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Neutron Physics NEUTRON PHYSICS Prof. J. K. Goswamy UIET, Panjab University Chandigarh OVERVIEW Road to Discovery of Neutron. Neutron Sources. Passage of Neutrons through Matter. Detection of Neutrons. Neutron Activation Analysis. ROAD TO NEUTRON DISCOVERY The Nucleus: Discovery . In 1898, J.J. Thomson proposed THOMSON MODEL that the atom is basically a spherical cloud of positively charged matter with electrons embedded in it like the seeds in a watermelon. This was a static model of atom with intrinsic electrostatic instability. It failed to explain the energy levels of the atom. Rutherford Experiments . Rutherford, a former student of Thomson, performed experiments with the scattering of alpha- particles from thin metal foils. The scattered alpha-particles were detected through tiny light flashes produced by them on ZnS screen. Most of the alpha-particles travelled without deflection through the foil. Small fraction suffered deflection through a large angle (upto 90o). Very few alpha particles were deflected back. Angular Distribution of Scattered -particles . Most of the α-particles pass through foil with deflection less than 8o indicating that atom predominantly has empty space. The large angle deflections (~90o) suffered by small fractions of α-particles, indicated that the positive charge in atom was concentrated in a very small volume at the centre of atom. The very small fraction of backscattered α- particles was possible as the central core accounted for nearly whole mass of atom. From angular distribution of scattered - particles Rutherford concluded existence of positively charged core of atom then called nucleus. The size of the nucleus was much smaller (10- Angular distribution of alpha-particles from gold foils. 14m) than size of the atom (10-10m) . Electron-Proton Theory After the Rutherford model of atom, the nucleus was postulated to be constituted by electrons and protons. Nucleus (A, Z) = A Protons + (A-Z) Electrons Drawbacks Ground state spin of most of the nuclei could not be reproduced. Magnetic moments of nuclei were predicted to be much higher than the observed values. Discovery of Neutron The process of discovery of neutrons started with the capture of α-particles by 9Be and the reaction was supposed to be 9Be(α,γ)13C. Bothe and Becker, through absorption of gamma-rays in lead, estimated the photon energy to be 7MeV. Later Curie and Joliot showed that emitted gamma- rays could knock out protons from paraffin and other hydrogenous materials. They estimated the energy of the emitted photon to be 55MeV. Chadwick performed a series of experiments to study recoil energy of different nuclei stuck by gamma rays emitted in this reaction. It was concluded that photon energy depended on nuclei which recoiled due to photon impact. This was surprising and not acceptable. Chadwick removed this anomaly by the hypothesis that emitted particle in this reaction is actually not gamma ray but an electrically neutral particle with mass nearly same as of proton. This discovered particle was called neutron. Neutron Proton Theory o Rutherford model suggests that the atomic mass is nearly equal to the mass of the nucleus, which contains +ve charged particles called protons. o The number of protons is equal to the number of electrons, often called atomic number Z of atom. o For light nuclei, the atomic mass is Mass of neutron (1.6748 x 10-27kg) is slightly more than proton. approximately twice the mass of protons and Neutron is uncharged but has an this ratio is more in case of heavier nuclei. internal structure. o This discrepancy was resolved in 1932 by Spin of neutron is h/4π James Chadwick who discovered neutron of Due to internal structure and spin, its mass nearly equal to that of proton. magnetic moment of -1.91µN. o A nucleus is made up of protons and neutrons: Free neutron undergoes β-decay with a half life of 12.5 minutes as A = N + Z n p SOURCES OF NEUTRONS Neutron Sources Pure isotopic sources of neutrons do not exist as no radioactive decay process causes emission of neutrons. Neutron Sources Fission Sources Isotopic (,n) Sources Photo neutron Sources Other Sources Spontaneous Fission Sources Many trans-uranium nuclei have high spontaneous fission probability. The products of spontaneous fission process are: Heavy fission products. β- and γ-activities of fission products. Prompt fast neutrons. These sources are usually encapsulated in a sufficiently thick container so that only fast neutrons and gamma rays escape from the source. 252Cf Half life = 2.65 years. Modes of decay: >90% α-decay and <10% spontaneous fission. Neutron yield = 0.116 neutrons/second per Becquerel of activity. Intense yield of 2.3 million neutrons/second per microgram of the sample. Energy Distribution: 0.5 -10 MeV. Spectrum of Neutrons Type of Neutrons Energy Range Thermal Neutrons 0.025 eV-0.5 eV Epithermal Neutrons 0.5 eV-100 keV Fast Neutrons 100 keV-25 MeV Radio-Isotope (α, n) Sources • These are small self-contained neutron sources obtained by mixing Source Half life Eα Yield an α-emitting source with Be like elements. • Usually the actinide elements are α- 239Pu/9Be 24000y 5.14MeV 65 npm emitters and form stable alloy with beryllium. Sources are prepared 241Am/9Be 433y 5.48MeV 82 npm through metallurgical process. • The α-particles, emitted by actinide, 238Pu/9Be 87.4y 5.48MeV 79 npm interact with Be nuclei within alloy without much loss of energy. Photo-Neutron Sources . Some radio-isotopes, which are γ-ray emitters, produce neutrons when Aluminum Encapsulation combined with appropriate target material. The gamma-rays produced in a radioactive decay, are absorbed by the emitter target nucleus thereby getting excited sufficiently to emit neutron. Two commonly used reactions for Neutron Emitting target producing photo-neutrons are: 9 8 Be(γ, n) Be Eγ>1.666MeV 2 1 H(γ, n) H Eγ>2.226MeV . Relatively mono-energetic neutrons are emitted. Accelerator Based Neutron Sources Deutron Induced Reactions are source of neutrons 2He(2He, n)3He 2H(3H, n)4He These reactions are possible through artificially accelerated particles. As coulomb barrier of light target nuclei for incident deutrons is low so it can be overcome through small acceleration. Charged particle Induced reactions yielding neutrons are 9Be(p,n) 7Li(p,n) 3H(p,n) Neutron Generators 3H(d,n)4He Deutrons are accelerated to 200 kV 14MeV neutrons in reactions: (n,p), (n,α), (n,2n)) Neutron yields: 1011/s/mA, Neutron flux: 109/cm2/s Research Reactors Thermal power: 100 kW-10 MW Thermal neutron flux: 1012-1014 n/cm2 s INTERACTION OF NEUTRONS Interaction of Neutrons Neutrons are uncharged particles and Neutrons interact with the nuclei of the can travel large distance without absorber atoms in which they may (a) interacting with absorber’s atoms. Disappear resulting in production of secondary radiations or (b) their energy Neutrons or direction is changed significantly. Slow Fast Neutrons Neutrons Elastic scattering resulting in Elastic scattering causing moderation of neutron energy. recoil of secondary radiations. Cause (n,p), (n,r) reactions. Inelastic scattering causing excitation of absorber nuclei. Neutron Flux Attenuation If a neutron beam passes through a slab of material, it suffers attenuation through scattering as well as absorption by the material nuclei. sc ab Absorption of Neutrons o Direct Nuclear Reaction: Neutrons interact with matter via direct nuclear reaction. The probability of reaction process depends upon the energy of neutrons and the nature of target nuclei. o Compound Nuclear Reaction: Fast neutrons get captured to form a compound nucleus which has excitation energy equal to the sum of neutron’s kinetic and binding energy of nucleus. This energy is subsequently released in the form of reaction products, gamma-rays and neutrons. Scattering of Neutrons o Secondary Radiation Production: Neutron may get scattered and portion of its energy is transferred to the recoiling nucleus. o Moderation: Slow neutrons suffer multiple scattering to slow down to thermal energies often called moderation. DETECTION OF NEUTRONS Principle of Neutron Detection A neutron detector does not record the presence of neutron directly but responds through secondary radiation (charged particles or gamma rays) which are emitted due to neutron induced nuclear reaction in the detector medium. For slow and thermal neutrons commonly employed reactions on light nuclei are (n, p) (n, α) (n, fission) For fast neutrons of several MeV energy, the scattering off a light target nuclei can give enough energy to the recoiling nucleus for detection as secondary radiation. Slow Neutron Detectors Boron Fluoride Proportional Counter 10 The isotope B is commonly used in the form of BF3 gas inside a proportional counter. This gas serves both as Target for nuclear reaction and Counter fill gas. The neutron causes the reaction 10B(n,α)7Li. The outgoing particle and recoiling nucleus cause ionizations in the detector gas. These ionization serve as a signal for neutron detection. Count rates are proportional to neutron density at the detector. 3He Proportional Counter 3He acts are target as well as counter fill gas. This utilizes the reaction 3He(n,p)3H. Reaction cross-section is high but energy of outgoing particles is low. Fission Counters The fission cross-sections of 233U, 235U and 239Pu are relatively large at low neutron energies and thus these materials can be used. The detectors using these materials yield much larger output pulse amplitude than any other detector used for slow neutrons. These detectors are mostly in the form of ionization chamber with its inner surface coated with fissile material. Self Powered Detectors In these detectors, materials having high cross-section for neutron capture are used which subsequently emit β- or γ-rays. The β-decay current following neutron capture determines the neutron flux. Fast Neutron Detectors .
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