G. A. Armantrout

What Can Be Expected from High-Z Semiconductor Detectors?

It has been hoped that high-Z semiconductors would offer efficient ¿-ray detection at or near ambient temperatures with energy resolution significantly better than NaI (T1) scintillators. For use at X-ray energies, this goal has been achieved with both HgI2, CdTe, and GaAs detectors. However, at higher energies (~660 keV) all current detectors have one or more significant deficiencies in terms of attainable volume, charge collection efficiency, and polarization effects. Starting with first principles, all potential compounds which can be formed by the binary combination of elements from the periodic chart were considered as possible detector materials. A rank-ordered listing of the most promising materials for further development is given as well as an assessment of the prospects for future success.

Recent Advances with HgI2 X-Ray Detectors

HgI2 x-ray detectors up to 16 mm3 have been made from single crystals grown from the vapor phase. Crystal growth, detector fabrication, and transport properties are described. Resolutions of 850 eV FWHM at 5.9 keV and 4.3 keV FWHM at 122 keV have been measured at 25° C in air.

Theoretical Band Structure Analysis on Possible High-Z Detector Materials

Theoretical energy band structure calculations have been utilized to investigate several high-Z materials for potential use as ambient temperature radiation detectors. Using the pseudopotential technique, the band structure for HgI2 has been determined and the effective masses of the holes and electrons have been estimated. Theoretical mobilities of the electrons and holes as a function of temperature have been computed for HgI2 and CdTe and are compared to experimental data.

High‐resolution Hgl2 x‐ray spectrometers

Several high‐resolution HgI2 x‐ray detectors have been made from single crystals grown from the vapor phase. Hole and electron transport properties and detector electrical characteristics are described. Measured mobility‐lifetime product for electrons is 8×10−5 cm2/V with a μe of 3 cm2/V sec. The mean energy per electron‐hole pair is 4.33 eV at 122 keV. Detectors up to 1 mm3 are described, and a resolution as low as 850 eV at 300 °K in air has been obtained for 5.9‐keV x rays, which suggests possible x‐ray flourescence detection applications.

Prognosis for High-Z Semiconductor Detectors

Considerable interest has been generated recently in the use of high-Z semiconductors for x-ray spectroscopy. To aid in this study, a Monte-Carlo computational model has been used to simulate x-ray spectral response in semiconductor detectors. The model employs one-dimensional charge collection in an arbitrary electric field profile and includes trapping and electronics system effects. Spectra are calculated for several materials, including HgI2 and CdTe, and are compared to experimental results.

Delay Line Readouts for High Purity Germanium Medical Imaging Cameras

High purity germanium offers excellent potential for use in nuclear medical imaging cameras. A position and energy readout technique using two inexpensive delay lines has been developed for these cameras. Results obtained with a 1-cm2 , 4 mm deep, 5×5 strip high purity germanium detector are 2.1 mm full width spatial resolution, a measured single strip resolution of 0.65 mm full width half maximum (FWHM), a 25 element uncollimated energy resolution of 2.95 keV FWHM, and 2.65 keV FWHM for a single central element at 140 keV.

Two-Detector, 512-Element High Purity Germanium Camera Prototype

A gamma-ray camera consisting of two 3.2×3.2×1-cm3 HPGe detectors has been assembled. Shallow orthogonal grooves define 512 2×2-mm2 elements. Square hole collimators have been fabricated with design parameters that exploit the unique characteristics of the detector. Intrinsic spatial resolution is a square function with 2-mm width, and energy resolution is approximately 2.5% FWHM at 140 keV. Evidently superior images are obtained when this instrument is compared to state-of-the-art scintillation cameras.

Spectrum Degradation Effects in Ge(Li) Detectors

The mechanisms responsible for the degradation of the performance of Ge(Li) detectors (which include the effects of uniform trapping, non-uniform trap distribution, and locally weak collection fields) are considered, and simulated spectra have been computed. Experimental measurements have been made to identify these sources of degradation. The important results include a measurement of the effective capture cross sections of the most important traps and a determination of the main sources of non-uniform trap distribution and weak collection fields in Ge(Li) detectors.

Imaging with a Small Ultra Pure Germanium Gamma-Camera

Semiconductor detector gamma-cameras promise marked improvement in spatial resolution, compared to NaI based system. In addition, solid state systems offer the potential of simultaneous imaging of multiple isotopes, or polychromatic nuclides. Beciuse semiconductor systems offer improved resolution, and expand the spectrum of radionuclides applicable to diagnostic imaging, their impact on nuclear medicine will be significant.

A High-Efficiency Multidetector System for Tumor Scanning

A high-efficiency detector system developed especially for medical imaging has three speciall cut Ge(Li) coaxial detectors (total volume 249 cm3). At 122 keV, the peak efficiency is 93% of that of a 7.6 × 7.6 cm NaI (T1) detector. Degradation of the paralleled energy resolution is avoided and resolution is improved by 35% over that of conventional output-summing techniques by gating the detector outputs. In effect this multiplexes them to a single line output.