M. Amann

Characterization of the HgI2 surface layer after KI etching

Abstract The formation and properties of the chemical complex which forms on the HgI 2 surface during etching in KI have been investigated. The amount of complex, identified as [KHgI 3 ,H 2 O], remaining on the surface has been determined as a function of the KI concentration, the time of etching in KI and the time of rinse in water. This complex has been found to be very unstable and strongly hygroscopic. Its resistivity after drying is about 10 5 times lower than that of bulk HgI 2 .

Current instability in mercuric iodide devices

Abstract The evolution with time of the leakage current in mercuric iodide devices has been investigated. It is shown that this evolution can be closely associated with the accumulation of an electric charge which modifies the electric field in the bulk. Under particular biasing conditions, release of this charge leads to the rise of a current peak after a time delay which depends on the experimental parameters. The evolution of this peak has been investigated as a function of injected charge, voltage and temperature. As this phenomenon can be correlated with the short time behaviour of the devices, it can be used to characterize the materials devoted to the preparation of devices such as nuclear radiation detectors.

Influence of water and water vapour on the characteristics of KI treated HgI2 detectors

Abstract After being cleaned using a potassium iodide solution in water followed by a water rinse, the surface of mercuric iodide is covered by a chemical complex identified as being KHgI3·H2O. This compound can adsorb large quantities of water and its electrical properties are strongly sensitive to water and water vapour. The consequences on the manufacturing and storing conditions (especially the relative humidity), of mercuric iodide-based devices are therefore of great concern. They are illustrated by the study of the electrical and spectrometric properties of HgI2 nuclear radiation detectors.

Photoetching effects in mercuric iodide

Abstract Photoetching effects during the chemical dissolution of mercuric iodide in an iodide solution may produce noticeable surface roughening while the mean etch rate, to a first approximation, is not modified. As this phenomenon is directly related to the formation during etching of a surface chemical complex that absorbs light, the presence or not of light is an important factor in the case of KI and NH4I etching. In contrast, the surface roughness produced after HI etching is essentially light insensitive.

Influence of humidity on the electrical properties of potassium triiodomercurate

The electrical properties of potassium triiodomercurate crystals (KHgI3·H2O) are considerably modified in the presence of water vapour. As a result, this material presents a very high sensitivity to relative humidity (RH). Resistance changes of several orders of magnitude have been observed in triiodomercurate samples (length, cm; section, mm2) within a few tenths of second in the range 25–65% RH under a dc electric field of less than 0.5 V/cm. This behavior has been attributed to the increase of the ionic conductivity in a thin surface layer surrounding the high resistivity core of the samples. This could be induced by fast triiodomercurate dissociation in the presence of water molecules.

Prototype of a mercuric iodide X-ray camera: Preliminary results

Abstract We present a prototype of an X- and γ-ray camera based on a semiconductor crystal of mercuric iodide (HgI 2 ) having a cross-sectional area of 10 × 10 mm 2 and a thickness of 0.5 mm. Two orthogonal sets of 20 charge-collecting μstrips are deposited on the two opposite surfaces of the detector giving a pixel size of 0.5 × 0.5 mm 2 . Using commercial hybrid preamplifiers several experimental tests have been performed in order to assess the performance of this device. Preliminary results on the spectroscopic capabilities together with an evaluation of the attainable spatial resolution are presented.

HgI2 X- and γ-ray camera with spectrometric capabilities

Abstract A HgI 2 microstrip two-dimensional array with total area of 10 × 10 mm 2 and a thickness of 0.5 mm has been tested. By using a coincidence-anticoincidence technique the effective pixel area has been identified and spectroscopic performance and some indications about the spatial resolution have been extracted. Values like 25% (FWHM) at 6 keV and 8.5% at 32 keV for the energy resolution have been obtained. For the X-ray energies used, ranging from 32 to 81 keV, the absence of an intense lateral diffusion of the produced charge allows performing the position readout in a quantized fashion, where the event position is that of the struck strip, leading to a spatial resolution corresponding to the pixel size.

A resistive charge division based microstrip detector for X- and γ-ray spectrometers

Abstract A new-design linear array detector with Pd strips deposited onto a Ge surface resistive layer covering a HgI2 bulk has been constructed. The analog readout of the strip outputs and a centroid finding algorithm allow one to identify the impact position of 241Am γ-rays (Eγ = 60 keV) with a spatial resolution of ≈ 100 μm. This device is developed with the aim to make available a high efficiency position-sensitive detector with the maximum possible active detection area to be used in applications requiring good spatial and energy resolution (for example a Bragg X-ray spectrometer). Project calculations, fabrication techniques of a prototype and preliminary tests are reported and discussed.

High spatial resolution HgI2 linear array based on resistive charge division

Newly designed Hg12 linear arrays based on resistive charge division, obtained by depositing a Ge surface resistive layer (with typical interstrip resistances of ≈ 4 and 6 MΩ) between the read-out Pd strips, have been fabricated. These detectors, coupled with specially designed front-end electronics, have shown a high detection efficiency over a large active detection area and an energy resolution of 20% for X-ray energy Ex = 22 keV, estimated to become 100 keV. Further, a spatial sensitivity of ≈ 10 μm and a spatial resolution of ≈ 100 μm have been obtained at 30 keV, by making use, for a fine spatial characterization of the devices, of a laser beam spot 50 μm in diameter with wavelength tuned to match the crystal band gap (582 nm). These devices could be successfully employed in basic research (for example Bragg X-ray spectrometry) and for radiological and high-energy (Ex > 100 keV) astrophysical applications.

HgI{sub 2} two-dimensional arrays based on resistive charge division readout

Newly designed HgI{sub 2} two-dimensional arrays based on resistive charge division, obtained by depositing a Ge surface resistive layer between the readout Pd strips, have been fabricated. These detectors, coupled with very low noise preamplifiers, have shown a high detection efficiency over a large active area and an energy resolution of 15% for X-ray energy E{sub x} = 60 keV ({sup 241}Am), becoming {approx} 9% at E{sub x} = 122 keV ({sup 57}Cl). Further, a spatial sensitivity of {approx} 5 {micro}m and a spatial resolution of {approx} 40 {micro}m in both the directions parallel to the anode and to the cathode strips have been obtained at an energy equivalent to {approx} 40 keV, by making use, for a fine spatial characterization of the devices, of a laser beam spot with wavelength tuned to match the crystal bandgap (582 nm). These devices, with optimized cathode layer resistances, could be successfully employed in basic research (for example Bragg X-ray spectrometry) and for imaging in radiological and space applications, at least in the important energy range 40 < E{sub x} < 120 keV.