Transcript Slide 1
Detector Design Principles 2 • Ionization chambers (gas and solid-state) • Proportional counters • Avalanche counters • Geiger-Müller counters • Cloud/bubble chambers • Track detectors Radiation Detectors Ionization Detectors Associated Techniques • • • • Scintillation Detectors • Phosphorescence counters • Fluorescence counters (inorganic solid scintillators, organic solid and scintillators) • Čherenkov counters Photo sensors and multipliers Charged-coupled devices Electronic pulse shape analysis Processing/acquisition electronics W. Udo Schröder, 2009 Primary Ionization Track (Gases) incoming particle ionization track ion/e- pairs Minimum-ionizing particles (Sauli. IEEE+NSS 2002) Different counting gases Helium GAS (STP) Argon Xenon CH4 DME 0.32 2.4 6.7 1.5 3.9 <n> (ion pairs/cm) 6 25 44 16 55 Radiation Detectors 3 dE/dx ( keV /cm ) e- I+ Statistical ionization process: Poisson statistics Detection efficiency e depends on average number <n> of ion pairs e 1e DE <n> ≈Linear for DE«E W. Udo Schröder, 2009 n GAS (STP) Helium Argon Higher e for slower particles thickness e 1 mm 45 2 mm 70 1 mm 91.8 2 mm 99.3 Driven Charge Transport in Gases Electric field E = DU/Dx separates +/- charges (q=ne+, e-) Pe(x) Charge Diffusion in Electric Field dN dx 5 t0 Radiation Detectors Pe(x) x t1 >t0 x Pe(x) t2 >t1 x E ( x w t )2 exp 4 D t 4Dt N0 1 F e E drift velocity 2m 2m v mean time between collisions w kT w : mobility e E Mass dependence ! D FluctuationDissipation Theorem Cycle: acceleration – scattering/ionization Drift (w) and diffusion (D) depend on field strength E and gas pressure p (or r). w w(E p); D D(E p) x W. Udo Schröder, 2009 Further ionization possible Amplification Direct-Ionization and Amplification Counters Single-wire gas counter (PC) signal gas 7 C + Radiation Detectors - W. Udo Schröder, 2009 R - U0 counter gas + Signal Generation in Ionization Counters Primary ionization: Gases I 20-30 eV/IP, Si: I 3.6 eV/IP Radiation Detectors d Capacitance C 8 - Ge: I 3.0 eV/IP x Energy loss De: n= nI =ne= De/I number of primary ion pairs n at x0, t0 x0 Force: Fe =-e·E= -e·U0/d = -FI Energy content of detector capacitor C: + R 0 DU(t) Cs Signal U0 C 2 U0 U 2 t CU DU t 0 2 2) W t neFe xe t x0 nI FI x I t x0 neU0 x I t xe t d 1) W t w 1) 2) DU t W. Udo Schröder, 2009 W t CU0 t t t0 ne w t w t t t0 Cd Time-Dependent Signal Shape Total signal: e & I components DU t t De w t w t t t0 Cd 10 3 w t Both components measure De. Both components measure position of primary ion pairs 9 w Drift velocities (w+, w->0) Radiation Detectors De C DU(t) De x0 C d x0 = w-(te-t0) For fast counting use only electron component. t0 W. Udo Schröder, 2009 te~s tI~ms t Make position independent with Frisch Grid Frisch Grid Ion Chambers Shielding e- charge-collecting electrode from primary ionization track e- signal no x dependence x Radiation Detectors 11 cathode d x0 particle Wire mesh (Frisch grid) at position xFG @ UFG=const. e- produce signal only inside sensitive anode-FG volume, ions are not “seen”. dFG 0 De DU t w t t tFG CdFG Anode/FG signals out W. Udo Schröder, 2009 not x dependent. “Drift chambers” exploit x-dependence Electronic Pulse Shape t event 1 Intrinsic pulse shape for PC : event 2 time t , PC wire length L 12 DU ( t ) event 4 event 4 event 2 event 1 DU Radiation Detectors Use only electronic pulse component (differentiate electronic signal in pre-amp) t0 e / CU 0 , mobility wdrift / E e counter gas dielectric constant t Differentiation DU W. Udo Schröder, 2009 C DU R q t ln(1 ) 4e L t0 DU long decay time of pulse pulse pile up, summary information differentiate electronically, RC-circuitry in shaping amplifier, individual information for each event (= incoming particle) Bragg-Curve Sampling Counters Sampling Ion Chamber with divided anode 13 DE1 DE2 Eresidual Anodes FG Radiation Detectors isobutane 50T DE W. Udo Schröder, 2009 Sample Bragg energy-loss curve at different points along the particle trajectory improves particle x identification. ICs have excellent resolution in E, Z, A of charged particles but are “slow” detectors. Gas IC need very stable HV and gas handling systems. DE (channels) Radiation Detectors 14 IC Performance Energy resolution e2 F nip F De I F<1 Fano factor Eresidual (channels) W. Udo Schröder, 2009 Proportional Counter gas counter gas C Voltage U0 (300-500) V R - U0 + RA W. Udo Schröder, 2009 signal + - Anode Wire Radiation Detectors 15 Rc Anode wire: small radius RA 50 m or less Field at r from wire RI E (r ) E(RI)=UI RI U0 1 ln( RC RA ) r Avalanche RI RA, several mean free paths needed e- q+ Pulse height mainly due to positive ions (q+) Solid-State IC n Radiation Detectors 17 c + + - Solids have larger density higher stopping power dE/dx more ion pairs, better resolution, smaller detectors (also more damage and little regeneration max accumulated dose ~ 1023 particles + Semiconductor n-, p-, i- types i p cs R Si, Ge, GaAs,.. (for e-, lcp, g, HI) DU(t) Band structure of solids: E U0 e- Capacitance Si : 2.2 C 3.7 Conduction EF h+ rnU0 pF mm2 - r pU0 pF mm2 Bias voltage U0 creates charge-depleted zone W. Udo Schröder, 2009 Valence + Ionization lifts e- up to conduction band free charge carriers, produce DU(t). Surface Barrier Detectors 20 EF Metal Junction CB Semi conductor Different Fermi energies adjust to on contact. Thin metal film on Si surface produces space charge, an effective barrier (contact potential) and depleted zone free of carriers. Apply reverse bias to increase depletion depth. Radiation Detectors VB Insulation Metal film Insulating Mount Silicon wafer depleted dead layer Ground +Bias Front: Au Back: Al evaporated electrodes W. Udo Schröder, 2009 Possible: depletion depth ~ 300 dead layer dd 1 V ~ 0.5V/ Over-bias reduces dd Metal case Connector ORTEC HI detector Charge Collection Efficiency in SSDs High ionization density at low electric fields: Edeposit > Eapp Lower apparent energy due to charge recombination, trapping. Low ionization density (or high electric fields): Edeposit Eapp Typical charge collection times: t ~ (10-30)ns 21 Pulse height defect Moulton et al. EPhD : Edeposit Eapp Fit : Radiation Detectors a( Z , A) EPhD Edeposit 10 b(Z , A) Edeposit a( Z ) 2.230 10 5 2 Z 0.5682 b( Z ) 14.25 / Z 0.0825 a( A) 3.486 10 6 2 A 0.5728 b( A) 28.40 / A 0.0381 Affects charge collection time signal rise time. Exploit for A, Z identification W. Udo Schröder, 2009 Position-Sensitive Semiconductor Detectors Gerber et al., IEEE TNS24,182(1977) Double-sided x/y matrix detector, resistive readout. xn Q1 Q Lx 22 R Radiation Detectors y R Q x R R ~2000 Wcm, 300 U0160V W. Udo Schröder, 2009 ym Q3 Q Ly (Lx xn ) Q2 Q Lx Q4 Q (Ly ym ) Ly Q1 Q2 Q3 Q4 Q DE Si-Strip Detectors Radiation Detectors 23 Typically (300-500) thick. Fully depleted, thin dead layer. Annular: 16 bins (“strips”) in polar (q) , 4 in azimuth (f) (Micron Ltd.) Rectangular with 7 strips W. Udo Schröder, 2009 Ge gray Detectors (SS-IC) Ge detectors for g-rays use p-i-n Ge junctions. Because of small gap EG, cool to -77oC (LN2) Radiation Detectors 24 Ge Cryostate (Canberra) W. Udo Schröder, 2009 Ge cryostate geometries (Canberra) Properties of Ge Detectors: Energy Resolution Superior energy resolution, compared to NaI Radiation Detectors 25 DEg ~ 0.5keV @ Eg =100keV W. Udo Schröder, 2009 Size=dependent mall detection efficiencies of Ge detectors 10% solution: bundle in 4arrays GammaSphere,Greta EuroGam, Tessa,… Townsend (Gas) Avalanche Amplification Gas or SS avalanche counters _ Avalanche Region 26 Radiation U0 M Radiation Detectors IC Region Nonlinear Region d + U0~kV/cm I U0 Amplification M W. Udo Schröder, 2009 M n 1 i(t )dt ; nip primary IP nip nip : nM d 1.Townsend coefficient Avalanche Formation Townsend Coefficient Electron-ion pairs through gas ionization dn n dx n( x ) n0 e x for const Electrons in outer shells are more readily removed, ionization energies are smaller for heavier elements. n( x ) n0 exp ( x)dx x 0 Avalanche Quenching A. Sharma and F. Sauli, Nucl. Instr. and Meth. A334(1993)420 28 in Argon Reduce and energy of ions by collisions with complex organic molecules (CH4, …). Radiation Detectors Excitation of rotations and vibrations already at low ion energies Organic vapors = “self quenching” CH4 W. Udo Schröder, 2009 Parallel Plate Counters: t-Resolution cathode sensitive layer eanode + R Charges produced at different positions along the particle track are differently amplified. non-linearity nip(DE) W. Udo Schröder, 2009 ff ff U p PPAC + PPAC Radiation Detectors 29 d~1/ PPACs for good time resolution, U(p,f)f Sparking and Spark Counters 30 /p g Different cathode materials Impact ionization Probability g + d Radiation Detectors Amplification by impact ionization n e d M n0 1 g e d 1 Prevent spark by reducing for ions: collisions with large organic molecules quenching additives, self-quenching gases W. Udo Schröder, 2009 Sparking :g e d 1 p (101 103 ) Torr 31 Radiation Detectors W. Udo Schröder, 2009 Avalanche Quenching A. Sharma and F. Sauli, Nucl. Instr. and Meth. A334(1993)420 32 in Argon Reduce and energy of ions by collisions with complex organic molecules (CH4, …). Radiation Detectors Excitation of rotations and vibrations already at low ion energies Organic vapors = “self quenching” CH4 W. Udo Schröder, 2009 Multi-Wire Proportional Counters Magic Gas: Ar(75%), isobutane (24.5%), freon (0.5%) HV:kV/cm Important for detection of high-energy particles, beam profile,.. (Charpak 1968-80) 33 Equipotential Lines Radiation Detectors dac Anode Wires s Anode Wires Cathode Wire Planes Field at ( x, y ) (0, 0) C 2 2 V ( x, y ) U 0 ln 4 sin x sinh 4e s s 2e Capacitance C ; d ac d ac s ln( d s) W. Udo Schröder, 2009 y s Field strength close to anode wires: d V(x,y) 1/r Position-Sensitive Semiconductor Detectors Gerber et al., IEEE TNS24,182(1977) Double-sided x/y matrix detector, resistive readout. xn Q1 Q Lx 34 R Radiation Detectors y R Q x R R ~2000 Wcm, 300 U0160V W. Udo Schröder, 2009 ym Q3 Q Ly (Lx xn ) Q2 Q Lx Q4 Q (Ly ym ) Ly Q1 Q2 Q3 Q4 Q DE Frisch Grid Ion Chambers Radiation Detectors 35 x cathode d x0 particle dFG 0 Anode/FG signals out W. Udo Schröder, 2009 Suppress position dependence of signal amplitude by shielding charge-collecting electrode from primary ionization track. Insert wire mesh (Frisch grid) at position xFG held constant potential UFG. e- produce signal only when inside sensitive anode-FG volume, ions are not “seen”. De DU t w t t tFG CdFG not x dependent. x-dependence used in “drift chambers”. Electronics: Charge Transport in Capacitors Simple charge motion, no secondary ionization/amplification 36 q- U Radiation Detectors conducting electrodes q+ t Connected to circuitry, q- R e Electronics W. Udo Schröder, 2009 Charges q+ moving between parallel conducting plates of a capacitor induce tdependent negative images q- on each plate. current of e- from negative electrode is proportional to charge q+. Electron Transport Radiation Detectors 37 Multiple scattering/acceleration produces effective spectrum P(e) calculate effective and : 2 e P e 1 w E e d e D e v e P e de 3m e v e 3 W. Udo Schröder, 2009 Simulations v e 2e m w- ~ 103 w+ Electron Transport: Frost et al., PR 127(1962)1621 V. Palladino et al., NIM 128(1975)323 G. Shultz et al., NIM 151(1978)413 S. Biagi, NIM A283(1989)716 http://consult.cern.ch/writeup/garfield/examples/gas/trans2000.html#elec Radiation Detectors 38 Stability and Resolution W. Udo Schröder, 2009 • Anisotropic diffusion in electric field (Dperp >Dpar). • Electron capture by electro+negative gases, reduces energy resolution • T dependence of drift: Dw/w DT/T ~ 10-3 • p dependence of drift: Dw/w Dp/p ~ 10-3-10-2 • Increasing E fields charge multiplication/secondary+ ionization loss of resolution and linearity Townsend avalanches Free Charge Transport in Matter P(x) 1D Diffusion equation P(x)=(1/N0)dN/dx t0 39 x x Radiation Detectors P(x) t1 >t0 x P(x) x2 exp Gaussian 4Dt 4D t N0 : rms x 2 x 2Dt 1 D v 3 D diffusion coefficient, <v> mean speed mean free path Thermal velocities : t2 >t1 x W. Udo Schröder, 2009 dN dx v 8kT m D(ion) 8 3 D(e ) v2 Maxwell+Boltzmann velocity distribution Small ion mobility - Solid-State Gamma Detectors V Radiation Detectors 40 + W. Udo Schröder, 2009