A summary is presented of the present status of several compound semiconductor radiation detectors, including detectors fabricated from GaAs, HgI2, CdTe, Cd1−xZnxTe and PbI2

Room-temperature compound semiconductor radiation detectors

A /spl gamma/-ray detector, designed around a silicon drift chamber having a diameter of 3 mm and coupled to a CsI(Tl) scintillator of the same diameter and 10 mm thick, has been realized and extensively tested. This detector may operate over a wide energy band with a minimum energy threshold that may be set around 10 keV at room temperature or at around 4 keV at 0/spl deg/C. At the typical energy of 662 keV used to characterize a /spl gamma/-ray spectrometer, an energy resolution of 4.4% full-width at half-maximum has been measured at room temperature. This detector shows high energy resolution over the full energy range explored, 6 keV-1.27 MeV, and to our knowledge the results reported are the best ever obtained with a scintillator.

Gamma ray spectroscopy with CsI(Tl) scintillator coupled to silicon drift chamber

The authors present initial results for a position-sensitive photodiode/BGO (bismuth germanate) detector for high-resolution, multilayer positron emission tomography (PET). Position sensitivity is achieved by dividing the 3 mm*20 mm rectangular photosensitive area along the diagonal to form two triangular segments. Each segment is individually connected to a low-noise charge amplifier. The photodiodes and crystals are cooled to -100 degrees C to reduce dark current and increase the BGO signal. With an amplifier peaking time of 17 mu s, the sum of the signals (511-keV photopeak) was 3200 electrons with a full width at half maximum (FWHM) of 750 electrons. The ratio of one signal to the sum determined the depth of interaction with a resolution of 11 mm FWHM. >

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Initial characterization of a position-sensitive photodiode/BGO detector for PET

A detailed study of the potential of CsI(Tl)—photodiode detectors for applications in low-level spectroscopy was carried out. The energy resolution of small crystals (few cm 3 ) for γ-rays in the MeV range can be parametrized as ΔE (FWHM)/ E = 5% × E [MeV] −0.69 . In kg size detectors, where resolution is limited by electronic noise, the use of new large-area low-capacitance pin Si photodiodes led to a significantly improved energy precision of ΔE (FWHM) = 150 keV, nearly independent of energy. Unfavourable light collection geometries, like in large-acceptance well detectors, do not substantially degrade the energy measurement. Scintillation light yields vary slowly with temperature and time so excellent long-term stability can be expected. For the first time a systematic investigation of inherent crystal radioactivity was possible due to the improved energy resolution obtained with large crystals. Six α-active radionuclides of the Th-decay-chain were identified. The isotopes are in equilibrium with each other but not in secular equilibrium with 232 Th. The total α-activity varies from crystal to crystal between 2.4 and 5.5 decays/h cm 3 ) corresponding to 228 Th concentrations of 1.1–2.5×10 −18 g/g. Activity from 238 U-chain isotopes is not sent at a level two orders of magnitude below the Th-activity. Prospects are discussed for further improved energy resolution and for applications in low-level spectroscopy, specifically for a search for 100 Mo double beta decay.

Prospects of CsI(Tl)-photodiode detectors for low-level spectroscopy

Abstract A detailed study of the potential of CsI(Tl)—photodiode detectors for applications in low-level spectroscopy was carried out. The energy resolution of small crystals (few cm 3 ) for γ-rays in the MeV range can be parametrized as ΔE (FWHM)/ E = 5% × E [MeV] −0.69 . In kg size detectors, where resolution is limited by electronic noise, the use of new large-area low-capacitance pin Si photodiodes led to a significantly improved energy precision of ΔE (FWHM) = 150 keV, nearly independent of energy. Unfavourable light collection geometries, like in large-acceptance well detectors, do not substantially degrade the energy measurement. Scintillation light yields vary slowly with temperature and time so excellent long-term stability can be expected. For the first time a systematic investigation of inherent crystal radioactivity was possible due to the improved energy resolution obtained with large crystals. Six α-active radionuclides of the Th-decay-chain were identified. The isotopes are in equilibrium with each other but not in secular equilibrium with 232 Th. The total α-activity varies from crystal to crystal between 2.4 and 5.5 decays/h cm 3 ) corresponding to 228 Th concentrations of 1.1–2.5×10 −18 g/g. Activity from 238 U-chain isotopes is not sent at a level two orders of magnitude below the Th-activity. Prospects are discussed for further improved energy resolution and for applications in low-level spectroscopy, specifically for a search for 100 Mo double beta decay.

Prospects of CsJ(Tl)-photodiode detectors for low level spectroscopy

Scintillation spectrometers with mercuric iodide as the photodetector were evaluated for their spectral response to gamma rays in the energy range of 60 keV to 1332 keV. A /sup 137/Cs (662-keV) photopeak resolution of better than 5% FWHM was obtained due to the low noise characteristics and high quantum efficiency of the mercuric iodide photodetector. Photopeak broadening due to the scintillator, electronic noise, and statistical noise vs. gamma energy are detailed. >

J.M. Markakis

Papers

Mercuric iodide photodetector-cesium iodide scintillator gamma ray spectrometers