A.J. Dabrowski

Mercuric iodide (HgI2) platelets for x-ray spectroscopy produced by polymer controlled growth

Abstract The low temperature red form of mercuric iodide has been grown by a new chemical transport method which introduces organic monomers or polymers during the crystal growth process. Resulting crystals are in the form of platelets which are more directly useful in radiation detector device application. Platelets near one centimeter in width and 200 μm in thickness have been grown in periods of a few days using only 99.9% (unpurified) starting material. Measurements of X-ray and gamma ray energy resolution from detectors fabricated from platelets have yielded a 1.15 keV (FWHM) value for the 59.5 keV Am-241 line and 400 ev (FWHM) for the 5.9 keV Fe-55 line. These are again among the highest resolution values ever measured for HgI2. Electrical carrier transport property values of HgI2 so grown equal the best values previously measured from vapor grown material.

A High Resolution Detection System for Positron Tomography

The feasibility of using a hybrid detection system, consisting of multiple narrow bismuth germanate (BGO) crystals coupled to one photomultiplier for coincidence timing, and individual mercuric iodide (HgI2) photodetectors coupled to each BGO crystal to identify crystals of interaction, was investigated for application to positron tomography. The match between the emission spectrum of BGO and the photoresponse spectrum of HgI2 in combination with recently developed HgI2 x-ray detector and low noise preamplifier technology provided the appropriate high sensitivity, low noise environment required to detect the low levels of scintillation light produced in the absorption of 511 keV gamma-rays in BGO. Energy resoluton of 24% for 511 keV was obtained with the BGO-HgI2 detector, compared to 18% for BGO with a PMT. Coincidence resolving time for the HgI2-BGO detector was measured to be 105 nanoseconds. This performance of a non-optimized detector system is nominally adequate for appropriately designed tomographs.

Toward the Energy Resolution Limit of Mercuric Iodide in Room Temperature Low Energy X-Ray Spectrometry

Development of mercuric iodide nuclear detectors fabricated from vapor grown single crystals continues with emphasis in our work on their capabilities as room temperature operable low-energy X-ray spectrometers. The influence of trapping in the detector crystal, electronic noise and statistical fluctuations on energy resolution of the detectors are discussed. A room temperature energy resolution value of 490 eV (FWHM) has been measured for Fe-55 (with a corresponding 420 eV value of electronic noise) using a 2mm2 HgI2 detector. The K¿ and Kß lines of Fe-55 at 5.9 keV and 6.5 keV have been separated, to our knowledge, for the first time at room temperature. A new Fano-factor value of 0.46 has been established for mercuric iodide. In the room temperature HgI2 detector - FET preamplifier combination the latter is the largest contributor to the width of the spectral line in the low energy region. Our efforts are therefore directed toward reducing room temperature preamplifier noise. A room temperature preamplifier noise level of 360 eV (HgI2) has been attained. We are currently investigating use of low noise, low capacitance FET devices in a more optimized preamplifier configuration which should further reduce the noise value (H.E. Kern, IEEE Trans. Nucl. Sci. NS-17). Also under investigation is the effect of Peltier cooling of the input stage of the FET preamplifier.

Progress in energy resolution of mercuric iodide (HgI2) X-ray spectrometers☆

Abstract A brief discussion of the main factors which effect the energy resolution of HgI 2 X-ray spectrometers is presented. A short review of the progress made in the process of improving the energy resolution is given. The latest experimental results obtained with a state-of-the-art cryogenically cooled low-noise preamplifier are shown. Ways of lowering the currently attainable room-temperature electronic noise level other than cryogenic cooling are discussed.

Mercuric Iodide Photodetectors for Scintillation Spectroscopy

We have measured the responses to 137Cs (662 keV) of both a 1-inch-diam by 2-inch-thick NaI(Tl) scintillator optically coupled to a 1-inch-diam by 800-¿m-thick mercuric iodide (HgI2) photodetector, and a 1-cm-diam by 1-cm-thick CaWO4 scintillator coupled to a 1.3-cm-diam by 600-¿m-thick HgI2 photodetector. Best spectral resolution to 137Cs was 7.8% FWHM for the NaI(Tl)-HgI2 and 12.5% FWHM for the CaWO4-HgI2 detectors; peak-to-valley ratios were 26:1 and 16:1, respectively. HgI2 detectors operate at room temperature and their use in scintillation spectroscopy presents the ultimate miniaturization of scintillation detectors, limited mainly by the size of the scintillation crystal.

Scintillation spectrometry with HgI2 as the photodetector

Abstract A mercuric iodide (HgI2) photodetector has been used to detect light pulses from gamma-ray and alpha-particle interactions in scintillators. The photocurrent response of an HgI2 photodetector to light has been measured and found to be favorable for detecting light from most scintillators, which have their maximum emission between 400 and 560 nm. Energy spectra for alpha particles or gamma-rays from combinations of an HgI2 photodetector with various scintillators are presented. The energy resolution of the photopeak from annihilation gamma-rays is 19% with a CsI(Tl) crystal and 24% with a BGO crystal. Fabrication of HgI2 photodetectors and their optical coupling to a scintillator crystal will be described. The advantages of this new solid-state radiation detector compared to the combination of scintillator and PMT, and some proposed applications, are discussed.

A Novel Radiation Detector Consisting of an HgI2 Photodetector Coupled to a Scintillator

HgI2 photodetectors have been used in conjunction with CsI(Tl) and BGO to detect the light pulses from gamma rays and alpha particles. The photocurrent response to light of a typical HgI2 photodetector is presented and discussed. The spectral response is appropriate for most important scintillators, which have their maximum emission between 400 and 560 nm. Energy spectra obtained with an HgI2 photodetector coupled to a CsI(Tl) scintillator crystal are presented for gamma rays from 137Cs, a 68Ga positron source, 241Am and 99mTc, as well as for the K x-rays from Pb. The photopeak energy resolution value for 511 keV annihilation gamma rays with the CsI(Tl)-HgI2 combination was about 10%. Spectra obtained with an HgI2 photodetector coupled to a BGO scintillator crystal are presented for the annihilation gamma rays from a 68Ga positron source (19% photopeak resolution) and the alpha particles from a 244Cm source. Estimates of the quantum efficiencies for an HgI2 photodetector coupled to CsI(Tl) and BGO scintillator crystals give values in excess of 70%. A brief discussion is given of the limits on energy resolution set by the electronic noise. Potential applications of this novel radiation detection device and the advantages over photomultiplier-based devices are discussed.

A Study of Low-Noise Preamplifier Systems for Use with Room Temperature Mercuric Iodide (Hgi2) X-Ray Detectors

An analysis of different preamplification systems for use with room temperature mercuric iodide x-ray detectors has been performed. Resistor-, drain-, and light-feedback preamplifiers have been studied. Energy resolution values of 295 eV (FWHM) for an Fe-55 source (5.9 keV) and 225 eV (FWHM) for a pulser have been obtained with both the detector and the input FET at room temperature using a pulsed-light feedback preamplifier. Improvement in energy resolution by cooling the input FET using a small Peltier element has been discussed.

Hybrid Mercuric Iodide (HgI2) - Gadolinium Orthosilicate (GSO) Detector for PET

Gadolinium orthosilicate (GSO), a high density (6.71 g/cm3), high effective Z (59) scintillation detector with faster decay time (60 ns) and higher light yield than bismuth germanate (BGO), was evaluated as a replacement for BGO for a hybrid detection system for high resolution Positron Emission Tomography (PET). The detection system consists of multiple GSO detectors attached to a single photomultiplier tube (PMT) and a HgI2 photodetector attached to each GSO crystal. The PMT signal provides coincidence timing and energy discrimination and the photodetector signal identifies the crystal of interaction. GSO light yield was 1.7 times that of BGO with energy resolution consistent with improved photon statistics (17.8 to 13.8% FWHM). Resolution of GSO coupled to a HgI2 photodetector was 13.6% FWHM. Coincidence timing was 2.3 ns FWHM. Timing between PMT and HgI2 was 136 ns FWHM.

Large Area Mercuric Iodide Photodetectors

Results of an investigation of large area mercuric iodide (HgI2) photodetectors are reported. Different entrance contacts were studied, including semitransparent metallic films and conductive liquids. Theoretical calculations of electronic noise of these photodetectors were compared with experimental results. HgI2 photodetectors with active area up to 4 cm2 were matched with NaI(Tl) and CsI(Tl) scintillation crystals and were evaluated as gamma-radiation spectrometers. Energy resolution of 9.3% for gamma radiation of 511 keV with a CsI(Tl) scintillator and energy resolution of 9.0% for gamma radiation of 622 keV with a NaI(Tl) scintillator have been obtained.