The development of large area xenon drift chambers as imaging systems for the advanced Gamma-Ray Astronomy Telescope Experiment (AGATE), sensitive in the energy range 20 MeV - 100 GeV, is presented here. AGATE is visualized as the successor to the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory, and will add to the wide range of important results currently being obtained by EGRET. Experiments were carried out with a laboratory prototype consisting of a stack of sixteen 1/2m X 1/2m active area drift chambers using both xenon and argon gas mixtures. The spatial resolution of the drift chamber stack was measured with a multi-wire readout plane using atmospheric muons traversing the active volume. A spatial resolution of about 0.23 mm was measured with drift chambers using xenon- methane gas mixtures. The experiments with the argon-isobutane gas mixtures yielded a spatial resolution of about 0.14 mm.

The charge collection and energy resolution in liquid rare gas ionization chambers can be improved by doping the liquid with photoionizing molecules. We have observed the remarkable improvement, especially at a low electric field region, in liquid xenon doped with triethylamine (TEA) and trimethylamine (TMA). Furthermore, in order to apply this effect to real gamma-ray detectors, time stability and temperature effect have been investigated.

The W_ph values in liquid Ar and Xe, which are defined to be the average energy required to produce an electron-ion pair by ionizing radiation, are theoretically estimated using the data for 1 MeV electrons, alpha-particles, fission fragments and relativistic heavy ions and are compared with the experimental values recently obtained.

Construction of cylindrical ionization chamber with shielding mesh is considered. The chamber has sensitive volume 2 liters filled by xenon with pressure about 50 atm. Main characteristics of this detector such as energy resolution and efficiency of gamma-rays with energy 0.1 - 2.0 MeV are presented. It is shown that the detector energy resolution for Eequals0,662 MeV line at optimal electric field strength in the chamber is about 4%. Comparison of experimentally measured characteristics of this detector and standard scintillator NaI(T1) is fulfilled. Applications of the high pressure xenon cylindrical ionization chamber with a shielding mesh in geology, geophysics and diagnostic of oil and gas wells are considered.

The experiment LXe-CAT (Liquid Xenon-Coded Aperture Telescope), which we have proposed for (gamma) -ray astrophysics observations in the 300 keV - 10 MeV energy range, uses a Liquid Xenon Time Projection Chamber (LXe-TPC) as a position sensitive (gamma) -ray detector, and a coded aperture mask to provide a telescope with an angular resolution of 30 ft over a field of view (FOV) of 28 degree(s) X 28 degree(s). The point source localization accuracy is about 2 ft for a 10(sigma) source. To demonstrate the spectroscopy and imaging capabilities of the LXe-TPC as a (gamma) -ray detector we have designed, built and are currently testing a 10 liter prototype with a sensitive area of approximately 400 cm² and an active liquid depth of 5 cm. Studies on stability, spectroscopy and imaging response of this detector to various (gamma) -ray sources will be presented. The unique capability of the LXe-TPC to identify and reject background events based on Compton kinematics reconstruction has also been demonstrated.

We review the status quo and the history of development of Ce doped GSO (Gd₂SiO₅:Ce) single crystals, a typical heavy and fast scintillator of large light output. GSO:Ce has 6.7 g/cm³ in density, 1.38 cm in radiation length and 30-60 ns in decay constant. The light output is two times as large as BGO. It shows, overall, the best performance among all the scintillators developed until now. The radiation damage caused by gamma ray from Co, proton in GeV order and thermal neutrons were studied on GSO:Ce crystals with various Ce concentrations. A description is given on the luminescence mechanism by Ce in GSO and growth technology for crack-free large GSO crystals. We believe that GSO crystals are scintillators having all the excellent characteristics required for gamma ray detectors.

This report reviews the design characteristics of crystal gamma ray detectors for high energy physics. The unique physics capability of these detectors is the result of their excellent energy resolution, uniform hermetic coverage and fine granularity. To maintain crystal's resolution in situ radiation hardness is a principle requirement. The performance of various heavy crystal scintillators is discussed. A technical approach to solve radiation damage problem by optical bleaching in situ is elaborated.

This paper reviews the progress achieved in recent years in the field of cadmium telluride X and gamma-ray detectors. Both material and structure of detectors are considered as well as recent progress in pulse handling.

Simulations of spectroscopic performances of CdTe detectors by means of a Monte Carlo code have been recently reported. The computer code was demonstrated to be an extremely useful tool in simulating the spectral response of CdTe to gamma rays and in understanding the detailed effects of physical and electronic parameters on the experimental performances. In the present work, the attention is focused on the various aspects of the CdTe response to Co gamma rays, in an energy range which is extremely interesting for practical applications of CdTe spectroscopy. Spectra are presented for different experimental conditions particularly in connection with the rise time distributions and with the spatial distribution of the electric field in the detector. A precise correlation is established between the energy distribution and the pulse rise time distribution: the spectra obtained by a selection of a particular rise time interval are presented and discussed, and the conditions for reaching the best energy resolution are indicated. These conditions depend on the material quality and on the electrical field distribution.

Room temperature cadmium zinc telluride (CdZnTe) and mercuric iodide (HgI₂) semiconductor hard X-ray detectors are currently being evaluated at NASA Goddard Space Flight Center for use in future balloon and satellite applications. PoRTIA, a small engineering prototype hard X-ray (20 - 150 keV) balloon instrument will contain both a CdZnTe and a HgI₂ detector, each 6.5 cm² x .15 - .2 cm and sharing the same 5 degree(s) field-of-view. PoRTIA will be launched from Alice Springs, Australia in the Spring of 1995 as a piggyback instrument aboard the GRIS balloon payload. PoRTIA will provide valuable information about detector efficiency, durability and material dependent detector background components at balloon altitudes as it observes the Crab Nebula. In addition, a CdZnTe research and development program has been initiated to develop the capability to produce improved CdZnTe detectors for astrophysics applications. The work at Goddard continues in an effort to develop CdZnTe detectors with improvements in electronics, contacts and packaging methods.

The paper presents an application of photodiodes of typical series production, specially selected for detector array in a X, gamma probe for checking the nonuniformity of structures. The probe is used in connection to a measuring unit, as a portable instrument.

Cadmium telluride (CdTe) constitutes an unique material for X-ray imaging, due to its outstanding properties: room temperature operation, high atomic numbers giving high photoelectric absorption properties, long term stability under irradiation. This paper deals with a procedure we have developed for the use of CdTe detectors in imaging purposes. For demonstration purpose, a 64 linear array has been realized, all detectors being biased at the same voltage. Each pixel is equivalent to a capacitance and a resistance in parallel. The loaded capacitance is disconnected just before irradiation by the X-rays by means of a MOS switching transistor. The detector's equivalent capacitance discharges under irradiation trough the parallel resistance by an amount directly proportional to the charges deposited in the detector, i.e. to the received radiation. In order to measure the corresponding charges, the capacitances of all pixels are successfully loaded for a short time, by switching on a shift register. Each charging current generates between the terminals of a common resistor, a voltage pulse proportional to the beam absorbed. This approach offers several advantages, in particular, to correct the dispersion introduced by differences in detectors, as well as by electronic circuits. Furthermore, it facilitates the use of a dummy line in order to reduce the dark current and parasitic commutation signals. Images of phantoms exposed to the X-ray beam have been visualized with success by means of this system.

Cadmium Telluride (CdTe) has been investigated for many years as detector operating in the current mode, when illuminated by X- or gamma-ray beams. Indeed, applications in Non Destructive Testing (NDT) and the nuclear medicine field would be quite of interest, due to the relatively lower dose of radiation needed for a similar investigation. In these earlier investigations, three limitations, three limitations appeared drastically: photomemory effect (afterglow), high dark current and instability of this dark current. In a systematic investigation, we have measured by Photo-Induced Current Transient Spectroscopy (PICTS) the afterglow effect of a large number of detectors using various starting materials, with different kinds of compensation or chemical dopants as well as different surface and contact treatments. The main results of this study will be presented, correlated with the microscopic analysis of the material by both PICTS and Thermally Stimulated Current (TSC), in order to reach a clearer picture of the contribution from bulk and surface handlings to the afterglow effect.

Two electric field profile analysis methods have been considered for evaluating the internal field distribution in cadmium telluride nuclear radiation detectors. A theoretical model is given, followed by two experimental measurements, using the induced radiation current pulse profile and the optical rotation by the Pockels effect of polarized IR light. Finally, theoretical and experimental results are discussed.

The IR absorption spectrum of KI etched HgI₂ single crystals shows a great number of bands in the range 400 - 4000 cm^-1, indicating the formation of a complex. This spectrum presents similarities and differences with that of KHgI₃.H₂O. The index of refraction and the relative evolution of the complex layer thickness have been determined optically.

The Tracking and Imaging Gamma-Ray Experiment (TIGRE) uses multilayers of silicon strip detectors both as a gamma-ray converter and to track Compton recoil electrons and positron-electron pairs. The silicon strip detectors also measure the energy losses of these particles. For Compton events, the direction and energy of the Compton scattered gamma ray are measured with arrays of small CsI(TI)-photodiode detectors so that an unique direction and energy can be found for each incident gamma ray. The incident photon direction for pair events is found from the initial pair particle directions. TIGRE is the first Compton telescope with a direct imaging capability. With a large (pi) -steradian field-of-view, it is sensitive to gamma rays from 0.3 to 100 MeV with a typical energy resolution of 3% (FWHM) and a 1-(sigma) angular resolution of 40 arc-minutes at 2 MeV. A small balloon prototype instrument is being constructed that has a high absolute detection efficiency of 8% over the full energy range and a sensitivity of 10 milliCrabs for an exposure of 500,000 s. TIGRE's innovative design also uses the polarization dependence of the Klein-Nishina formula for gamma-ray source polarization measurements. The telescope will be described in detail and new results from measurements at 0.5 MeV and Monte Carlo calculations from 1 to 100 MeV will be presented.

We present measurements on purity and long-term stability of a 60 l liquid Xenon TPC, showing electron lifetimes in the liquid higher than some milliseconds for a period of one month. The electron lifetime is measured with a monitor chamber inside the TPC. The method is based on the measurement of the number of electrons extracted from a photocathode and collected on the anode after the drift through an electric field. We also report on preliminary results of the calibration of the TPC with gamma-ray sources.

This comparative study of the performances of CdTe and Cd₁--x)Zn_xTe detectors at temperatures in the 25 - 70 degree(s)C range shows that Cd_0.8Zn_0.2Te has a better performance as a gamma-ray detector at elevated temperatures. Peak position shifts as function of temperature are less pronounced in spectra obtained with Cd_0.8Zn_0.2Te detectors than in spectra from CdTe. The Cd_0.8Zn_0.2Te detectors exhibited a FWHM of 24% (at 60 degree(s)C) at 81 keV without significant deterioration due to the heating cycles.

Impurities in mercuric iodide (HgI₂) x-ray spectrometers are investigated by photoionization (PI) spectroscopy. The observed spectrum is shown to be a function of the particular impurities present in the material both in as-grown material and in material intentionally doped with Cu and Ag. The effect of contaminants on detector performance is also explicitly demonstrated as is the need for high purity starting materials and carefully controlled processing conditions.