2. Radiation Detectors W. Udo Schröder, 2007 2 Detector Design Principles Ionization chambers (gas and solid-state) Proportional counters Avalanche counters Geiger-Müller counters Cloud/bubble chambers Track detectors Ionization Detectors Phosphorescence counters Fluorescence counters (inorganic solid scintillators, organic solid and scintillators) Čherenkov counters Scintillation Detectors Associated Techniques Photo sensors and multipliers Charged-coupled devices Electronic pulse shape analysis Processing/acquisition electronics

3. Radiation Detectors W. Udo Schröder, 2007 3 Primary Ionization Track (Gases) incoming particle ionization track  ion/e - pairs Argon DME (ion pairs/ cm ) 25 55 dE/dx (keV/cm) GAS (STP) 2.4 3.9 Xenon 6.7 44 CH 4 1.5 16 Helium 0.32 6 Minimum-ionizing particles (Sauli. IEEE+NSS 2002) Different counting gases Statistical ionization process: Poisson statistics Detection efficiency  depends on average number of ion pairs thickness  Argon GAS (STP) 1mm91.8 2mm 99.3 Helium 1mm45 2mm 70 Higher  for slower particles e-e- I+I+ E  ≈Linear for E«E

4. Radiation Detectors W. Udo Schröder, 2007 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

5. Radiation Detectors W. Udo Schröder, 2007 5 Driven Charge Transport in Gases x P(x) x t0t0 t 1 >t 0 t 2 >t 1 Electric field E = U/x separates +/- charges (q=ne +, e - ) x P(x) E x Cycle: acceleration – scattering/ionization Drift (w) and diffusion (D) depend on field strength E and gas pressure p (or ).

6. Radiation Detectors W. Udo Schröder, 2007 6 Ion Mobility GAS ION µ + (cm 2 V -1 s +1 ) @STP Ar Ar + 1.51 CH 4 CH 4 + 2.26 Ar+CH 4 80+20 CH 4 + 1.61 Ion mobility   = w + /E Independent of field, for given gas at p,T=const. Typical ion drift velocities (Ar+CH 4 counters): w + ~ (10 -2 – 10 -5 ) cm/s  slow! E. McDaniel and E. Mason The mobility and diffusion of ions in gases (Wiley 1973)

7. Radiation Detectors W. Udo Schröder, 2007 7 Signal Generation in Ionization Counters Primary ionization: Gases I  20-30 eV/IP, Si: I  3.6 eV/IP Ge: I  3.0 eV/IP Energy loss  n= n I =n e = /I number of primary ion pairs n at x 0, t 0 Force: F e = -eU 0 /d = -F I Energy content of capacitor C: Capacitance C + - U0U0  U(t) 0 x0x0 x d R CsCs Signal

8. Radiation Detectors W. Udo Schröder, 2007 8 Time-Dependent Signal Shape t 0 t e ~s t I ~ms t U(t) Drift velocities (w + >0, w - <0) Total signal: e & I components Both components measure  and depend on position of primary ion pairs x 0 = w - (t e -t 0 ) For fast counting use only electron component.

9. Radiation Detectors W. Udo Schröder, 2007 9 Amplification Counters Single-wire gas counter U0U0 C - - + + counter gas gas signal R

10. Radiation Detectors W. Udo Schröder, 2007 10 Proportional Counter Anode wire: small radius R A  50  m or less Voltage U 0  (300-500) V counter gas e - q + RARA RIRI UIRIUIRI Anode Wire Avalanche R I  R A, several mean free paths needed Pulse height mainly due to positive ions (q + ) U0U0 C - - + + gas signal R RcRc E(R I )=

11. Radiation Detectors W. Udo Schröder, 2007 11 Pulse Shape t t UU UU long decay time of pulse  pulse pile up, summary information differentiate electronically, RC-circuitry in shaping amplifier, individual information for each event (= incoming particle) R C event 1 event 2 event 4 event 1 event 2 event 4 UU

12. Radiation Detectors W. Udo Schröder, 2007 12 isobutane 50T Bragg-Curve Sampling Counters Sampling Ion Chamber with divided anode Sample Bragg energy-loss curve at different points along the particle trajectory improves particle identification. E 1 E 2 E residual Anodes EE x

13. Radiation Detectors W. Udo Schröder, 2007 13 IC Performance E residual (channels) E (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 F<1 Fano factor

14. Radiation Detectors W. Udo Schröder, 2007 14 Solid-State IC 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 ~ 10 23 particles i Semiconductor n-, p-, i- types Si, Ge, GaAs,.. (for e -, lcp, , HI) Band structure of solids: Valence Conduction E EFEF + - e-e- h+h+ Ionization lifts e - up to conduction band  free charge carriers, produce U(t). Bias voltage U 0 creates charge-depleted zone U0U0 + + + - n p U(t) c R

15. Radiation Detectors W. Udo Schröder, 2007 15 Particles and Holes in Semi-Conductors Fermion statistics: 0  FF Valence Band Conduction Band e-e- h+h+ GG VV CC Small gaps  G (Ge)  large thermal currents. Reduce by cooling.

16. Radiation Detectors W. Udo Schröder, 2007 16 Semiconductor Junctions and Barriers 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. 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 Similar: Homogeneous n(p)- type Si with reverse bias U 0 also creates carrier-free space d n,p : up to 1mm possible. + + + + - - - - o o o n p e-e- h+h+ Donor Acceptor ions space charge Si Bloc e - Potential d

17. Radiation Detectors W. Udo Schröder, 2007 17 Surface Barrier Detectors Metal film Silicon wafer Metal case Insulation Connector EFEF Junction Metal CB Semi conductor VB 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. Ground +Bias Front: Au Back: Al evaporated electrodes Insulating Mount depleted dead layer Possible: depletion depth ~ 300 dead layer d d  1 V ~ 0.5V/ Over-bias reduces d d ORTEC HI detector

18. Radiation Detectors W. Udo Schröder, 2007 18 Charge Collection Efficiency High ionization density at low electric fields: E deposit > E app Lower apparent energy due to charge recombination, trapping. Low ionization density (or high electric fields): E deposit  E app Typical charge collection times: t ~ (10-30)ns Moulton et al. Affects charge collection time  signal rise time. Exploit for A, Z identification

19. Radiation Detectors W. Udo Schröder, 2007 19 Si-Strip Detectors Typically (300-500) thick. Fully depleted, thin dead layer. Annular: 16 bins (“strips”) in polar (), 4 in azimuth () (Micron Ltd.) 5 cm Rectangular with 7 strips circuit board

20. Radiation Detectors W. Udo Schröder, 2007 20 Ge ray Detectors Ge detectors for -rays use p-i-n Ge junctions. Because of small gap E G, cool to -77 o C (LN 2 ) Ge Cryostate (Canberra) Ge cryostate geometries (Canberra)

21. Radiation Detectors W. Udo Schröder, 2007 21 Properties of Ge Detectors: Energy Resolution Size=dependent mall detection efficiencies of Ge detectors  10%  solution: bundle in 4- arrays GammaSphere,Greta EuroGam, Tessa,… Superior energy resolution, compared to NaI E  ~ 0.5keV @ E  =100keV

22. Radiation Detectors W. Udo Schröder, 2007 22 Townsend Gas Avalanche Amplification U0U0 M IC Region Non- linear Region Amplification M Radiation U0U0 I d + U 0 ~kV/cm _

23. Avalanche Formation Townsend Coefficient Electron-ion pairs through gas ionization Electrons in outer shells are more readily removed, ionization energies are smaller for heavier elements.

24. Radiation Detectors W. Udo Schröder, 2007 24 Parallel Plate Counters: t-Resolution sensitive layer d~1/ e-e- cathode - anode + R + PPAC U p ff PPACs for good time resolution, U(p,f)f Charges produced at different positions along the particle track are differently amplified.  non-linearity n ip (E)

25. Radiation Detectors W. Udo Schröder, 2007 25 Sparking and Spark Counters  /p  Impact ionization Probability  Prevent spark by reducing  for ions: collisions with large organic molecules  quenching additives, self-quenching gases d Different cathode materials - +

26. Radiation Detectors W. Udo Schröder, 2007 26

27. Radiation Detectors W. Udo Schröder, 2007 27 Avalanche Quenching in Argon A. Sharma and F. Sauli, Nucl. Instr. and Meth. A334(1993)420 Reduce and energy of ions by collisions with complex organic molecules (CH4, …). Excitation of rotations and vibrations already at low ion energies CH 4 Organic vapors = “self quenching”

28. Radiation Detectors W. Udo Schröder, 2007 28 Multi-Wire Proportional Counters Magic Gas: Ar(75%), iso- butane (24.5%), freon (0.5%) HV:kV/cm Anode Wires Equipotential Lines Anode Wires Cathode Wire Planes s d ac d Field strength close to anode wires: V(x,y)  1/r (Charpak 1968-80) Important for detection of high-energy particles, beam profile,..

29. Radiation Detectors W. Udo Schröder, 2007 29 Position-Sensitive Semiconductor Detectors Gerber et al., IEEE TNS- 24,182(1977) x y Double-sided x/y matrix detector, resistive readout. R R R R Q n-Si Au ~2000 cm, 300  U 0  160V

30. Radiation Detectors W. Udo Schröder, 2007 30 Frisch Grid Ion Chambers 0 d FG x0x0 d x Anode/FG signals out cathode Suppress position dependence of signal amplitude by shielding charge-collecting electrode from primary ionization track. Insert wire mesh (Frisch grid) at position x FG held constant potential U FG. e - produce signal only when inside sensitive anode-FG volume, ions are not “seen”.  not x dependent. x-dependence used in “drift chambers”. particle

31. Radiation Detectors W. Udo Schröder, 2007 31 Electronics: Charge Transport in Capacitors Charges q + moving between parallel conducting plates of a capacitor induce t- dependent negative images q + on each plate. t U Connected to circuitry, current of e - from negative electrode is proportional to charge q +. q+q+ q+q+ q+q+ conducting electrodes Electronics R e+e+ Simple charge motion, no secondary ionization/amplification

32. Radiation Detectors W. Udo Schröder, 2007 32 Electron Transport Multiple scattering/acceleration produces effective spectrum P()  calculate effective and  : Simulations http://consult.cern.ch/writeup/garfield/examples/gas/trans2000.html#elec 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 w - ~ 10 3 w +

33. Radiation Detectors W. Udo Schröder, 2007 33 Stability and Resolution Anisotropic diffusion in electric field (D perp >D par ). Electron capture by electro+negative gases, reduces energy resolution T dependence of drift: w/w  T/T ~ 10 -3 p dependence of drift: w/w  p/p ~ 10 -3 -10 -2 Increasing E fields  charge multiplication/secondary+ ionization  loss of resolution and linearity  Townsend avalanches

34. Radiation Detectors W. Udo Schröder, 2007 34 Free Charge Transport in Matter x P(x) t0t0 x t 1 >t 0 x P(x) t 2 >t 1 1D Diffusion equation  P(x)=(1/N 0 )dN/dx D diffusion coefficient, mean speed   mean free path Thermal velocities : Maxwell+Boltzmann velocity distribution Small ion mobility

35. Radiation Detectors W. Udo Schröder, 2007 35 Solid-State Gamma Detectors V - +