The modes of operation of semiconductor detectors are reviewed, together with the influence of charge carrier collection in developing a signal pulse for spectroscopic applications. Because of the importance of charge trapping in many semiconductors of interest in the fabrication of room temperature radiation detectors, the effects of incomplete charge collection are quantified. Calculated results are presented for the expected pulse height and energy resolution under a variety of charge collection conditions.

In the paper the development and usage of radiation detection systems based on room temperature semiconductors are described. The detection systems are intended for use by safeguards inspectors to verify nuclear material. Because of their high intrinsic efficiency, these detectors have advantages compared to classical germanium or Nal detectors. Several measurement tasks which cannot be handled in an optimum manner by using standard detectors have been solved. The main problem in the application of room temperature semiconductor detectors is the limitation in the availability of large detector volumes.

High-purity germanium (HPGe) for gamma-ray spectroscopy is a mature technology that continues to evolve. Detector size is continually increasing, allowing efficient detection of higher energy gamma rays and improving the count rate and minimum detectable activity for lower energy gamma rays. For low-energy X rays, entrance window thicknesses have been reduced to where they are comparable to those in Si(Li) detectors. While some limits to HPGe technology are set by intrinsic properties, the frontiers have historically been determined by the level of control over extrinsic properties. The point defects responsible for hole trapping are considered in terms of the “standard level” model for hole capture. This model originates in the observation that the magnitude and temperature dependence of the cross section for hole capture at many acceptors in germanium is exactly that obtained if all incident s-wave holes were captured. That is, the capture rate is apparently limited by the arrival rate of holes that can make an angular-momentum-conserving transition to a s ground state. This model can also be generalized to other materials, where it may serve as an upper limit for direct capture into the ground state for either electrons or holes. The capture cross section for standard levels σS.L. is given by

where g is the degeneracy of the ground state of the center after capture, divided by the degeneracy before capture. Mc is the number of equivalent extrema in the band structure for the carrier being captured, mo is the electronic mass, m* is the effective mass, and T is the temperature in degrees Kelvin.

The advancement of room-temperature mercuric iodide detector technology to the point of spectroscopy-grade energy resolution coupled with the advancement in long-term stability and the high efficiency of these detectors now make it's use in large scale array systems for x- and gamma-ray imaging applications viable. Camera design criterion differ depending upon the application. A HgI2 gamma-ray camera developed at EG&G Energy Measurements has been used for imaging with various apertures, and the energy dispersive imaging aspects have been examined. Concepts for applications such as medical diagnostic imaging and a satellite based telescopes for x- and gamma-ray mapping have been objectified.

In this paper we describe the development of a robotic system that uses Mercuric Iodide (HgI2) sensors to create a map of radiation intensity in the environment. The sensors are mounted at the end-effector of a PUMA 560 manipulator. Data from these sensors is used to compute the radiation intensity. A control system is developed that moves the end-effector along constant intensity contours. The experiments use light sources (instead of radiation sources) and demonstrate the efficacy of such a system to create radiation maps.

A method has been developed to analyze mercuric iodide (HgI2) for elemental contamination using Inductively Coupled Plasma/Mass Spectroscopy (ICP/MS). This paper will discuss the ICP/MS method, the effectiveness of purification schemes for removing impurities from Hgl2, as well as preliminary correlations between HgI2 detectorperformance and elemental contamination levels.

The Ion Micro-Analysis Group (IMAG) in Livermore conducts quantitative trace elemental analysis with PIXE and depth profiling with IBS using an MeV ion microbeam. The system has the capability to produce two-dimensional trace element and IBS images. PIXE analyses have been conducted on HgI2 and PbI2 crystals and detector materials in order to identify and quantify near surface trace contaminants. IBS measurements have been conducted to investigate elemental depth distributions in various materials. The results of measurements on several different samples are reported and a discussion of factors affecting quantitative in vacuo microanalysis of these materials is presented.

A short review will be given of the methods by which mercuric iodide is prepared and purified to obtain material suitable for the growth of single crystals. The method used in our laboratory to grow single crystals up to 1,000 grams in weight from the vapor will be discussed. The effects of gravity on the growth process will be described.

A crystal growth system suitable for operation in the reduced gravity environment of space was designed, and crystal growth experiments were performed during the flights of Spacelab 3 (April 1985) and the International Microgravity Laboratory (IML-1) (January 1992). The structural quality and electronic properties of the ground-based and spacegrown crystals were compared, and the results will be presented.

Recently, substantial progress has been made in the development of HgI2 detectors and associated electronics for space instrumentation. The many technical challenges in the instrument development involving spectroscopy performance, power consumption, weight, size, operation over wide ranges of temperature and ambient gas pressure, and others have been overcome. Descriptions of instrumentation under development such as: x-ray and alpha backscattering spectrometer, scanning electron microscope and particle analyzer, Mössbauer spectrometer, and gamma-ray spectrometer are given.

A laboratory XRF analysis system employing an HgI2 spectrometer detector has been built. The system consists of an x-ray generator tube, a rotating carousel holding multiple XRF targets, and a probe head containing the HgI2 detector. Tests have been performed on several samples, under vacuum, helium and nitrogen ambient atmospheric conditions. This is of particular interest, since the applications for such an XRF system include operation under different ambient conditions. For applications in terrestrial and space planetary exploration, the expected temperatures and gas ambient will vary widely. Comparative analyses using the various gases were made on spectra obtained from standard geological samples.

A HgI2 linear array detector with a surface resistive layer between the readout strips has been constructed with the aim of achieving the maximum possible active detection area and a spatial resolution of ≈ 50 µm, by using an analog readout of the strip outputs and a centroid finding algorithm.

This device has been developed as a high efficiency position-sensitive detector for use in a Bragg X-ray spectrometer, designed for detecting X-ray photons with energies up to ≈ 30 keV.

Calculations performed by taking into account the various sources of electronic noise have allowed the optimization of the interstrip distance and resistance.

Fabrication techniques and preliminary results on energy and spatial resolution are reported and discussed.

The processing steps associated with purification of source material, crystal growth, and attachment of electrical contacts can introduce defects into mercuric iodide (HgI2) that degrade the performance of detectors. We have employed low-temperature photoluminescence (PL) spectroscopy to study radiative recombination centers in the interfacial region between a thin semitransparent film of silver and mercuric iodide. The Ag film was found to introduce a new broad emission band centered at 5490 Å in the photoluminescence spectrum of HgI2. This PL feature can be used as a signature to identify the existence of Ag as a contaminant in HgI2 crystals and detectors. Experiments were also conducted on mercuric iodide surfaces that had been doped with silver, and the results showed that Ag is a rapid diffuser in bulk HgI2. Detectors with silver electrodes were also fabricated and tested using an americium-241 gamma-ray source. Large increases in the leakage currents were observed for the Ag-doped HgI2 devices, indicated that Ag impurities are electrically active in HgI2. These measurements show that silver is unacceptable as an electrode material for mercuric iodide x-ray and gamma-ray detector applications. In addition, they reveal that caution must be taken during handling of mercuric iodide source material, crystals, and detectors to avoid contact with silver, silver compounds, or with any material that contains silver as a contaminant.

The incorporation of extrinsic defects into mercuric iodide substrates during detector fabrication can be extremely detrimental to device performance. In particular, extrinsic defects can act as trapping and recombination centers, and they can reduce charge collection efficiencies, decrease gt product, or contribute to polarization effects in nuclear detectors. In this paper we present results of processing, photoluminescence, and electromigration experiments that clearly show that extrinsic defects can be incorporated in mercuric iodide during detector fabrication. By observing the luminescence features characteristic of Cu and Ag in mercuric iodide, we show that both these materials are taken up by mercuric iodide crystals during etching with KI if the etching solution is contaminated with these elements. The migration of material from contacts into the crystal, as shown by resistance measurements, is also presented. We infer from this work that other defects which are detrimental to device performance may also be incorporated in mercuric iodide if insufficient care is taken during device fabrication. Suggestions are therefore made as to some of the precautions that must be taken in order to realize the highest quality detectors.

We present theoretical studies of electronic structures, optical responses, and phonon modes of undoped HgI2 in its red tetragonal form. The electronic band structure is studied via an empirical nonlocal pseudopotential model, including the spin-orbit interaction. The electron and hole effective masses, optical matrix elements for interband transitions, and complex dielectric function are computed. Excitonic effects on the absorption coefficient near the fundamental band gap are included within the effectivemass approximation. The resulting absorption spectra and their polarization dependence are compared with experiment with favorable agreement. The phonon modes of HgI2 are studied with a microscopic model and a good fit to the neutron scattering data is obtained.

During the past four years the yield of high-resolution mercuric iodide (HgI2) gamma ray spectrometers produced at EG&G/EM has increased dramatically. Data are presented which demonstrate a strong correlation between starting material and spectrometer performance. Improved spectrometer yields are attributed to the method of HgI2 synthesis and to material purification procedures. Data are also presented which show that spectrometer performance is correlated with hole mobility-lifetime products. In addition, the measurement of Schottky barrier heights on HgI2 spectrometers has been performed using I-V curves and the photoelectric method. Barrier heights near 1.10 eV have been obtained using various contacts and contact deposition methods. These data suggest the pinning of the Fermi level at midgap at the HgI2 surface, probably due to surface states formed prior to contact deposition.

Mercuric iodide detectors are leading candidates for room-temperature radiation detection applications. The inherently reactive nature of mercuric iodide limits the number of materials suitable for fabrication of electrical contacts. The theoretical stabilities of elemental contact materials on mercuric iodide were evaluated at 25°C. Additionally, the stabilities of transparent conductive compounds, for photodetector applications, were studied. Calculations were based on Gibbs free energy data, estimates and a series of hypothesized reactions with mercuric iodide. Leading candidate materials were identified and compared to experimental results.

At present a 10% KI solution is commonly used as an etchant for HgI2 crystals. Recent photoluminescence (PL) spectra from such etched samples show that impurities contained in the KI solution dope the HgI2 during the etching process. Some of these dopants are known to cause carrier trapping in the detectors fabricated from the material. Thus, it is desirable to find an alternative etchant that does not create new defects in HgI2. Etchants studied here include deionized water, 10% potassium iodide (KI), methanol, and acetone. The results of acetone as an etchant reveal only small changes in the PL spectra after etching. Methanol etching causes the incorporation of a deep radiative recombination center in HgI2.

X-ray topography and rocking curve experiments were performed on α-mercuric iodide samples. As-grown crystals were examined for intrinsic defects and crystallinity. Orientation of certain defects depends on the direction of crystal growth. The propagation of as-grown crystalline features was documented. The extent of crystal damage introduced during various steps of device fabrication such as sawing, polishing, etching and contact deposition was explored. Coefficients of linear thermal expansion of α33 = 54 ± 5 (10−6/°C) along the tetragonal c-axis, \001] direction and ±ll = 11 ± 4 (10−6/°C) in the \100] direction were measured.

The surface modifications that occur on °-mercuric iodide surfaces during the fabrication process are crucial for the development of a low noise, high resolution X-ray spectrometer. In this study, we discuss the effects of surface treatments on the dark currents and the results are correlated with surface morphology studies that were carried out by the atomic force microscopy (AFM) technique.

The goal of this work is to compare and analyze the transient responses to X-ray pulses of different cadmium telluride detectors:

- CdTe: Cl grown by the vertical Bridgman method (B.).

- CdTe: Cl grown by the Travelling Heater Method (T.H.M.).

- Cd0,8 Zn0,2 Te grown by the High Pressure Bridgman method (H.P.B.).

Photoconductive detectors were subjected to very short (20 ns) 100 keV X-ray pulses and short (4 µs) 2 MeV pulses. The transient response of the photoconductors CdTe: Cl and CdZnTe under high X-ray energy beam were characterized in terms of sensitivity, linearity and dynamic range vs different parameters such as bias voltage and X-ray beam fluence. Bridgman and T.H.M. detectors show similar behaviors while H.P.B. detectors which are highly resistance show shorter transient behavior after the 20 ns pulses irradiations and larger afterglow after the 4 µs pulses irradiations.