This hence that is the most relevant energy scale for range of wavelengths are relevant for communication. The radio waves range in spectroscopic studies. wavelength is taken to be from 1mm to km. This corresponds to the energy range that X- ray spectrum has the wavelength spreads from feV(femto electron volts ) to meV range 0.1 A0 to 100A0. Corresponding energy (milli electron volts). range is eV to keV. These energies are relevant in spectroscopy and nuclear physics. Here we are using eV as a unit of energy . One eV is the amount of energy gained by the Gamma ray spectrum has the charge of a single electron moved across an wavenlength less than 0.1 A0. The electric potential difference of one volt. corresponding enegy range is keV to MeV and are relevant in nuclear physics.

Energies beginning with these and The temperature can also be expressed in the beyond are relevant for particle physics and eV. hence particle physics is also called high energy One degree Kelvin = 8.6173324 10 5 eV physics. I eV =11604. 505 oK

The actual energy figures for above Some thermodynamical energies are listed mentioned branches of physics may not be the below. same as mentioned above but there is a reason for that. Accumulation of energy may take place States Temperature In unit of because of large number of particles involved or in Kelvin(K) eV some energy figures might be smaller than Triple point 13.81 1.19 10 3 above mentioned because of energy attenuation. of hydrogen Inspite of this the most relevant energies are the Boiling 27.102 2.34 10 3 ones that have been enumerated in the point of beginning of the section. neon 3 Triple point 54.361 4.68 10 In the following sections we summarise of oxygen actual energies involved in various branches Triple point 273.16 2.35 10 2 of physics. These energies are spread out around of water above energies. It will be nice excersice to Boiling 373.15 3.22 10 2 analyse the physical mechanisms giving rise to point of this spread. water

Freezing 692.73 5.97 10 2 Thermal physics point of Zinc

Freezing 1235.08 0.11 Thermal physics is essentially the study point of of heat, temperature and heat transfer. Thermal silver energy is due to chaotic molecular motion. Freezing 1337.58 0.12 There are three factors affecting thermal energy point of gold the temperature ,sample size and composition.

Anything that changes temperature, sample size Specific heat is the heat required to raise or composition of an object can change its the temperature of 1 gram of material by 1 thermal energy. In all most every energy degree Kelvin . Different materials have transformation some thermal energy is produced different specific heats. Following table have in the form of heat. Heat is a transfer of thermal enlisted specific heats of some common energy due to temperature difference. thermal elements. energy is not measurable but heat is measurable.

Material(at Specific in eV Unit The Boltzmann constant is the physical constant relating energy at the individual 298K, 1atm) heat(J/g K) 4 particle level with temperature. Ice 2.09 1.80 10 Water 4.11 3.54 10 4 Value of Boltzmann constant Sodium 1.86 1.60 10 4 23 5 kB =1.3806488 10 J/K Aluminium 0.9 7.76 10 Iron 0.45 3.88 10 5

The theorem of equipartition of energy very small band gaps or none, because the states that molecules in thermal equilibrium valence and conduction bands overlap. The have the same average energy associated with following list shows band gap of some common each independent degree of freedom of their semiconductors. motion and that the energy is 3/2 kBT per molecule or 3/2 RT per mole for three degrees Material Band of freedom. Where kB is the Boltzmann gap(in eV) constant, T is the temperature and R is the Silicon(Si) 1.11 universal gas constant. The average translation Selenium(Se) 1.74 energy possessed by free particle given by Germanium(Ge) 0.67 equipartition of energy is sometimes called the Silicon carbide(SiC) 2.86 thermal energy per particle. Aluminium phosphide(AlP) 2.45 Aluminium arsenide(AlAs) 2.16 The average kinetic energy of a gas Diamond(C) 5.5 molecule at room temperature is 0.026eV. Gallium(III) arsenide(GaAs) 1.43 Zinc oxide(ZnO) 3.37 Solid State Physics and Electronics Zinc sulfide(ZnS) 3.6

We can see the energy scale involved in Spectroscopy solid stated physics and electronics. There are different types of bonds. The bonds and their Spectroscopy is the study of the corresponding energies are listed below. interaction between matter and radiated energy. Spectroscopy is a very spacious field and is Bond Energy in eV divided according to nature of material( e.g. Van Der Waal bonds 0.044 atom, molecule, etc) and that emit or absorb the Hydrogen bonds 0.2 spectrum. The both spectroscopy have their own Ionic bonds 0.17-0.3 characteristics and limits. Atomic spectrum is a Single covalent bond 2-5 spectrum of radiation caused by electron transitions within an atom. These radiations lie Every solid has its own characteristic from radio to X-ray. The molecule emits the energy band structure. This variation in band radiation basically in three ways, due to structure is responsible for the wide range of transitions from electronic, vibrational, rotation electric characteristics observed in various levels.The different regions of spectra are listed materials, the band gap generally refers to the in table below. energy difference (in electron volts) between the top of the valence band and the bottom of the We are analyzing some spectroscopic conduction band in insulators and process and we can see the energy range which semiconductors. This is equivalent to the energy is involved. The ionization energy of an atom or required to free an outer shell electron from its molecule, is the energy required to remove orbit about the nucleus to become a mobile electrons from gaseous atoms or ions. The first charge carrier, able to move freely within the ionization energy of some metals are listed solid material. So the band gap is a major factor below. determining electric properties of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors, while conductors either have

Region of Frequency Energy range beta and gamma rays. These are helium nuclei spectra involved in electron and photon respectively. Nuclear eV transitions can emit gamma-rays with quantum energies in the MeV range. The binding energy Infra-red 3 1012 1.24 10 2 region is responsible for the energy produced in 3 1014 1.24 nuclear fission and fusion reaction. Some of the Visible 4.23 1014 1.74 binding energies per nucleon for some common region elements are shown in the following table. 7.5 1014 3.10

Ultraviolet 14 3.10 7.5 10 Element Binding region 2 3 1016 1.2 10 Energy(MeV) X-ray 1017 4.12 10 2 Deuterium 1.12 region 19 4 Helium 4 7.07 10 4.14 10 Lithium 7 5.74 Beryllium 9 6.46 Element First Iron 56 8.79 Ionization Silver 107 1 8.55 energy Iodine 127 8.45 (in eV) Lead 206 7.88

Hydrogen(H) 13.6 Polonium 210 7.83

Lithium(Li) 5.39 Uranium 235 7.59

Berillum 9.32 Uranium 238 7.57

Boron(B) 8.30

Carbon(C) 11.3 The gamma rays involved in the decay Oxygen(O) 13.6 of Cobalt-60 is of the energy 1.17MeV and Flurin(F) 17.4 1.33MeV and in Sodium-24 decay involves the Neon (Ne) 21.6 energies 1.368Mev and 2.753MeV Helium (He) 24.6 Particle Physics Nuclear Physics As we have seen that another name for Nuclear physics is the field of physics particle physics is high energy physics. The that studies the building blocks and interactions typical energies that are relevant for particle 9 of atomic nucleus. An important property of the physics are of the order of GeV(10 eV). High nucleus is that the mass of an atom's nucleus is energy physics deals basically with the study of always less than the sum of the individual the ultimate constituents of matter and the masses of the constituent protons and neutrons nature of interaction between them. The when separated, this is called the mass defect experimental studies in this field of science are and the amount of energy that this mass is carried out with giant particle accelerators. High converted into is called the binding energy.This energies are necessary for two reason first is that is the energy required to split a nucleus of an we are probing at very small scales of distances atom into its component parts. The binding and one requires radiation of smallest possible energies of nucleons are in the range of millions wavelength and highest possible energy. of electron volts(MeV). Three types of Secondly many of the fundamental constituents radioactive emission are identified as alpha , have larger masses and require correspondingly high energies for their creation.

The major accelerator facilities make use kinetic energies and letting them impact other of several types of devices to build up the particles. energy of the particles. Some of the types of apparatus used and maximum available energy are listed below. Accelerator Total Energy Stanford Linear 100 GeV Type Maximum Accelerator Collider achieved energy (SLAC) in Palo Alto Cockroft-Walton proton upto 2MeV Large Electron Positron 200 GeV Accelerators Collider(CERN-LEP)in van de Graff protons upto 10 Geneva Accelerators MeV Relativistic Heavy Ion 200 GeV Tandem van de Graaf protons upto Collider(BNL-RHIC)in Accelerators 20MeV Brookhaven cyclotron protons up to 25 Tevatron (FNAL) in 2 TeV MeV Chicago electron linac 100 MeV to 50 Large-Hadron 14 TeV GeV Collider(CERN-LHC) proton linac up to 70 MeV in Geneva Synchrocyclotron 700MeV Betatron Few hundred MeV We are curious about to know what is Linear accelerators 50GeV the world made of? Ordinary matter is made of synchrocyclotron protons up to 750 atoms, which are in turn made of just three basic MeV components electrons whirling around a nucleus proton synchrotron protons up to 900 composed of neutrons and protons. The electron GeV is a truly fundamental particle, but neutrons and electron synchrotron electrons from 50 protons are made of smaller particles, known as MeV to 90 GeV quarks. Quarks are, as far as we know, truly elementary. Particle with energy about 1 GeV (109 eV) is required to probe the structure inside Our current knowledge about the proton. Higher energy is required for smaller subatomic composition of the universe is system - about 1000 GeV is needed to probe summarized in what is known as the Standard into the quarks. The same amount of energy is Model of particle physics. It describes both the required to create many of the hypothetical fundamental building blocks out of which the particles. Below summarizes some features of world is made, and the forces through which the major accelerators in the world (all of them these blocks interact.There are twelve basic are colliders). A collider is a type of a particle building blocks. Six of these are quarks(up, accelerator involving directed beams of down, charm, strange, bottom and top). The particles. Colliders may either be ring other six are leptons,these include the electron, accelerators or linear accelerators, and may muon and the tauon, as well as three neutrinos. collide a single beam of particles against a stationary target or two beams head-on.Colliders There are four fundamental forces in the are used as a research tool in particle physics by universe: gravity, electromagnetism, and the accelerating elementary particles to very high weak and strong nuclear forces. Each of these is

76 produced by fundamental particles that act as candidate for a microscopic theory of gravity. It carriers of the force. The most familiar of these attempts to provide a complete, unified, and is the photon, a particle of light, which is the consistent description of the fundamental mediator of electromagnetic forces.The graviton structure of our universe('Theory of is the particle associated with gravity. The Everything'). The quantum effect of gravity strong force is carried by eight particles known become strong at planck energy scale which is as gluons. Finally, the weak force is transmitted an energy scale around 1019GeV . by three particles, the W ,W , and the Z 0 . We have discussed in this article about What is String Theory energy prespectives of different areas of physics. We have seen that typical energy The behavior of all of the above involved in thermal and solid states physics is mentioned particles and forces is described upto a few electron volts and that in with impeccable precision by the Standard spectroscopy reaches upto keV. The energy Model, with one notable exception gravity. For range relevant for nuclear physics is kev to technical reasons, the gravitational force, the MeV. Typical energy of particle physics is in most familiar in our every day lives, has proven Gev range whereas the enegy relevant for string very difficult to describe microscopically. This theory is 1019 GeV. Thus various branches of is one of the most important problems in physics are organized linearly with increasing theoretical physics to formulate a quantum energy. theory of gravity. In the last few decades, string theory has emerged as the most promising