Transcript Slide 1
Detector Design Principles Radiation Detectors 2 Ionization Detectors • Ionization chambers (gas and solid-state) • Proportional counters • Avalanche counters • Geiger-Müller counters • Cloud/bubble chambers • Track detectors Scintillation Detectors • Phosphorescence counters • Fluorescence counters (inorganic solid scintillators, organic solid and scintillators) • Čherenkov counters Associated Techniques • • • • Photo sensors and multipliers Charged-coupled devices Electronic pulse shape analysis Processing/acquisition electronics W. Udo Schröder, 2007 Primary Ionization Track (Gases) incoming particle ionization track ion/e- pairs Minimum-ionizing particles Different counting gases Helium 3 GAS (STP) Radiation Detectors e- I+ ≈Linear for DE«E W. Udo Schröder, 2007 Argon Xenon CH4 DME dE/dx ( keV /cm ) 0.32 2.4 6.7 1.5 3.9 <n> (ion pairs/cm) 6 25 44 16 55 Statistical ionization process: Poisson statistics Detection efficiency e depends on average number <n> of ion pairs e 1e DE <n> (Sauli. IEEE+NSS 2002) 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 Radiation Detectors 4 Effective Ionization Energies Mean energy per ion pair larger than IP because of excitations Large organic molecules have low-lying excited rotational states excitation without ionization through collisions =“quenching” additives W. Udo Schröder, 2007 Driven Charge Transport in Gases Electric field E = DU/Dx separates +/- charges (q=ne+, e-) P(x) Charge Diffusion in Electric Field t0 5 x P(x) Radiation Detectors t1 >t0 x E dN dx ( x w t )2 exp 4D t 4Dt N0 e E drift velocity 2m v mean time between collisions w D kT e : w mobility E x P(x) t2 >t1 x W. Udo Schröder, 2007 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) Ion Mobility GAS ION 6 Ar Ar+ CH4 CH4+ Ar+CH4 80+20 CH4+ µ+ (cm2 V-1 s+1) @STP 1.51 2.26 1.61 Ion mobility + = w+/E Independent of field, for given gas at p,T=const. Typical ion drift velocities (Ar+CH4 counters): Radiation Detectors w+ ~ (10-2 – 10-5) cm/s W. Udo Schröder, 2007 slow! E. McDaniel and E. Mason The mobility and diffusion of ions in gases (Wiley 1973) Signal Generation in Ionization Counters Primary ionization: Gases I 20-30 eV/IP, Si: I 3.6 eV/IP d Capacitance C 7 - x Energy loss De: n= nI =ne= De/I number of primary ion pairs n at x0, t0 x0 Force: Fe = -eU0/d = -FI Energy content of capacitor C: + R Radiation Detectors Ge: I 3.0 eV/IP Cs 0 DU(t) Signal C 2 U0 U 2 t CU0 DU t 2 2) W t ne Fe xe t x0 + nI FI x I t x0 1) W t + U0 W. Udo Schröder, 2007 neU0 x I t xe t d w 1) + 2) DU t W t CU0 + t t t0 ne + w t w t t t0 Cd Time-Dependent Signal Shape Total signal: e & I components Drift velocities (w+>0, w-<0) De w + t w t t t0 DU t Cd Radiation Detectors 8 w + t De C 10 3 w t Both components measure De and depend on position of primary ion pairs DU(t) x0 = w-(te-t0) De x0 C d For fast counting use only electron component. t0 W. Udo Schröder, 2007 te~s tI~ms t Amplification Counters Single-wire gas counter signal gas 9 C + Radiation Detectors - W. Udo Schröder, 2007 counter gas R - U0 + Proportional Counter gas 10 Rc + - counter gas Anode Wire Radiation Detectors C Voltage U0 (300-500) V R - U0 + RA W. Udo Schröder, 2007 signal 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+) Pulse Shape t event 1 Pulse shape : time t , wire length L DU (t ) t0 e / CU 0 , mobility wdrift / E DU e dielectric constant event 4 DU event 4 event 2 event 1 Radiation Detectors 11 event 2 C DU W. Udo Schröder, 2007 t q ln(1 + ) t0 4e L R t 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 Radiation Detectors 12 DE1 DE2 Eresidual Anodes isobutane 50T DE W. Udo Schröder, 2007 Sample Bragg energy-loss curve at different points along the particle trajectory improves particle x identification. DE (channels) 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. Energy resolution Radiation Detectors 13 IC Performance e2 F nip F F<1 Fano factor Eresidual (channels) W. Udo Schröder, 2007 De I Solid-State IC + + - n 14 i p 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 c + Semiconductor n-, p-, i- types DU(t) Band structure of solids: Si, Ge, GaAs,.. (for e-, lcp, g, HI) Radiation Detectors R 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, 2007 Valence + Ionization lifts e- up to conduction band free charge carriers, produce DU(t). Particles and Holes in Semi-Conductors e Conduction Band eC 0 eF 15 Fermion statistics: 2m 2 3 V ne e e fe e V volume 2 3 2 2m 2 3 V nh e e fh e ne nh !! 2 3 2 e F e C e G 2 e G 2 for e C : 0 eeG eV h+ Valence Band Occupation numbers f e + eG 2 e : fe e 1 + exp kT 1 e eF fe e 1 + exp kT Radiation Detectors e + e G 2 h : fh e 1 + exp kT W. Udo Schröder, 2007 1 e + eG 2 exp kT kT 25meV e G 1 + Small gaps eG (Ge) large thermal currents. Reduce by cooling. ne2 ne 2m 2 3 V ne nh 2 2 3 rms e exp G 2kT 2 e e exp G kT conductivity at T Radiation Detectors e- Potential Si Bloc 16 Semiconductor Junctions and Barriers eo-+ o+ +o Donor Acceptor ions n p Need detector with no free carriers. Si: i-type (intrinsic), n-type, p-type by diffusing Li, e- donor (P, Sb, As), or acceptor ions into Si. h+ o +o +o + + + - - -o -o -o - o - o - o o+ o+ + o + o +o +o + + - - -o -o -o - o - o - o o+ o+ + o + o +o +o + + - - -o -o -o - o - o - o space charge o o o o o o o o o o o o d Similar: Homogeneous n(p)type Si with reverse bias U0 also creates carrier-free space dn,p: up to 1mm possible. W. Udo Schröder, 2007 Trick: Increase effective gap Junctions diffuse donors and acceptors into Si bloc from different ends. Diffusion at interface e-/h+ annihilation space charge Contact Potential and zone depleted of free charge carriers Depletion zone can be increased by applying “reverse bias” potential dn, p 3.3 105 rn, pU0 m rn, p 20 k cm, U0 500V d 70 m Surface Barrier Detectors 17 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. VB Insulation Metal film Radiation Detectors Insulating Mount Silicon wafer depleted dead layer Ground +Bias Front: Au Back: Al evaporated electrodes W. Udo Schröder, 2007 Possible: depletion depth ~ 300 dead layer dd 1 V ~ 0.5V/ Over-bias reduces dd Metal case Connector ORTEC HI detector Charge Collection Efficiency 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 Pulse height defect 18 Moulton et al. EPhD : Edeposit Eapp Fit : a( Z , A) EPhD Edeposit 10 b(Z , A) Edeposit a( Z ) 2.230 10 5 2 Z + 0.5682 Radiation Detectors b( Z ) 14.25 / Z + 0.0825 W. Udo Schröder, 2007 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 Si-Strip Detectors Radiation Detectors 19 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, 2007 Ge gray Detectors Ge detectors for g-rays use p-i-n Ge junctions. Because of small gap EG, cool to -77oC (LN2) Radiation Detectors 20 Ge Cryostate (Canberra) W. Udo Schröder, 2007 Ge cryostate geometries (Canberra) Properties of Ge Detectors: Energy Resolution Superior energy resolution, compared to NaI Radiation Detectors 21 DEg ~ 0.5keV @ Eg =100keV W. Udo Schröder, 2007 Size=dependent mall detection efficiencies of Ge detectors 10% solution: bundle in 4arrays GammaSphere,Greta EuroGam, Tessa,… Townsend Gas Avalanche Amplification _ 22 Radiation Nonlinear Region U0 M IC Region Radiation Detectors d + U0~kV/cm I U0 Amplification M W. Udo Schröder, 2007 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 Parallel Plate Counters: t-Resolution cathode sensitive layer d~1/ 24 eanode + Charges produced at different positions along the particle track are differently amplified. non-linearity nip(DE) W. Udo Schröder, 2007 ff ff U p PPAC + PPAC Radiation Detectors R PPACs for good time resolution, U(p,f)f Sparking and Spark Counters /p g 25 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, 2007 Sparking :g e d 1 p (101 103 ) Torr 26 Radiation Detectors W. Udo Schröder, 2007