We have developed a new kind of phoswich counters that are capable of detecting low flux hard X-ray/γ-ray from localized sources. The counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). The well-shaped shielding part acts as an active collimator as well as an active shield. The whole assembly is viewed by a phototube from the exterior bottom surface of the shielding part. By using an appropriate pulse-shape discriminator (PSD), hard X-rays/gamma-rays that have deposited energy only in the detection part can be selected. The first counter was built by using a new scintillator (GSO) for the detection part and CsI(Tt) for the shielding part. A detector system consisting of 64 such phoswich counters (total area ~740cm²) was flown three times on board a balloon, setting a limit to the ⁵⁷Co line flux from SN1987A at around 10^-4cm^-2s, determining the pulsating hard X/γ-ray flux of PSR15O9-58, determining the hard X/γ-ray spectra of CenA and GX339-04. Analyses have revealed the fact that background counts due to the Compton scattering, nuclear reactions, and β-γ radioactivities in the detector are largely suppressed because they are likely to register at least one extra count in the shielding part. The ultimate sensitivity of the detector will then be determined by the level of radioactive contamination. Other scintillator combinations such as GSO/BGO, NaI(Tl)/CsI(Tl), and YAIO₃/BGO have also been studied. Efforts to reduce the radioactive contamination in scintillators have also been actively pursued. In near future we expect to reach a sensitivity (3σ) around a few times 10^-7cm^-2s^-1keV^-1 for continuum in a 10⁴s balloon observation.

p-Terphenyl (PTP) and 2,2' dimethyl-p-terphenyl (DMT) have been investigated for use as wavelength shifters (WLS) for barium fluoride (BaF₂). These two organic fluors are attractive due to their: high quantum efficiency, fast fluorescence decay time, and emissions above the wavelength cut-off of borosilicate glass. Measurements of these WLS dissolved in cyclohexane are presented. Coincidence time spectra, and fluorescence decay spectra measured with BaF₂ as the pump source, are shown. The fluorescence decay time and the ratio of fast to slow intensities were measured by a time correlated single photon technique. The increase in signal resulting from using a WLS between the photomultiplier tube (PMT) and the BaF₂ crystal is measured. The motivation behind this work is the development of a gamma-ray spectrometer suitable for high count rate applications. We are investigating the possibility of separating the fast and slow emissions of BaF₂ using optical techniques. For our measurements, PTP an DMT do not facilitate spectral separation due to the nature of their absorption and emissions spectra. A discussion of WLS properties needed to spectrally separate BaF₂ emissions is presented.

We plan to use an array of 9600 CsI(Tl) crystals as the electromagnetic (EM) shower detector for the asymmetric B-Factory at KEK. In order to determine the length of a crystal, we studied energy resolution, electron/pion separation, and other properties of 25 cm and 30 cm long CsI(Tl) crystals of a 3 X 3 matrix at GeV energy region. When electrons with energy of 1 GeV (4 GeV) is injected into the crystals, the energy resolution of 25 cm and 30 cm length is 2.7% (3.0%) and 2.4% (1.8%), respectively. The misidentification of a pion as an electron is less than 0.5% for energies above 1 GeV.

The gamma-ray excited, temperature dependent scintillation characteristics of CsI(Tl) are reported over the temperature range of -100 to +50 degree(s)C. The modified Bollinger-Thomas and shaped square wave methods were used to measure the rise and decay times. The emission spectra were measured using a monochromator and corrected for monochromator and photocathode spectral efficiency. The shaped square wave method was also used to determine the scintillation yield as was a current mode method. The thermoluminescence emissions of CsI(Tl) were measured using the same current mode method. At room temperature, CsI(Tl) was found to have two primary decay components with decay time constants of (tau) ₁ equals 679 +/- 10 ns (63.7%) and (tau) ₂ equals 3.34 +/- 0.14 microsecond(s) (36.1%) and to have emission bands at about 400 and 560 nm. The (tau) ₁ luminescent state was observed to be populated by an exponential process with a resulting rise time constant of 19.6 +/- 1.9 ns at room temperature. An ultra-fast decay component with a < 0.5 ns decay time was found to emit about 0.2% (about 100 photons/MeV) of the total scintillation light. At -100 degree(s)C (tau) ₂ was too long to be resolved and (tau) ₁ was determined to be 3.52 +/- 0.39 microsecond(s) , while the 400 nm emission band was not observed. At +50 degree(s)C the decay constants were found to be 628 ns (70%) and 2.63 microsecond(s) (30%) and both emission bands are present. Four different commercially available CsI(Tl) crystals were used. Minimal variations in the measured scintillation characteristics were observed among these four crystals. Thermoluminescence emissions were observed to have peak yields at -90, -65, -40, +20, and possibly -55 degree(s)C. The relative magnitudes and number of thermoluminescence peaks were found to vary from crystal to crystal.

We have developed a low background hard X-ray/gamma-ray telescope for balloon-borne experiments. The telescope called Welcome-1 (Well type Compound Eye) utilizes newly developed well-type phoswich counters. In the well-type phoswich counter, the background from external and internal sources are reduced significantly, Welcome-1 is designed for observation in the energy range from 60 keV to 800-1000 keV. The effective area of Welcome-1 is 740 sq cm at 122 keV and 222 sq cm at 511 keV line. We flew Welcome-1 in 1990 and 1991 to detect hard X-rays from SN1987A, PSR1509-58, Cen-A and others. The background levels at an altitude of 4g/sq cm are 1 x 10 exp -4/sq cm photons/s/keV at 122 keV and 3 x 10 exp -5 photons/sq cm/s/keV at 511 keV. The data obtained during the flight shows that the detector in fact has the 3 sigma sensitivity of about a few x 10 exp -6 photons/sq cm/s/keV and about 10 exp -4 photons/sq cm/s in a 10 exp 4 s observation for the continuum spectrum and line spectrum, respectively.

We have developed a microstrip gas chamber (MSGC) by using multi-chip technology which enables high-density assembly of bare LSI chips on a silicon board. Our MSGC was operated steadily with approximately 10³ gain more than one week. An energy resolution of 15% (FWHM) for 5.9 keV x ray of ⁵⁵Fe was obtained. With a very thin polyimide substrate of 16 micrometers thickness, two interesting phenomena were observed; one is a strong dependence of gains on the back plane potential, and the other is little time variation of gains. A new type of MSGC with a guarding mask of a thin polyimide layer on the cathode edges has been examined to reduce incidental electrical discharges between anode and cathode strips. Furthermore, a new approach to reduce the resistivity of the substrate has been examined. By these approaches, the stability of the high gain operation of approximately 10⁴ has been drastically improved. In addition, we discuss the possibility of the application of MSGC to the coded mask x-ray imaging detector for astrophysics.

We describe a balloon-borne hard X-ray telescope called SIGHT (Scintillation Imaging Gas-filled Hard X-ray Telescope). SIGHT is a high sensitivity, good energy resolution instrument that images in the 30 to 300 keV region. We discuss the development of a large area, 20 atmosphere, position sensitive xenon gas scintillation drift chamber which is the gamma-ray detector at the heart of the telescope package. Results of the development of the novel waveshifting fiber readout for this chamber are presented.

Recent operation results of a three-ton liquid argon time projection chamber for the ICARUS project are reported. This elecronic continually sensitive, self-triggering bubble-chamber is capable of providing 3D imaging of any ionizing event in conjunction with a good calorimetric response.

The feasibility of a large volume liquid xenon time projection chamber (LXe-TPC) for three- dimensional imaging and spectroscopy of cosmic gamma-ray sources, was tested with a 3.5 liter prototype. The observation of induction signals produced by MeV gamma rays in liquid xenon is reported, with a good signal-to-noise ratio. The results represent the first experimental demonstration with a liquid xenon ionization chamber of a non-destructive read- out of the electron image produced by point-like charges, using a sense wire configuration of the type originally proposed in 1970 by Gatti et al. An energy resolution as good as that previously measured by us with millimeter size chambers, was achieved with the large prototype of 4.4 cm drift gap.

The charge collection and energy resolution in liquid rare gas ionization chambers can be improved by doping the liquid with photoionization molecules. We have observed the improvement in liquid Xe doped with triethylamine (TEA). The improvement is remarkable especially at a low electric field region. This effect is used to develop gamma-ray detectors with good energy resolution as well as high detection efficiency for gamma rays.

A small prototype cadmium telluride (CdTe) detector optimized for the detection of gamma-rays having an energy of the order of 500 keV was designed. This detector was arranged as an array of 5x5 CdTe crystals, each having dimensions of 2x2x10 cu mm, and was constructed primarily in order to verify its technical feasibility and performances. The initial aim in developing this type of detector was for use in space applications such as measurement of annihilation radiation (511 keV photons) from the galactic center and possibly from extragalactic objects as for example AGN's (Active Galactic Nucleus). Many other applications may also benefit from the use of CdTe position sensitive detectors, in linear or square configuration, such as nuclear medicine diagnostics, nondestructive testing in industrial quality assurance processes, and safety inspection and controls.

A discussion of recent results in the preparation of Cd_1-xZn_xTe crystals by a high pressure Bridgman (HPB) method and use of the crystals for gamma- and x-ray detectors is presented. Resistivities in excess of 10¹¹ ohm-cm are achieved in Cd_1-xZn_xTe without impurity doping. The consequential low detector leakage currents lead to excellent energy resolution, 8.4% (FWHM) at 30 keV, for example. Useful energy spectroscopy can be performed at temperatures up to 100 degree(s)C. The dependence of leakage current on temperature from -40 degree(s)C to 100 degree(s)C implies a Fermi level at mid-gap for x equals 0.2. HPB grown Cd_1-xZn_xTe crystals exhibit relatively low defect content, as evidenced by etch-pit-densities <EQ 10⁴ cm^-2, double-crystal-rocking-curve linewidths of 10 - 15 arc-seconds and sharp, bright emission lines, with excitonic features, in low temperature photoluminescence measurements. Results of flash x ray experiments indicate high current sensitivity, low leakage current, and good temporal response. Preliminary results of Cd_1-xZn_xTe imaging detector array studies are discussed.

The requirements of a semiconductor material intended to operate in a gamma-ray detector at room temperatures are discussed, and the status of the search for alternative materials is reviewed. The important material characteristics of a semiconductor gamma-ray detector material are high average atomic number, material's uniformity, resistivity, and electron and holes transport properties. Materials under investigation include GaAs, InP, TlBr, and PbI2. Theoretically, it is considered to be feasible to built a large volume semiconductor gamma-ray detector capable of good energy resolution at room temperature. But it is very unlikely that a semiconductor detector with germanium-like performance will be available in the next five years.

New technology developments in PbI₂ nuclear detectors are reviewed with particular attention given to crystal growth, purification, and detector processing steps as they related to device performance. It is concluded that PbI₂ is a promising candidate for the manufacturing of semiconductor gamma-ray detectors.

Silicon positive-intrinsic-negative (p-i-n) diodes have been used in plasma diagnostics by the Los Alamos and Lawrence Livermore National Laboratories (LANL and LLNL) since the early seventies. Since the response bandwidth of these detectors is relatively poor (typically, approximately 5 ns FWHM for 1 cm² sensitive area and 250 micrometers depletion depth), they are too slow for high-speed applications. GaAs photoconductive detectors (PCD) have been developed since the early eighties at LANL and later at LLNL, and can be tailored by judicious neutron damage to provide the required high-speed bandwidth. Unfortunately, for surface absorbed or non-penetrating radiation, we have discovered that the PCD sensitivity is not flat across its gap, where the incident radiation is perpendicular to the bias electric field. This response non-uniformity can lead to erroneous measurements in cases where the radiation is spatially varying. To overcome this problem, we reoriented the GaAs chip to allow the radiation to be incident through the electrode and parallel to the bias electric field. Then to increase bandwidth, we doped the GaAs crystal with chromium to create trapping sites and provide large resistivity (approximately 10⁹ (Omega) cm), thus creating a semi-insulator detector (SID). We present and discuss the merits of the SID versus PCD and p-i-n diode by showing pulse response data of each detector characterized with 16 MeV endpoint gamma and electron radiation created by the EG&G/EM linear accelerator (Linac) and 5 to 16.5 MeV proton radiation produced by the LLNL Tandem Van de Graaff (TVDG). Application of the SID in Compton electron spectrometry also is discussed.

Photoconductive detectors have been developed from a new wide bandgap (E_g equals 2.01 eV) semiconductor, InI, and these photodetectors are intended for use in scintillation spectroscopy of nuclear radiation. InI single crystals were produced using the Bridgman process and these crystals were characterized by measuring their optical transmission spectrum, micro-hardness, electrical resistivity, and charge transport properties. Photodetectors were fabricated from InI crystal slices by evaporating thin (< 100 angstrom) Pd front contacts, and applying graphite back contacts. These photodetectors showed considerable promise due to their high quantum efficiency (> 60%) in the 300 nm to 600 nm wavelength region and their uniform photo-response over the active detector area. Finally, these photodetectors were coupled to CsI(Tl) scintillator and were successfully tested as spectrometers at room temperature by irradiating the scintillator with 5.5 MeV (alpha) particles (²⁴¹Am source) and 662 keV (gamma) rays (¹³⁷Cs source).

Photosensitive gaseous detectors, first developed for Cherenkov Light Imaging, are showing increasing potentialities for the detection and imaging of photons emitted by scintillators. The progress is coming from new scintillators emitting ultraviolet photons, the availability of solid photocathodes associated with high-gain gaseous detectors, and the progress in the amplifying structure of gaseous detectors permitting the imaging of single photoelectrons.

A massive thermal detector consisting of a 73 g TeO₂ crystal, to be used to search for double beta decay of ¹³⁰Te and to detect high energy gamma-rays, is operating at aprox. 15 mK in the Gran Sasso Underground Laboratory. The FWHM resolution achieved with this detector is 5-8 KeV, slightly dependent on energy from 100 KeV to 3 MeV, showing the competitive performances of this detector in gamma-ray spectroscopy comparable with those of conventional Ge solid state detectors. Moreover this is the first massive high-resolution gamma-ray detector of atomic number larger than 32, which implies a peak to Compton ratio comparable to that of a Ge diode of a mass larger by an order of magnitude.

Recent advances in photomultiplier tube technology have led to the availability of position sensitive photomultiplier tubes (PSPMTs). These tubes make it possible to build a new generation of imaging instruments for gamma rays and other types of ionizing radiation. We have investigated the use of these tubes for the construction of several prototype instruments. The first application investigated measures the quantity and distribution of radioactive compounds on filter papers used in microbiology research. The intent of this instrument is to replace film autoradiography with an electronic imaging system which can analyze samples 75 to 110 times faster than film. The second application involved the development of an intraoperative imaging probe to help surgeons identify cancerous tissue and ensure its complete removal. This instrument will replace a non-imaging probe now in use at many hospitals. A third prototype instrument under evaluation is an imaging nuclear survey system which obtains both a video and gamma ray image for the purpose of locating and quantifying radioactive materials. This system would be used at nuclear power plants and radioactive materials preparation facilities. A modification of this system could be built into robots used for inspecting and repairing power plants.

The purpose of this instrument is to distinguish the temporal variations on the order of nanoseconds of a faint gamma emitter in the energy range of 10 to 20 MeV despite a coincident gamma background of lower photon energy yet orders of magnitude greater power. The instrument itself is a combination of a Compton electron monochromator comprised of two dipole magnets, a dual lens total internal reflection Cherenkov optical system, and microchannel plate photomultiplier detectors. Two units, called H1 and H2, were built differing only in magnet strength and viewing the following energy bins: H1 gamma energy bin: 15.7 to 17.7 MeV, H2 gamma energy bin: 13.7 to 15.7 MeV. The H1 design aims at a large signal-to-noise through-put and fast temporal response, and this limits the energy resolution. The H1 unit has a 671 MHz bandwidth (FWHM equals 463 ps) for the energy bin shown above.

The present status of research and development of liquid Xe calorimeters for homogeneous ionization and scintillation in the range from 1 to 100 GeV is discussed. Designs of two calorimeters, namely, a liquid Xe ionization and a liquid Xe scintillation calorimeters, are described.

A scintillating fiber sampling calorimeter telescope (SSCT) intended to be operated onboard a satellite for cosmic gamma-ray observation in the range from 200 MeV to 200 GeV is presented. SSCT consists of a target part and a main part that includes a calorimeter with imaging capability, which are separated by a distance of 60-70 cm. A gamma ray enters the target and is converted into a pair of electrons which in turn develops into an electron-photon cascade that is observed in the main part. The gamma-ray direction is measured by pointing the energy-weighted centers observed in the two parts. The shower particle are detected using 2 mm square fibers which are bundled into a 2 mm thick flat belt. A pair of belts crossing at a right angle is inserted between lead plates. Experimental results show that the energy resolution of the calorimeter is 20 percent/sq root E(GeV) below a few GeV. The angular resolution is 1.6 deg at 1 GeV and 0.34 deg at 10 GeV.

Silicon drift photodiodes have been developed over the last five years and are a derivative of silicon drift chambers. These devices, while lacking the position sensitivity of the silicon drift chamber, retain the qualities of low capacitance and large area. These properties make them attractive for use in applications requiring low noise high efficiency photodetection, such as for scintillation light detectors in nuclear spectroscopy. These devices might also find other uses in photonics; as replacements for silicon p-i-n photodiodes in other applications demanding low noise operation. We report on our progress in fabricating silicon drift photodiodes for use as scintillator photodetectors, specifically optimized for detecting the 550 nm emission from CsI(Tl). The design we chose to build was a square photodiode 1 cm² in area, having the general features of that described by Avset et al. Although some technical problems have temporarily delayed us from producing working drift photodiodes, we have made some diagnostic measurements on our devices and have made observations that may be of general interest.

High resistivity(over 1O.Okohm.cm) extrinsec P-type Si was produced from high pure P-type Si(resistivity between 1.0 and 2.Skohm.cm,lifetime over 1 ms),using the neutron transmutation doping technique.Material obtained by this method,used for detectors,has better properties in dopant homogeneity and controllability than conventionally doped ones. Improved radiation detectors of surface barrier type were obtained by deposition of amorphous Ge and Al,Au, respectively, for contacts.Results obtained by testing two types of detection structures are presented.First type,for gamma radiation are of sensitive area of 100 sqxnm sensitive depth of 300 μm.They present a good response for Cs-137, higher than 400 p/s/cGy/h. The second type, for alfa-radiation (2...9 MeV) and beta-radiation (225keV...2.26MeV) are of sensitive area of 5sqcm and sensitive depth of 250 μm. Calibration was done obtaining: R_∞=min.1 p.sqcm/s.Bq for Am-241 (5.45 MeV) and R_β=min. 0.7 p. sqcm/ s .Bq for Sr-9 0+ Y-9 0 (2.26 MeV). Original results concerning functioning tests of these detectors in special conditions, such as temperature between -25°C and +45°C, humidity 90% at 30°C and mechanical shocks are presented.

A high pressure xenon parallel plate ionization chamber was designed for measurements of cosmic gamma-ray lines. The 3 liter chamber was filled with 0.6 g/cu cm density of xenon, mixed with hydrogen for increasing the drift velocity of the electrons. The noise-subtracted energy resolution was 1.3 percent FWHM at energy 1 MeV. This chamber was installed on the space station 'MIR', currently in orbit and the measurements are being carried out. No energy resolution degradation has been observed for two years, despite the strong irradiation by the Brazil anomaly.

Detectors made of GaAs are being studied for use on the focal plane of a Compton spectrometer which measures 1 MeV to 25 MeV gamma rays with high energy resolution (1% or 100 keV, whichever is greater) and 200 ps time resolution. The detectors are GaAs chips (Cr-doped or un-doped) that have been neutron-damaged to improve the time response. The detectors are used to measure fast transient signals in the current mode. The properties of various GaAs detector configurations are being studied by bombarding sample detectors with short pulses of 4 MeV to 16 MeV electrons at the Linac Facility at EG&G Energy Measurements, Inc., Santa Barbara, California Operations. Measurements of detector sensitivity and impulse response versus detector bias, thickness, and electron beam energy and intensity have been performed and are presented.

A method for imaging a target with Compton scattered gamma radiation is presented. This method accepts simultaneously, in the detector energy spectrum, radiation scattered from the entire target in a single measurement. The energy spectrum provides the information necessary for reconstructing the electron density distribution within the target. Models on which the reconstruction process is based are presented and tested against Monte Carlo simulation and experimental data. Image reconstruction from simulations demonstrate the feasibility of the method. The problems encountered in reconstructing images from experimental spectra are addressed along with some workable solutions.

Traditionally, researchers developing improved gamma ray detectors have used analytical techniques or, rarely, computer simulations to predict the performance of new detectors. However, with the advent of inexpensive personal computers, it is now possible for virtually all detector researchers to perform some form of numerical computation to predict detector performance. Although general purpose code systems for semiconductor detector performance do not yet exist, it is possible to perform many useful calculations using commercially available, general purpose numerical software packages (such as `spreadsheet' programs intended for business use). With a knowledge of the rudimentary mechanics of detector simulation most researchers, including those with no programming skills, can effectively use numerical simulation methods to predict gamma ray detector performance. In this paper we discuss the details of the numerical simulation of gamma ray detectors with the hope of communicating the simplicity and effectiveness of these methods. In particular, we discuss the steps involved in simulating the pulse height spectrum produced by a semiconductor detector.

Optical fibers are being used to obtain full coverage over a range of energies in a multi- channel, time-resolved gamma ray spectrometer. Gamma rays are incident upon a beryllium foil 60 cm from the entrance port of a Sm-Co magnet. Compton electrons from the foil are focussed according to their energy onto quartz optical fibers arrayed in close-packed configuration behind a low-Z vacuum window at the focal plane. Cerenkov radiation produced inside each of the fibers propagates down the fiber which is brought out of the magnet. Fibers are grouped into preselected energy bins corresponding to streak record channel assignments. The light from the fibers in an energy bin are combined into one signal and then transmitted to a streak camera with a specified number of channels. This unique optical fiber array serves both as the detector and as a means to define energy bins of our choosing for a streak camera recording system.

We are using a Sm-Co based permanent magnet spectrometer to analyze Compton electrons ejected from a Be converter foil that is illuminated by a gamma-ray beam. The distance along the focal plane at which a mono-energetic electron beam entering the spectrometer will cross the focal plane is proportional to the square root of the electron momentum. This design achieves a very broad range of energies that are analyzed (from momentum of about 1 MeV/c up to 30 MeV/c) while maintaining good energy resolution (the electron momentum resolution is the larger of 0.1 MeV/c or 1% of the momentum). In addition this design has a fairly large acceptance. By restricting the angular acceptance of the spectrometer for the Compton electrons, incoming gamma ray energies and ejected electron momenta are simply related. The electron optical properties of this spectrometer are discussed as well as some aspects of the overall system design and testing.

The construction of a cylindrical ionization chamber, filled with high pressure xenon (55 bar) is described. The main characteristics of this detector are given. It is shown that the energy resolution of this detector, at 662 keV, is 4% FWHM for the optimal parameters of the gas mixture, applied electric field and shaping time. The detector was tested within a temperature interval between 20°C and 170°C. The dependence of the energy resolution of the cylindrical gamma-ray detector is quite stable. This detector can be used in geology, geophysics, oil and gas well logging.

Analytical calculation has been done to obtain energy resolution and pulse distribution of compressed gaseous xenon detector. Cylindrical chamber was considered. Calculation takes into consideration energy of primary gamma-ray, fluctuation of number of electron-ions pairs, space distribution of electrons to be produced into chamber, walls of chamber, recombination, H excess, parameters of electronic. Calculation was performed for xenon density less than 0.6 g/cm and H excess less than 2%.