1. Paul Sellin, Radiation Imaging Group Advances in Compound Semiconductor Radiation Detectors a review of recent progress P.J. Sellin Radiation Imaging Group Department of Physics University of Surrey

2. Paul Sellin, Radiation Imaging Group CZT/CdTe Review of recent developments in compound semiconductor detectors: CdZnTe (CZT) continues to dominate high-Z room temperature devices: a range of electrode configurations to overcome poor hole transport lack of monocrystalline whole-wafer material High Pressure Bridgman CZT from eV Products still the major volume supplier HPB CZT also from Bicron (US), LETI (France), also LPB CZT good results from CdTe Schottky diodes CdTe from a number of suppliers (eg. Acrotech, Eurorad, Freiburg) CZT/CdTe pixel array detectors under development: hard X-ray astronomical imaging gamma cameras for nuclear medicine custom ASICs for CZT/CdTe starting to appear 3. Paul Sellin, Radiation Imaging Group Material Properties Summary of some material properties: Z EG W i at RT (eV) (eV/ehp) () Si 14 1.12 3.6 ~104 Ge 32 0.66 2.9 50 InP 49/15 1.4 4.2 107 GaAs 31/33 1.4 4.3 108 CdTe 48/52 1.4 4.4 109 CdZn0.2Te 48/52 1.6 4.7 1011 HgI2 80/53 2.1 4.2 1013 TlBr 81/35 2.7 5.9 1011 Diamond 6 5 13 >1013 Also: SiC, PbI2, GaSe 4. Paul Sellin, Radiation Imaging Group Detection Efficiency Vast majority of compund semiconductor detector development is driven by improved photoelectric absorption for hard X-rays and gamma rays: Exceptions are radiation hard detector programmes - SiC and Diamond 5. Paul Sellin, Radiation Imaging Group Material Quality in CdZnTe High Pressure Bridgman CdZnTe is the new material of choice for medium resolution X-ray and gamma ray detection Material suffers from mechanical defects - monocrystalline pieces are selected from wafers - no whole-wafer availability CZT material grown by High Pressure Bridgman from eV Products (Growth and properties of semi-insulating CdZnTe for radiation detector applications, Cs. Szeles and M.C. Driver SPIE Proc. 2 (1998) 3446). New growth methods have developed very recently - eg. Low Pressure Bridgman CZT from Yinnel Tech (US) and Imarad (Israel) 6. Paul Sellin, Radiation Imaging Group Hole tailing in a 5mm thick CdZnTe detector Poor hole transport causes position- dependent charge collection efficiency hole tailing characteristic of higher energy gamma rays in CdZnTe GF Knoll, Radiation Detection and Measurement, Ed. 3 7. Paul Sellin, Radiation Imaging Group Scanning of CCE vs depth using lateral Ion- beam induced charge microscopy 400 Vcathode -400 V cathode Pulse height spectra as a function of depth +400 V -400V Image of CCE using 1m resolution 2MeV scanning proton beam 8. Paul Sellin, Radiation Imaging Group Induced signals due to charge drift In a planar detector the drifting electrons and holes generate equal and opposite induced charge on anode and cathode In CZT the holes are quickly trapped: hole component is much reduced interactions close to the anode have low CCE Reviewed in Z. He et al, NIM A463 (2001) 250 9. Paul Sellin, Radiation Imaging Group The coplanar grid detector Z Coplanar electrodes produce weighting fields maximised close to the contacts The subtracted signal from the 2 sets of coplanar electrodes gives a weighting field that is zero in the bulk The subtracted signal is only due to electrons - generally holes do not enter the sensitive region First applied to CZT detectors by Luke et al. APL 65 (1994) 2884 cathode anode 1 anode 2 h o l e s e l e c t r o n s 10. Paul Sellin, Radiation Imaging Group Depth sensing Coplanar CZT detectors provide depth position information: signal from planar cathode distance D from coplanar anodes and event energy E : SC D x E signal from coplanar anode is depth independent: SA E so the depth is simply obtained from the ratio: D = SC / SA Z. He et al, NIM A380 (1996) 228, NIM A388 (1997) 180 Benefits of this method: -ray interaction depth allows correction to be made for residual electron trapping 3D position information is possible, for example useful for Compton scatter cameras 11. Paul Sellin, Radiation Imaging Group Interaction Depth position resolution from CZT Position resolution of ~1.1 mm FWHM achieved at 122 keV Collimated gamma rays were irradiated onto the side of a 2cm CZT detector - 1.5 mm slit pitch: Z. He et al, NIM A388 (1997) 180 12. Paul Sellin, Radiation Imaging Group CZT pixel detectors In a pixel detector, the weighting field from the small pixel effect acts similarly to a coplanar structure: the pixel signal is mainly insensitive to hole transport depth dependent hole trapping effects are minimised the pixel signal decreases dramatically when the interaction occurs close to the pixel - the missing hole contribution becomes important: A. Shor et al, NIM A458 (2001) 47 13. Paul Sellin, Radiation Imaging Group Correcting for electron trapping Knowing the depth of the interaction, spectral degradation due to electron trapping can be compensated for: Energy vs position plot for 133 Ba spectrum: Resolution @356keV improves from 1.7% FWHM to 1.1% FWHM 14. Paul Sellin, Radiation Imaging Group 3D pixel array detectors A 3D sensitive CZT pixel array has been developed: non-collecting guard rings plus small pixels form a single-polarity sensing device depth information allows pulse height corrections due to trapping and non- uniformity Z. He et al., NIM A422 (1999) 173 The coplanar grid detector acts as a form of 2D strip detector - with all electrodes on one side of the device: small pixel anodes are connected orthogonally across guard ring anode strips relatively complex design V.T. Jordanov et al., NIM A458 (2001) 511 15. Paul Sellin, Radiation Imaging Group CZT/CdTe pixel array detectors Outstanding issues: CZT-compatible flip-chip bonding: low temperature indium or polymer material uniformity and cost for large area arrays - requirement for large area mono-crystalline CZT or CdTe motivation is astronomical X-ray imaging and nuclear medicine gamma ray imaging Goal for astronomy: 20x20mm active area with