Detectors for X-Ray Astronomy 48 Proportional Counters
Detectors for X-ray astronomy 48 Proportional Counters
Workhorses of X-ray astronomy for >10 years 1962-1970: Rockets and Balloons 1962 Sco X-1 and diffuse X-ray sky background discovered by Giacconi sounding rocket Limited by atmosphere (balloons) and duration (rockets) • 1970 → Satellite era Uhuru: First dedicated X-ray Satellite e.g. Ariel V, EXOSAT e.g. Ginga e.g. XTE e.g. ROSAT
49 Adapted from Hill, Urbino 08 Proportional counters
Simplest proportional counter is made of a gas chamber with an anode to collect charges
"Proportional counter avalanches" by Dougsim - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons Gas Detectors (Ar, Xe) Incident X-ray interacts with a gas atom and a photoelectron is ejected Photoelectron travels through the gas making an ionisation trail Trail drifts in low electric field to high E-field In high E-field multiplication occurs (Townsend avalanche) Charge detected on an anode
50 Adapted from Hill, Urbino 08 One can add 2nd chamber to discriminate between X-ray photons and cosmic rays
51 Gain of 105 or more Duration about 1 ns → short pulse!
52 Quenching molecules
The positive atom moves slowly to the cathode wall of the chamber. They are neutralized by gaining an electron
This causes an energy release which can further ionize the gas, creating a charge pulse!
To eliminate (or quench) this signal, one uses a 10-15% proportion of organic gas (e.g., methane), which has low electron affinity
Thus, the positive ions will take electrons from the quenching gas molecules, and only the ionized molecules will reach the cathode
The recombination of the ionized molecules does liberate some energy, but insufficient to ionize the gas, but it breaks the molecules
Such proportional counters have, thus, a limited time life, unless they are constantly replenished by quenching molecules (gas flow counters)
53 Escape peak
If the incident X-ray photon has higher energy, E0, than the K-shell
fluorescence energy of gas, Ef, an escape peak can appear at E’=E0-Ef Fluorescence X-ray photon can escape the counter, leaving a smaller energy to be measured
Ar-filled PC NB: Ef(Ne)=0.85 keV No No escape peak as escape the fluorescence peak, E (Ar)=3 keV energy is almost E 54 Typical Characteristics 0.4 Townsend Avalanche Energy Resolution is limited by: The statistical generation of the charge by the photoelectron By the multiplication process Quantum Efficiency: Low E defined by window type and thickness High E defined by gas type and pressure 55 Adapted from Hill, Urbino 08 Typical Characteristics Position sensitivity Non-imaging case: Sensitivity Area Limited by source confusion to 1/1000 Crab Imaging case: track length, diffusion, detector depth, readout elements Timing Resolution Limited by the anode-cathode spacing and the ion mobility: ~ µsec Timing variations: Sensitivity Area High Gain: 103-105 56 Adapted from Hill, Urbino 08 Background origin Charged particles Cosmic rays, sub-relativistic electrons, particles from interactions with detector/telescope Photons Forward Compton scattering of gamma rays in the gas can deposit 0.1-10 keV X-rays and gamma-rays created by cosmic rays Fluorescent X-rays created in structure Background is generally flat 57 Background rejection techniques Energy Selection Reject events with E outside of band pass Rise-time discrimination Rise time of an X-ray event can be characterised. The rise-time of a charged particle interactions have a different characteristic. • Anti-coincidence Use a sub-divided gas cell with a shield of plastic scintillator Co-incident pulses indicate extended source of ionisation 58 Adapted from Hill, Urbino 08 Capella ROSAT PSPC ( ~ 1-2) 59 Micro-channel plates 60 Gain is very high 106-108 CGCD=crossed grid charge detector Chandra HRC Micro-channel plate 61 Fine position determination 62 Low63 QE and no energy response X-ray CCDs 1977 – ASCA XMM Swift XRT CCD Chandra Swift Suzaku 64 Adapted from Hill, Urbino 08 CCDs Charge Coupled Devices invented in the 1970s Sensitive to light from optical to X-rays In practice, best use in optical and X-rays CCDs make use of silicon chips The CCD consists of (1) a p-type doped silicon substrate, (2) the charge storage (depletion) layer, which is covered by (3) a SiO2 insulating layer; upon this is (4) an array of closely spaced electrodes, which can be set to pre-defined voltage value 65 Si array Si array n-type Also Sb,P Si array p-type Also Al,Ga 66 Reminder of solid state physics Electrons in a lattice do not have discrete energies. They form energy bands: Valence band Conduction band For semi-conductors, the Fermi level is just in the middle of the conduction and valence bands. At finite temperature, some electrons of p h n the valence band can jump into the conduction o o t t o o n h band (current noise) p EG(Si)=1.1 eV (IR), EG(Ge)=0.72 eV E (C)=5.5 eV (insulator) G Hole Electron 67 (From http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html) The pn junction 68 Reverse-biased pn junction Forward-biased pn junction 69 4.1 eV 2.5 eV 1.8 eV 1.2 eV Transverse cut of CCD with buried channel Blecha (cours instrumentation) 70 The electrode has positive potential to attract the generated photoelectrons in a potential well The above MOS capacitor is 1 pixel 71 Front-illuminated CCDs Back-illuminated CCDs s s n n o o t t o o h h p p g g n n i i m m o o c c n n I I Anti-reflective coating n p 15m n 625m p They have a low Quantum The QE can approach 100%. These Efficiency due to the reflection thinned CCDs become transparent to and absorption of light in the near infra-red light and the red response surface electrodes. Very poor blue is poor. Response can be boosted by response. The electrode structure the application of an anti-reflective prevents the use of an anti- coating on the thinned rear-side. These reflective coating that would coatings do not work so well for front- otherwise boost performance. illuminated CCDs due to the surface bumps created by the surface electrodes 72 Courtesy of S. Tulloch Quantum Efficiency Comparison The graph below compares the quantum of efficiency of a thick frontside illuminated CCD and a thin backside illuminated CCD. 73 Courtesy of S. Tulloch Back-illuminated CCDs BI CCD FI CCD Thinner dead layers higher low-E QE Thinner active region lower high-E QE Increased noise, charge transfer inefficiency higher FWHM From C. Grant, X-ray Astronomy School 2007 Structure of a CCD The diagram shows a small section (a few pixels) of the image area of a CCD. This pattern is repeated. Channel stops to define the columns of the image Plan View Transparent horizontal electrodes to define the pixels One pixel vertically. Also used to transfer the charge during readout Electrode Insulating oxide n-type silicon Cross section p-type silicon Every third electrode is connected together. Bus wires running down the edge of the chip make the connection. The channel stops are formed from high concentrations of Boron in the silicon. 75 Courtesy of S. Tulloch CCD Analogy VERTICAL RAIN (PHOTONS) CONVEYOR BELTS (CCD COLUMNS) BUCKETS (PIXELS) MEASURING CYLINDER HORIZONTAL (OUTPUT CONVEYOR BELT AMPLIFIER) (SERIAL REGISTER) 76 Courtesy of S. Tulloch Exposure finished, buckets now contain samples of rain. 77 Courtesy of S. Tulloch Conveyor belt starts turning and transfers buckets. Rain collected on the vertical conveyor is tipped into buckets on the horizontal conveyor. 78 Courtesy of S. Tulloch Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn into the measuring cylinder . 79 Courtesy of S. Tulloch After each bucket has been measured, the measuring cylinder is emptied , ready for the next bucket load. ` 80 Courtesy of S. Tulloch 81 Courtesy of S. Tulloch 82 Courtesy of S. Tulloch 83 Courtesy of S. Tulloch 84 Courtesy of S. Tulloch 85 Courtesy of S. Tulloch 86 Courtesy of S. Tulloch A new set of empty buckets is set up on the horizontal conveyor and the process is repeated. 87 Courtesy of S. Tulloch 88 Courtesy of S. Tulloch 89 Courtesy of S. Tulloch 90 Courtesy of S. Tulloch 91 Courtesy of S. Tulloch 92 Courtesy of S. Tulloch 93 Courtesy of S. Tulloch 94 Courtesy of S. Tulloch 95 Courtesy of S. Tulloch 96 Courtesy of S. Tulloch 97 Courtesy of S. Tulloch 98 Courtesy of S. Tulloch 99 Courtesy of S. Tulloch 100 Courtesy of S. Tulloch 101 Courtesy of S. Tulloch 102 Courtesy of S. Tulloch 103 Courtesy of S. Tulloch 104 Courtesy of S. Tulloch Eventually all the buckets have been measured, the CCD has been read out. 105 Courtesy of S. Tulloch Charge Collection in a CCD Photons entering the CCD create electron-hole pairs. The electrons are then attracted towards the most positive potential in the device where they create ‘charge packets’. Each packet corresponds to one pixel y y r g r a s n a i d n d l o n l m n e t u e o u x o i o x c o i h p b n p b i p n-type silicon Electrode Structure Charge packet p-type silicon SiO2 Insulating layer 106 Courtesy of S. Tulloch Charge Transfer in a CCD 1. In the following few slides, the implementation of the ‘conveyor belts’ as actual electronic structures is explained. The charge is moved along these conveyor belts by modulating the voltages on the electrodes positioned on the surface of the CCD. In the following illustrations, electrodes colour coded red are held at a positive potential, those coloured black are held at a negative potential. 1 2 3 107 Courtesy of S. Tulloch Charge Transfer in a CCD 2. +5V 2 0V -5V +5V 1 0V -5V +5V 3 0V -5V 1 2 3 Time-slice shown in diagram 108 Courtesy of S. Tulloch Charge Transfer in a CCD 3. +5V 2 0V -5V +5V 1 0V -5V +5V 3 0V -5V 1 2 3 109 Courtesy of S. Tulloch Charge Transfer in a CCD 4. +5V 2 0V -5V +5V 1 0V -5V +5V 3 0V -5V 1 2 3 110 Courtesy of S. Tulloch Charge Transfer in a CCD 5. +5V 2 0V -5V +5V 1 0V -5V +5V 3 0V -5V 1 2 3 111 Courtesy of S. Tulloch Charge Transfer in a CCD 6. +5V 2 0V -5V +5V 1 0V -5V +5V 3 0V -5V 1 2 3 112 Courtesy of S. Tulloch Charge Transfer in a CCD 7. +5V 2 0V -5V Charge packet from subsequent pixel enters from left as first pixel exits to the right. +5V 1 0V -5V +5V 3 0V -5V 1 2 3 113 Courtesy of S. Tulloch Charge Transfer in a CCD 8. +5V 2 0V -5V +5V 1 0V -5V +5V 3 0V -5V 1 2 3 114 Courtesy of S. Tulloch Photoelectric Absorption in Silicon Depth of active region in typical X-ray CCD Best X-ray sensitivity From C. Grant, X-ray Astronomy School 2007 115 Photoelectric Absorption Photoelectric interaction of X-ray with Si atoms generates electron-hole pairs. On average: Ne = Ex/w Ne = number of electrons Ex = energy of X-ray photon w ~ 3.7 eV/e- (temperature dependent) X-ray creates a charge cloud which can diffuse and/or move under influence of an electric field 116From C. Grant, X-ray Astronomy School 2007 From C. Grant, X-ray Astronomy School 2007 Event Processing CCD output rate (~10 Mbits/sec/CCD) exceeds telemetry resources Raw CCD frames must be processed on-board to find prospective X-ray events Event selection: CCD bias level determined and removed Pixel pulse height greater than threshold value (“event threshold”) Pixel is a local maximum (3 x 3 pixels on ACIS) For each event, record position, time and pulse heights of event island (3 x 3 pixels for ACIS) Events are assigned a grade which characterizes the shape of the event. 118From C. Grant, X-ray Astronomy School 2007 Grading Events Event grade can be used to ✓ discriminate between X-ray and cosmic ray events In general, X-ray events split ✓ into simpler/smaller shapes (single, singly-split) ✓ ✓ Cosmic ray events are more ✓ complex ASCA code grades are one example On-board grade filtering can ✓ further reduce telemetry Grade 7 - everything else Grade filtering can improve spectral resolution - split events are noisier than singles From C. Grant, X-ray Astronomy School 2007 ACIS X-ray/Particle Discrimination Blobs/streaks - charged particles. Small dots - X-ray events. From C. Grant, X-ray Astronomy School 2007 CCD Quantum Efficiency Transmission through dead layers (channel stops, gates, oxide layers) -i ti T = i e Absorption in depleted region A = 1 - e- Sid - d - t QE = (1 - Si ) i i is linear absorption coefficient e i e t thickness of dead layer d depletion depth From121 C. Grant, X-ray Astronomy School 2007 Optical Blocking Filters Imager Filter: Al/Polyimide/Al:120/200/40 nm Spectrometer Filter: Al/Polyimide/Al:100/200/300 nm CCDs sensitive to optical photons Cause noise and pulse-height calibration issues Filter materials usually plastic and aluminum From122 C. Grant, X-ray Astronomy School 2007 Filter Transmission CCD+flter CCD only CCD+filter Bare CCD At low energies (< 0.5 keV), > 50% reduction in efficiency 123From C. Grant, X-ray Astronomy School 2007 CCD X-ray Spectroscopy: The Basic Idea Photoelectric interaction of a single X-ray photon with a Si atom produces “free” electrons: Ne E X w (w 3.7 eV/e ) 2 e F Ne (F 0.12; not a Poisson process) Spectral resolution depends on CCD readout noise and physics of secondary ionization: 2 2 FWHM (eV) 2.35 w e read CCD characteristics that maximize spectral resolution: Good charge collection and transfer efficiencies at very low signal levels Low readout and dark-current noise (low operating temperature) High readout rate (requires trade-off vs. noise) 124From C. Grant, X-ray Astronomy School 2007 ASCA SIS (1993): 4 e- RMS Chandra ACIS (1999): 2 e- RMS From C. Grant, X-ray Astronomy School 2007 Spectral Redistribution Function Input spectrum has three spectral lines: Mn-L (0.7 keV), Mn-K (5.9 keV), and Mn-K (6.4 keV) Instrument produces Si-K fluorescence and escape peaks, low-energy features Off nominal features ~2% of total 126From C. Grant, X-ray Astronomy School 2007 (Prigozhin et al., 1999, Nuclear Instruments and Methods) From C. Grant, X-ray Astronomy School 2007 Low-Energy Detection Efficiency Many astrophysically interesting problems require good low-energy (< 1 keV) efficiency (pulsars, ISM absorption, SNR, …) Low energy X-rays are lost to absorption in gate structures and filter Solutions: Thinned gates, open gates (XMM EPIC-MOS, Swift) Back-illumination (Chandra ACIS, XMM EPIC-PN, Suzaku XIS) From128 C. Grant, X-ray Astronomy School 2007 Chandra ACIS Focal Plane The Advanced CCD Imaging Spectrometer (ACIS) contains 10 planar, 1024 x 1024 pixel CCDs ; four arranged in a 2x2 array (ACIS-I ) used for imaging, and six arranged in a 1x6 array (ACIS-S ) used either for imaging or as a grating readout. Two CCDs are back-illuminated (BI ) and eight are front- illuminated (FI ). From C. Grant, X-ray Astronomy School 2007 XMM-Newton EPIC-MOS The MOS EEV CCD22 is a three-phase frame transfer device on high resistivity epitaxial silicon with an open-electrode structure; it has a useful quantum efficiency in the energy range 0.2 to 10 keV. The low energy response of the conventional front illuminated CCD is poor below ~700 eV because of absorption in the electrode structure. For EPIC MOS, one of the three electrodes has been enlarged to occupy a greater fraction of each pixel, and holes have been etched through this enlarged electrode to the gate oxide. This gives an "open" fraction of the total pixel area of 40%; this region has a high transmission for very soft X- rays that would have otherwise be absorbed in the electrodes. In the etched areas, the surface potential is pinned to the substrate potential by means of "pinning implant". High energy efficiency is defined by the resistivity of the epitaxial silicon (around 400 Ohm-cm). The epitaxial layer is 80 microns thick (p-type). The actual mean depletion of the flight CCDs is between 35 to 40 microns: the open phase region is not fully depleted. Image and caption taken from http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/technical/EPIC 130From C. Grant, X-ray Astronomy School 2007 The field of view of the EPIC MOS 1 cameras: The camera detector co-ordinate frames are noted. 7 CCD’s each 10.9 x 10.9 arcmin 131 XMM-Newton EPIC-PN The PN-CCDs are back-illuminated. In the event of an X-ray interaction with the silicon atoms, electrons and holes are generated in numbers proportional to the energy of the incident photon. The average energy required to form an electron-hole pair is 3.7 eV at -90° C. The strong electric fields in the pn-CCD detector separate the electrons and holes before they recombine. Signal charges (in our case electrons), are drifted to the potential minimum and stored under the transfer registers. The positively charged holes move to the negatively biased back side, where they are 'absorbed'. The electrons, captured in the potential wells 10 microns below the surface can be transferred towards the readout nodes upon command, conserving the local charge distribution patterns from the ionization process. Each CCD line is terminated by a readout amplifier. The picture shows the twelve chips mounted and the connections to the integrated preamplifiers. Image and caption from http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/technical/EPIC/index.shtml#2.2 132From C. Grant, X-ray Astronomy School 2007 CCD’s Onboard XMM-Newton The field of view of the EPIC pn camera; The EPIC prime boresight is marked with a small box. 12 CCD’s each 13.6 x 4.4 arcmin. 133 Capella ROSAT PSPC ( ~ 1-2) 134 Capella ASCA SIS0 ( ~ 10-20) 135 Photon Pileup If two or more photons interact within a few pixels of each other before the image is readout, the event finding algorithm may regard them as a single event Increased amplitude Reduction of detected events Spectral hardening of continuum Distortion of PSF Correcting for pile-up is complicated Best to set up observation to minimize pileup 136From C. Grant, X-ray Astronomy School 2007 Photon Pileup Monochromatic Source Thermal Spectrum Bright c e s / s t Brighter n u o C Brightest Energy (From Chandra Proposer’s Observatory Guide) PSF Distortion From C. Grant, X-ray Astronomy School 2007 Readout Streak/Out-of-time Events 3C273 - Quasar with jet Transfer Direction Photons that interact while imaging array is transferring Assigned incorrect row/chip y value Events may have poor initial calibration Can be modeled & removed Streak events have higher time resolution, no pileup From C. Grant, X-ray Astronomy School 2007 CCD Operating Modes X-ray CCDs operated in photon-counting mode Spectroscopy requires ≤ 1 photon interaction per pixel per frametime Minimum frametime limited by readout rate Tradeoff between increasing readout rate and noise For ACIS, 100 kHz readout 3.2 s frametime Frametime can be reduced by reading out subarrays or by continuous parallel clocking (1D imaging) 1.5 s 0.8 s 0.4 s (CXC Proposer’s Observatory Guide) 139From C. Grant, X-ray Astronomy School 2007 Charge Transfer Inefficiency X-ray events lose charge to charge trapping sites. Leads to: Position dependent gain Spectral resolution degradation Al K Position dependent QE Caused by radiation damage or manufacturing defects Ti K Depends on: Density of charge trapping Mn K sites Charge trap capture and re- emission properties (temperature) Occupancy of charge traps (particle background) From C. Grant, X-ray Astronomy School 2007 Hot Pixels, Flickering Pixels Radiation damage or manufacturing defects can cause pixels to have anomalously high dark current Can regularly exceed event threshold and cause spurious events Extreme cases may be removed onboard, otherwise filtered in data analysis Strongly correlated with temperature More important for ASCA (–60C) and Suzaku (–90C) than ACIS (–120C) Unstable defects cause flickering pixels Lower frequency, more difficult to detect and remove 141 Fixed pattern noise Cal source Cal source De Plaa et al. (2006) 142 143 144 145 146 147 148 149 150 151 152 153 Microcalorimeter ASTRO-H SXS: Silicon bolometers 154 ASTRO-H Soft X-ray Spectrometer 155 Transition Edge Sensors Athena X-IFU e.g., Bi/Cu 156 e.g., Mo/Au Frequency Domain Multiplexing Athena baseline Time Domain Multiplexing 157 Event Grade 158 X-IFU Performance 159 Threshold for SQUID: 10-14 T B field of heart: 10-10 T B field of brain: 10-13 T http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/coop.html http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/squid.html 160 ≈1.4 eV (T=100 mK) Onboard ASTRO-E, ASTRO-E2/Suzaku, ASTRO-H/Hitomi (SXS) 161 162 Athena X-ray Integral Field Unit Launch foreseen in 2028! 163 NGC 1275 with Hitomi/SXS (and Suzaku XIS) 164 Superconducting Tunnel Junction Two films of superconducting metal (e.g., Nb, Ta, Hf) separated by a thin insulating layer Incoming photon perturbs equilibrium of junction. If bias voltage is applied across STJ with parallel magnetic field to suppress the Josephson current, electric charge can be Josephson junction measured, and is Operated below the superconductor's critical proportional to energy of temperature (typically below 1 K) photon Good also for optical, UV; high count rates Courtesy, LLNL, UC Courtesy, LLNL, UC E 10 100 eV (FWHM) Courtesy, LLNL, UC Detectors for gamma-ray astronomy gamma-ray interaction with matter Photoelectric effect h dN E = h- E dE e b Photopeak; Eb≈0 h E Compton effect e- = 0° =180° h dN dE e g d Compton edge e Compton continuum n o t p h‘ m o E Pair production C h e- h dN 2moc2 dE Double escape e+ peak h E (also single esc. peak) P. v. Ballmoos, CESR 170 gamma-Ray Spectra h2moc2 k a escape Compton e e scattered g p d o e photoeffect t o n dN h o t p dE p m multiple Comptoneffect o Compton Compton C e- scattering e- continuum pair production e+ 511 keV h E k e escape 511 keV e 2 e a h2moc p g p e a d a c p e c s o s t n e e o o • Compton edge t h e e l p l p b g dN m u n i o o E h h s C dE d e multiple 2 Compton c 2 h o c o Compton m scattering h( ) 2 2 m continuum h m c 1 2 0 h h 2 h E 2h /m0c Ee ( ) h 2 1 2h m0c 171 Adapted from P. v. Ballmoos mass attenuation for NaI 1 ] 5 I 10 Z a N m g / 2 0 m 10 c [ n o i t a photoel. 2 u -1 Z n 10 Z e t t a pair Compton 10-2 0.1 1 10 100 103 104 E [MeV] s o o m l l a x B n I I e , o 0 photo Compton pair v m o r F Three interactions from gamma-rays with matter 172 Evans (1955) 173 1 7 4 Detecting Gamma-rays • I conversion : Gamma-rays do not interact with matter unless they undergo a “catastrophic” interaction. In all cases of practical interest, Gamma-rays are detected by their production of secondary electrons. X or gamma-ray photon matter mean free path ≈ 10-1 m h e- fast electron mean free path ≈ 10-3 m • II ionization of detector medium by fast electrons → creation of large number of charge carriers • III collection (reconversion) of detector signal, amplify current and converted by an ADC P.v. Ballmoos 174 Gamma-ray detectors • Gas-filled detectors • Ionization chambers • Proportional counters • Geiger counters • Scintillators • Scintillators • Photomultipliers • Spectral resolution : scintillators vs semiconductors • Semiconductors • Narrow gap / low temperature semiconductors • Wide bandgap / high temperature semiconductors • Examples • Phonon detectors • Ultra-low temperature detectors for gamma-ray detection ? P. v. Ballmoos 175 Background cosmic gamma ray background cosmic rays trapped protons P. v. Ballmoos 176 Scintillators • Developed to counter the low efficiency of PC • Incident photon excites the scintillation material which emits photons in proportion to the photon energy (visible or UV) • This fluorescent light is collected by photomultipliers or photodiodes • Three main properties: scintillation efficiency, decay times of induced luminescence, and stopping power (related to Z value) • Two classes of materials: inorganic crystals and organic liquids 177 Inorganic scintillators • High scintillation efficiency and high-Z value • Crystalline insulators (empty conduction band) • Activators (impurities), e.g., Tl in NaI (COMPTEL) or CsI (IBIS- PICsIT) crystals, used to enhance efficiency and shift to visible • High efficiency (20%) vs pure crystals with natural impurities (e.g., Bi4Ge3O12=BGO with 2%; Integral SPI ACS, but high Z) Knoedlseder (2003) 178 Organic scintillators • Weak intermolecular interactions: excited states of individual molecules (indep. of state of material, i.e., gaseous, solid and liquid, or incorporated into plastics) • Aromatic compounds (planar molecules made of benzenoid rings: e.g., anthracene=C14H10; stylbene=C14H12) • Interactions with heavy or slow particles (p or n) suppresses fast fluorescence leading to pronounced tail of light pulse • Often used as particle anticoincidence shields, rarely as spectrometers (low scintillation efficiency and low Z-values) spin-forbidden Decay with ≈ps NB: repopulation to lowest of S10 from T10 vibrational state possible, leading (S10) to slow 10-4 – 1 s fluorescence with 2-30ns ≈s 179 Knoedlseder (2003) Photomultipliers Gain of 107-109 From Bruker-AXS 180 Semi-conductor detectors • See introduction on semiconductors in previous slides (conduction/valence bands) • Gamma-ray interaction produces secondary electrons; they produce electron-hole pairs in the conduction/valence bands • Electric field separates the pairs before recombination, drifting electrons to anode and holes to cathode. Charge is collected, which is proportional to energy deposited in detector • Energy required to generate electron-hole pair is =(14/5)Eg+c, where 0.5≤c≤ eV • Common detectors made of Ge (e.g., Integral SPI; Eg=0.74 eV, =2.98 eV), Si (=3.61 eV), CdTe (Integral ISGRI; Eg=1.6eV, =4.43 eV) 181 scintillator vs. semiconductor h = 1 MeV h = 1 MeV scintilator solid state detector e.g NaI e.g. HP Germanium conduction band p e a e conduction band g E 1 eV d Eg > 5 eV g n hvis a e valence band b e h+ hvis valence band e- e Eeh 20 eV => . - 5 104 e /hole pairs PA R PMT Coldfnger (80 K) +HV • Scintillation eff. ~ 12% => 120 keV ( V/UV) Energy to form e-/hole pair : Eeh ≈ 3 eV Vis. photon energy ~ 3eV => 40'000 V/UV ph N ≈ 106/3eV ≈ 300’000 charge • sem carriers on photocathode => 20'000 photons F ≈ 0.06-0.14 (Fano factor) • sem • quantum eff. QE ≈ 20% => 4’000 photo-e- (Nsci) R = 0.42 (N /F )1/2 ≈ 25 R = 0.42 (N /F )1/2 ≈ 500 • sci sci sem sem P. v. Ballmoos 182 Comparison : scintillator / semiconductor spectra Knoll, 1989 P. v. Ballmoos 183 For gamma-rays with energies of ~ 1 – 10 MeV, direct scintillation or solid state detection becomes inefficient. Photons interact with matter mainly through Compton scattering Have the gamma-ray undergo Compton scattering event in an upper detector layer (1); determine direction of motion and energy of the down- scattered photon in a second, lower detector layer (2). Need to also measure energy and direction of the recoil electron in layer 1 to uniquely determine gamma-ray 184direction. Pair production telescope 2 Theoretically for E > 2mec (i.e., E > 1,022 MeV), but in practice for E>30 MeV Trajectories are measured from the particle tracking detectors, while the detector at the bottom (e.g., calorimeter or scintillator) will give the energy. The particles trajectories of the electron-positron pair are similar to 2 the photon trajectory for E >> 2mec . For E>30 MeV Adapted from J. Paul (2000), GDR PCHE Fermi/GLAST LAT, 20 MeV-300 GeV 186 TeV astronomy Science with E>30 GeV TeV astronomy Air Showers: Secondaries or higher order particles created, when the primary CRs hit the Earth‘s atmosphere. – Photon-induced air showers: • Pure electromagnetic in nature • Only electrons, positrons and photons – Hadron-induced air showers: • Hadronic cascades • Decays of pions, hadrons • Muons and neutrinos produced in decays • Later electromagnetic cascades Tülün Ergin Graduate School Workshop "Physics at 188TESLA" October 2-5, 2001 Ground Based Atmospheric Cherenkov Detectors • Measure the Cherenkov radiation, which is produced by the passage of air showers through the atmoshere, with a photon detector. • Atmospheric Cherenkov Telescopes (ACT) are used – Huge detection areas (up to 0.1 km2 or more) – High detecting rates at TeV energies • But high background ! (hadronic CRs) Tülün Ergin Graduate School Workshop "Physics at TESLA" October 2-5, 2001 Particle speed exceeds light speed in medium: Vs>Cs 1 F blue radiation 2 ct n 1 cos ct n Spectrum dominated by blue photons Ground-Based Atmospheric Cherenkov Detectors • There are differences between the Cherenkov light emission produced by photon-induced air showers and hadron-induced air showers How can this difference be measured ? Take an image of the shower! 191 Atmospheric Cherenkov Imaging 20 km p Cherenkov Imaging gives the t h ability to distinguish compact g i e images of gamma-ray h c showers from more irregular i r e images from hadronic h p s showers o m t Arrival directions also A determined with high accuracy 5o e_ e+ _ To record the Cherenkov light images of gamma-ray initiated air showers, a large camera, consisting of an array of PMTs in the focal plane of a large optical reflector, is used. 192 Types of images Hadron Gamma seen by atmospheric ray Cherenkov camera Muon Ring Sky Noise 193 gamma-ray ~ 10 km shower 2 Seff = 1m at 1 GeV Crab flux: 5 nsec F(>E)=5x10-4 (E/1TeV)-1.5 ph/m2 hr gamma-ray telescopes – “fast” detectors 30 GeV – 30 TeV 4 2 100m Seff>3x10 m at 1 TeV in principle also at 10 GeV 194 From Aharonian (TeVPA 2008)