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Detection of Ionizing Radiation

Detection of Ionizing Radiation. Helium. Xenon. Argon. CH. DME. GAS (STP). 4. 3.9. 1.5. 6.7. 0.32. dE/dx. (. keV. /. cm. ). 2.4. 6. ). n. (ion pairs/. 16. 55. cm. 44. 25. I +. e -. thickness. . GAS (STP). 1. mm. 45. Helium. 70. 2. mm. 1. mm. 91.8.

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Detection of Ionizing Radiation

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  1. Detection of Ionizing Radiation

  2. Helium Xenon Argon CH DME GAS (STP) 4 3.9 1.5 6.7 0.32 dE/dx ( keV / cm ) 2.4 6 ) n (ion pairs/ 16 55 cm 44 25 I+ e- thickness  GAS (STP) 1 mm 45 Helium 70 2 mm 1 mm 91.8 Argon 99.3 2 mm Primary Ionization Track (Gases) incoming particle ionization track  ion/e- pairs Minimum-ionizing particles(Sauli. IEEE+NSS 2002) Statistical ionization process: Poisson statisticsDetection efficiency e depends on average number <n> of ion pairs DE  nLinear Higher e for slower particles W. Udo Schröder, 2004

  3. P(x) P(x) P(x) t0 x x x t1 >t0 t2 >t1 Free Charge Transport in Gases 1D Diffusion equation  P(x)=(1/N0)dN/dx D diffusion coefficient, <v> mean speed lmean free path Thermal velocities : Maxwell+Boltzmann velocity distribution Small ion mobility W. Udo Schröder, 2004

  4. P(x) P(x) P(x) t0 x E x x x l Driven Charge Transport in Gases Electric field E = DU/Dx separates +/- charges t1 >t0 Cycle: acceleration – scatteringDrift and diffusion depend on field strength and gas pressure p (or r). t2 >t1 W. Udo Schröder, 2004

  5. GAS ION µ+ (cm2 V-1 s+1) @STP Ar Ar+ 1.51 CH4 CH4+ 2.26 Ar+CH4 80+20 CH4+ 1.61 Ion Mobility Ion mobility m+ = w+/E Independent of field,for given gas at p,T=const. Typical ion drift velocities(Ar+CH4 counters): w+ ~ (10-2 – 10-5)cm/ms slow! E. McDaniel and E. Mason The mobility and diffusion of ions in gases (Wiley 1973) W. Udo Schröder, 2004

  6. Electron Transport Multiple scattering/acceleration produces effective spectrum P(e)  calculate effective land t: Simulations 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 W. Udo Schröder, 2004

  7. Stability and Resolution • 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 W. Udo Schröder, 2004

  8. q+ U conducting plates q+ t e+ R q+ Electronics Electronics: Charge Transport in Capacitors Charges q+ moving between parallel conducting plates of a capacitor influence t-dependent negative images q+on each plate. If connected to circuitry, current of e- would emerge from plate, in total proportionally to charge q+. W. Udo Schröder, 2004

  9. x + d x0 0 + DU(t) U0 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 De:n= nI =ne= De/Inumber of primary ion pairs n at x0, t0 Force: Fe = -eU0/d = -FI Energy content of capacitor C: Capacitance C W. Udo Schröder, 2004

  10. DU(t) t0 te~ms tI~ms t Time+Dependent Signal Shape Total signal: e & I components Drift velocities (w+>0, w+<0) Both components measure De and depend on position of primary ion pairs x0 = w-(te-t0) Use electron component only for fast counting. W. Udo Schröder, 2004

  11. x d x0 dFG 0 Anode/FG signals out Frisch Grid In Ion Chambers 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. Signal not x dependent. x+dependence used in “drift chambers”. W. Udo Schröder, 2004

  12. isobutane 50T DE/Dx x Bragg+Curve Sampling Counters Sampling Ion chamber with divided anodes Sample Bragg energy+loss curve at different points along the particle trajectory improves particle identification. W. Udo Schröder, 2004

  13. DE(channels) Eresidual (channels) IC Performance 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. W. Udo Schröder, 2004

  14. + + n + p + DU(t) E U0 Conduction e+ EF h+ Valence + + Solid+State IC Solids have larger density  higher stopping power dE/dx  more ion pairs, better resolution, smaller detectors (also more damage, max dose ~ 107 particles i Semiconductors ideal types: n, p, I Si, Ge, GaAs,.. Band structure of solids: Ionization lifts e+ up to conduction band  free charge carriers, produce D U(t). Bias voltage U0 creates charge+depleted zone W. Udo Schröder, 2004

  15. Donor Acceptor ions n p e- Si Bloc 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 o o o - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + space charge e- Potential o o o o o o o o o o o o d Semiconductor Junctions and Barriers Pure “intrinsic” Si can be made to n-type or p-type Si by diffusing e- donor (P, Sb, As) and acceptor ions into Si. Junctions occur when both are diffused into Si bloc from different sides. 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 U0 also creates carrier-free space dn,p: W. Udo Schröder, 2004

  16. Insulation Metal case Connector Surface Barrier Detectors Metal contact Silicon wafer W. Udo Schröder, 2004

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