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We have developed a prototype of a scalable high-resolution direction and energy sensitive gamma-ray detection system that operates in both coded aperture (CA) and Compton scatter (CS) modes to obtain optimal efficiency and angular resolution over a wide energy range. The design consists of an active coded aperture constructed from 52 individual CZT planar detectors each measuring 3×3×6 mm3 arranged in a MURA pattern on a 10×10 grid, with a monolithic 20×20×5 mm3 pixelated (8×8) CZT array serving as the focal plane. The combined mode is achieved by using the aperture plane array for both Compton scattering of high-energy photons and as a coded mask for low-energy radiation. The prototype instrument was built using two RENA-3 test systems, one each for the aperture and the focal plane, stacked on top of each other at a distance of 130 mm. The test systems were modified to coordinate (synchronize) readout and provide coincidence information of events within a user-adjustable 40-1,280 ns window. The measured angular resolution of the device is

We have developed high energy and high spatial resolution two-dimensional (2D) solid-state imaging pixel detectors and their custom integrated circuits (ICs). Solid-state pixel detectors and their readout ICs are now regarded to be an integral part of position-sensitive semiconductor detectors such as Si, CdTe and CdZnTe for x-ray and gamma-ray imaging. These detectors have a 2D structure. We have also developed one-dimensional (1D) detectors, which are mostly used for scanning type imaging. The new 2D pixel detectors we have developed can be used for both scanning and staring mode imaging applications. Because the requirements of various detector applications tend to be diverse, a custom IC is typically designed for a specific detector array. This often lengthens the time and raises the cost of system development. To help close the readout technology gap and facilitate advances in this field, we have been formulating and implementing strategies for instrumenting different detectors of a given application category with highly versatile ICs that meet a range of requirements. The solid-state pixel detectors that have been developed within this effort are presented below.

The RENA-3 (R Readout E eadout Electronics for N lectronics Nuclear A uclear Applications) is a multi-channel mixed-signal integrated circuit (IC) developed for the readout of position-sensitive solid-state detectors with excellent energy resolution. We will present results of experiments characterizing its performance as used with a variety of spectroscopy-grade detectors currently available in the industry, notably CZT pixel arrays as well as other detector configurations. The merits of specific RENA-3 design features vis-a-vis different detector applications will also be discussed.

We present the design and initial performance characterization of the XENA-2 readout IC for solid-state x-ray detector arrays. XENA-2 consists of 32 readout channels, each with charge-sensitive input amplifier, adjustable two-stage gain amplifier and five comparators with 16-bit pulse counters. Readout of the counters, over a 16-bit data bus, takes approximately 20 μs. Compared to the XENA chip, its predecessor, this new IC's main improvement is significantly reduced noise, which allows for lower comparator thresholds and increased count rates.

We have developed high energy and high spatial resolution 1D and 2D solid state imaging detectors and their custom integrated circuits (ICs). Readout ICs are now regarded to be an integral part of position-sensitive semiconductor detectors, especially for Si and CdZnTe for x-ray and gamma-ray imaging. These detectors have a 1D or 2D structure. The 1D structure types are mostly used for scanning purposes with some staring type imaging while 2D pixel detectors can be used for both scanning and staring mode imaging applications. Because the requirements of various detector applications tend to be diverse, a custom IC is typically designed for a specific detector array. This often lengthens the time and raises the cost of system development. To help close the readout technology gap and facilitate advances in this field, we have been formulating and implementing strategies for instrumenting different detectors of a given application category with highly versatile ICs that meet a range of requirements. The solid-state detectors and their ICs that have been developed within this effort are presented.

A simple x-ray detector that utilizes amorphous selenium (a-Se) directly deposited on a specially-designed CCD (charge coupled device) with a 25 micrometer del (detector element) pitch is described. This simple detector has been used to test the feasibility of creating digital mammography detectors. To enable the use of electron transport CCDs with a-Se, we have developed a-Se hole blocking layers to permit the transfer of electrons to the CCD while suppressing hole leakage current in the presence of the high negative bias (~1000 V) required to make the a-Se x-ray sensitive. We report measurements of the charge transfer efficiency (CTE), dark signal, x-ray sensitivity, x-ray signal linearity, and x-ray MTF (modulation transfer function) of the simple detector. As the thickness of the a-Se hole blocking layer was increased, the MTF decreased. For a thin (1 micrometer) blocking layer the MTF at a spatial frequency of 20 cycles/mm was 0.4.

Large area, high spatial resolution CdZnTe pixel detectors are being developed for hard X-ray astronomy. We have designed and fabricated custom readout chips and bump-bond these to pixelated CdZnTe crystals using indium bump bonding technology. The resulting detectors have 16 x 16 pixels with 300 micron pitch, enabling low noise operation and permitting detailed imaging. These devices are ideally suited for the focal plane of future high-resolution hard x-ray focusing telescopes now being considered, such as the HXT on Constellation-X. An initial demonstration using the sparse read-out capabilities of these detectors is presented.

The Keck Solar Two Gamma-Ray Observatory is a ground-based instrument is being developed to detect 20-300 GeV gamma rays by sampling the Cherenkov light generated as gamma rays and cosmic rays interact with the atmosphere. The observatory utilizes the Solar Two Pilot Power Plant in Barstow, California (Figure 1) which has the largest heliostat mirror area in the world. It has over 1,818 heliostats each with about 41 m2 mirror area. The total active area is over 75,000 m2. Thus, Keck Solar Two Gamma Ray Observatory has the potential to be the most sensitive ground-based gamma-ray detector between 20-300 GeV. The secondary mirror systems, each capable of viewing 32 heliostats has been designed. The secondary mirror systems also include the photomultiplier tube (PMT) camera, electronics, and heliostat field. The first secondary camera has been manufactured and it is being calibrated. Work on building the second secondary camera system with 32 heliostats has been started. When the second system is completed a 64 heliostat telescope will be ready to observe 50-300 GeV gamma rays. Further enlargement of the telescope to 128 or 256 heliostat is expected to lower the energy threshold to about 20 GeV.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Our collaboration is characterizing a prototype detector module designed for high energy X-ray astrophysics research covering the 20 - 250 keV energy range. The module consists of a three dimensional position sensitive CdZnTe detector, 25 mm X 25 mm X 2 mm, with 1 mm pitch crossed strip electrodes, an interleaved steering electrode, and an Application Specific Integrated Circuit (ASIC) for individual electrode readout. The newly developed readout system is compact, lightweight, has low power consumption and will lead to reduced system electronic noise. The detector is surrounded by a plastic anti-coincidence system for charged particles, and passive shielding that has been optimized based on results from two previous balloon flights. The first balloon flight test of the new detector module is scheduled for Fall 2000. In addition to our continuing balloon studies, we are investigating proton radiation damage effects and present preliminary results. After proton irradiation, the energy resolution is not significantly degraded, calibration photopeaks are down shifted by less than 10% in energy, and the depth of interaction dependence is nearly eliminated.

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A linear CdZnTe pad detector array with approximately 1 mm2 pad area has been developed. The detector has a wide energy range from about 20 to 200 keV. To read out these detector arrays, a fast, low-noise monolithic mixed signal ASIC chip has been developed. A prototype x-ray imaging system consisting of the CdZnTe detector array and the monolithic ASIC chip has been fabricated and tested. In this system, the detectors are abutted against each other to form an approximately 1 m long linear array. The system has been used to take preliminary scanned images of complex objects at various energies. New results from this system will be presented.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Digital mammography has demanding imaging requirements, including very high spatial resolution (50 micrometer) and SNR. To make efficient use of the radiation dose, it is also desirable that the DQE of the image receptor is high. To achieve these requirements, a prototype CCD read-out has been designed, which is hybridized to a semiconductor array to form a direct conversion detector that can be employed in a slot- scanned digital x-ray imaging system. The image quality of the detectors in which the CCD is hybridized to either a silicon photodiode array or a CdZnTe photoconductor array has been measured. A 1 mm thick silicon device has shown a DQE(f) of 0.64 at 0 mm -1 , falling to 0.14 at 10 mm -1 in the slot direction (20 keV). The CdZnTe hybrid device is very thin (150 micrometer) and has a theoretical DQE in excess of 0.9 at 20 keV. The resolution of the CdZnTe device is excellent, with an experimental MTF that is limited only by the detector element size, and the TDI scanning technique. However, the experimental DQE is lower than predicted, believed to be due to crystal non-uniformity, and excessive carrier trapping. Future work will investigate the improvement in image quality obtainable by using a very high purity single-crystal CdZnTe device.

The Solar Two Gamma-Ray Observatory is a ground-based instrument designed to detect 20-300 GeV gamma rays by sampling the Cherenkov light generated as gamma rays and cosmic rays interact with the atmosphere. The observatory utilizes the solar two pilot power plant in Barstow, California which has the largest heliostat mirror area in the world. It has over 2,000 heliostats each with about 41 m 2 mirror are. the total active are is over 75,000 m 2 . Thus, Solar Two Observatory has the potential to be the most sensitive ground-based gamma-ray detector between 20-300 GeV. The secondary mirror system, each capable of viewing 32 heliostats has been designed. The secondary mirror systems also include the photomultiplier tube camera, electronics, and heliostat field. The first secondary camera system is now being manufactured and tested.

The FiberGLAST scintillating fiber telescope is a large-area instrument concept for NASA's GLAST program. The detector is designed for high-energy gamma-ray astronomy, and uses plastic scintillating fibers to combine a photon pair tracking telescope and a calorimeter into a single instrument. A small prototype detector has been tested with high energy photons at the Thomas Jefferson National Accelerator Facility. We report on the result of this beam test, including scintillating fiber performance, photon track reconstruction, angular resolution, and detector efficiency.

FiberGLAST is a scintillating fiber gamma-ray detector designed for the GLAST mission. The system described below provides superior effective area and field of view for modest cost and risk. An overview of the FiberGLAST instrument is presented, as well as a more detailed description of the principle elements of the primary detector volume. The triggering and readout electronics are described, and Monte Carlo Simulations of the instrument performance are presented.

The FARXITE balloon payload concept contains 10 co-aligned, hard X-ray telescopes, each containing a set ofnested multilayer mirror modules and an actively shielded CZT strip detector at each focal plane. The 500 micronstrip pitch provides 26 arcsecond pixels at the 4m focal length of FARJ(ITE. The active shielding and advancedCZT detector techniques reduce the background at float altitudes to a few times iO counts/cm2 s keY. We describethese advanced detectors and how they allow us to meet the scientific objectives of the FAR3UTE program.Keywords: X-ray astronomy, CZT detectors, telescopes, balloons 1. INTRODUCTION The Fine Angular Resolution X-ray Imaging TelescopE (FARJ(ITE, pronounced "far sight") combines state-of-the-art hard X-ray mirrors and room temperature semiconductor (CZT) detectors, which are designed to fly on along duration balloon gondola with an advanced pointing system. One scientific goal of the FARXITE mission isimaging with sub-arcminute resolution deeper than ever before with iO ergs/s sensitivity in the 20-90 keV rangeat the Galactic center. This goal requires both spatial and spectral resolution at hard X-ray energies. FAR.XITE ischaracterized by

New, high spatial resolution CdZnTe (CZT) and silicon (Si) pixel detectors are highly suitable for x-ray astronomy. These detectors are planned for use in wide field of view, imaging x-ray, and low energy gamma-ray all-sky monitor (AXGAM) in a future space mission. The high stopping power of CZT detectors combined with low-noise front-end readout makes possible an order of magnitude improvement in spatial and energy resolution in x-ray detection. The AXGAM instrument will be built in the form of a fine coded aperture placed over two-dimensional, high spatial resolution and low energy threshold CZT pixel detector array. The preliminary result of CZT and silicon pixel detector test with low-noise readout electronics system are presented. These detectors may also be used with or without modification for medical and industrial imaging.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A scintillating fiber detector is currently being studied for the NASA Gamma-Ray Large Area Space Telescope (GLAST) mission. This detector utilizes modules composed of a thin converter sheet followed by an x, y plane of scintillating fibers to examine the shower of particles created by high energy gamma-rays interacting in the converter material. The detector is composed of a tracker with 90 such modular planes and a calorimeter with 36 planes. The two major component of this detector are the scintillating fibers and their associated photodetectors. Here we present current status of development and test result of both of these. The Hamamatsu R5900-00-M64 multianode photomultiplier tube (MAPMT) is the baseline readout device. A characterization of this device has been performed including noise, cross- talk, gain variation, vibration, and thermal/vacuum test. A prototype fiber/MAPMT system has been tested at the Center for Advanced Microstructures and Devices at Louisiana State University with a photon beam and preliminary results are presented.

A digital mammography system in which the x-ray sensitive device is a solid-state direct conversion detector is under development. This detector is a 1 mm thick silicon photodiode array hybridized to a CCD read-out, with a 50 micrometer pixel pitch. The detector is designed to be used in a slot-scanned system using time-delay integration (TDI) for signal acquisition. To handle the large signal generated in the photodiode, a novel read-out technique was used, in which charge was integrated 'on-chip' over a small number of rows, and the output of each of these sections was digitally summed 'off-chip' to produce the total integrated signal for each pixel in the image. This two-stage integration process not only allows easy acquisition of large signals, it effectively increases bit depth from 12 bits (for a single section) to approximately 16 (for the total integrated signal). The image quality of the device has been measured and compared to predictions based on cascaded linear systems theory. The resolution of the new detector was determined from the modulation transfer function (MTF) which was obtained from over-sampled edge spread functions (ESF). The ESF was measured in both the scan and slot directions from four repeated images of a tantalum edge. Noise power spectra (NPS) were determined from 40 repeated flat-field images at each of several x-ray exposures. By combining the MTF and NPS measurements, the detective quantum efficiency (DQE) was also determined. The MTF in the non-scanned direction was found to greater than 20% at 10 mm-1 and slightly lower in the scanned direction (approximately equals 10% at 10 mm-1). In all cases, the DQE was at least comparable to film-screen mammography receptors. The DQE at 120 mR detector exposure at zero spatial frequency ranged from 0.4 to 0.6 depending on the sample tested. Electronic noise was fairly low, contributing to less than plus or minus 7 ADU (out of a possible 98304 ADU). Future work will involve re-designing the prototype to use a photoconductor with higher density and atomic number to improve quantum interaction efficiency and reduce geometric constraints on image quality.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Recently the need for a higher level of integration in x-ray and gamma ray sensor systems has lead to several approaches of integrating read out electronics in a monolithic integrated circuit (IC). Typically these ICs are limited in their application to a specific problem. The Readout Electronics for Nuclear Applications (RENA) Integrated Circuit, presented here, is targeted for use in many energy sensor applications. The RENA IC has 32 parallel signal channels with, signal polarity control for use with either electron or hole collection from detectors. The input amplifier is optimized for a detector capacitance of 6 pF, but may be used with detector capacitances up to 50 pF. The Shapers' peaking time is digitally selectable, for optimum noise filtering, with peaking times geometrically spaced from 400 ns to 6 microsecond(s) . Up to 16 RENA ASICs may be daisy chained together to make a system with 512 detector channels. Various trigger modes are available with a user- defined threshold over the full signal range of 50K electronics. The circuits in the RENA are designed to be stable with no 'tweaking' control,s which allows an easy user interfaces.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A mixed signal Application Specific Integrated Circuit chip for front end readout electronics of position sensitive solid state detectors has been developed. It is called RENA. This chip can be used for large number of channels and high energy resolution astrophysics and nuclear physics detectors. It can also be used for medical and industrial imaging of x-rays and gamma rays. The RENA chip is a monolithic integrated circuit and has 32 channels with low noise charge sensitive amplifiers followed by a polarity amplifier and a high quality shaper circuit. It works in pulse counting mode with good energy resolution. It also has a self triggering output which is essential for nuclear applications when the incident radiation arrives at random. Different, externally selectable, operational modes that include a sparse readout mode are available to increase data throughput. It also has externally selectable shaping times. A full scale prototype RENA chip has been manufactured. The preliminary results of test done on the prototype chip are presented.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Coded mask imagers for future high energy x-ray astronomy missions will require detector planes with areas of hundreds to thousands of cm2 and position resolutions < 1 mm. Such detectors will enable coded mask imagers to discover and study thousands of high energy x-ray sources. The UCSD/WU/UCR/NOVA collaboration has been developing CZT detector systems with crossed-strip readout to meet these requirements. We report progress on a compact detector module with 41 cm2 area and 0.5 mm spatial resolution. The design includes the bias network and ASIC readout electronics, and allows modules to be combined in large area arrays with very high live-area factors. Results from laboratory and balloon flight tests are presented.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A wide field-of-view, arcsecond imaging, high energy resolution x-ray and low energy gamma ray detector is proposed for a future space mission. It is specifically designed to monitor and study gamma ray bursts (GRBs) with high energy and angular resolution and also find counterparts at other wavelengths. Detection of GRBs requires wide field-of-view ((pi) to 2 (pi) field-of-view) and high sensitivity. This is achieved by using high quantum efficiency CdZnTe pixel detectors with a low energy threshold (few keV) to observe the larger flux levels at lower energies, and large effective area (625 to 1,000 cm2) per coded aperture imaging module. Counterpart searches can only be done with ultra high angular resolution detectors (10 to 30 arcsecond FWHM) which gives 1 to 5 arcsecond position determination especially for strong GRBs. A few arcsecond size error box is expected to contain at most one object observed at another wavelength. This will be achieved by using ultra high spatial resolution pixel detectors (100 by 100 microns) and a similar resolution coded aperture to achieve the required angular resolution. AXGAM also has two other important advantages over similar detectors: (1) excellent low energy response (greater than 1 keV) and (2) high energy resolution (less than 6% at 5.9 keV, less than 3% at 14 keV, less than 4% at 122 keV). The low energy range may provide important new information on GRBs and the high energy resolution is expected to help in the observation and identification of emission and absorption lines in the GRB spectrum. The effective energy range is planned to be 2 to 200 keV which is exceptionally wide for such a detector. AXGAM will be built in the form of a 'bucky ball' using a coded aperture mask in a semi-geodesic dome arrangement placed over a two-dimensional, high resolution CdZnTe pixel detector array using newly developed p-i-n detector technology. The p-i-n structure decreases the electron and hole trapping effect and increases energy resolution significantly. The major scientific goals of the proposed mission in addition to continuously monitoring gamma- ray bursts, is to observe AGNs, transient phenomena, isolated and binary pulsars, and solar flares. A space deployed AXGAM detector is expected to observe several hundred gamma ray bursts per year.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We propose a new astrophysics space mission for a low energy gamma-ray-burst observatory (LEGO) that will fit the envelope of a small-explorer (SMEX) type mission. The LEGO instrument combines silicon pixel detectors with ultra-high energy resolution and a novel cost effective fine-pitch coded mask, to image the sky with sub-arcminute accuracy in the 0.3 - 30 keV range with a wide field-of-view. LEGO is well adapted to study hundreds of short transients such as gamma-ray bursts and soft gamma repeaters in the unexplored energy range below 5 keV. LEGO takes one of the next logical steps in GRB studies in the post-BeppoSAX era by attacking the astrophysics questions raised by recent discoveries of variable radio, optical, and x-ray counterparts to burst sources. In addition to monitoring the sky for gamma-ray bursts, LEGO would provide a first all-sky monitor in the 0.3 - 30 keV range. LEGO will be sensitive to all mCrab sources in the sky in a day and to 0.1 mCrab sources in a year, and thus, may provide daily light curves and sensitive spectral measurements on about 103 objects and yearly data on an order of magnitude more sources.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A fusion of two independent but complementary three- dimensional imaging techniques is proposed for detecting drugs in containers, cargo, mail and luggage. The containers, cargo, mail and/or luggage are scanned using a combined neutron and gamma ray source. A detector that can detect both neutrons and gamma rays is used to produce three dimensional images from both signals. The two images will be combined and analyzed by a fast host computer to detect drugs that may be concealed in the container, cargo and/or luggage. The two independent signatures from both neutrons and gamma rays, when analyzed simultaneously, may help determine the type of concealed material inside the containers. Containers, cargo and luggage are filled with a large variety of materials. Imaging them only in two dimensions may result in a poor contraband detection probability as different materials may shield each other. Therefore, a true three-dimensional imaging system is proposed, where the individual items inside the container or cargo can be resolved. This is expected to lead to reliable identification of the drugs even in small quantities. Such a system will also pinpoint the location of the suspected item and help expedite inspection by law enforcement agents. The proposed detection system produces two complementary three- dimensional images of the containers, cargo and/or luggage. These images are combined and analyzed by a specially developed algorithm to identify and locate the contraband automatically.

A compact integrated narcotics detection instrument (CINDI) has been developed at NOVA R&D, Inc. with funding provided by the U.S. Coast Guard. CINDI is designed as a portable sensitive neutron backscatter detector which has excellent penetration for thick and high Z compartment barriers. It also has a highly sensitive detection system for backscattered neutrons and, therefore, uses a very weak californium-252 neutron source. Neutrons backscatter profusely from materials that have a large hydrogen content, such as narcotics. The rate of backscattered neutrons detected is analyzed by a microprocessor and displayed on the control panel. The operator guides the detector along a suspected area and displays in real time the backscattered neutron rate. CINDI is capable of detecting narcotics effectively behind panels made of steel, wood, fiberglass, or even lead-lined materials. This makes it useful for inspecting marine vessels, ship bulkheads, automobiles, structure walls or small sealed containers. The strong response of CINDI to hydrogen-rich materials such as narcotics makes it an effective tool for detecting concealed drugs. Its response has been field tested by NOVA, the U.S. Coast Guard and Brewt Power Systems. The results of the tests show excellent response and specificity to narcotic drugs. Several large shipments of concealed drugs have been discovered during these trials and the results are presented and discussed.

We present a space mission concept for a low energy gamma-ray telescope, ATHENA, which is under investigation as the next major advance in gamma-ray spectroscopy following the current COMPTON Gamma Ray Observatory and the planned INTEGRAL missions. The instrument covers the nuclear line emission energy domain with dramatically improved sensitivity and spectral resolution. The baseline configuration combines a high resolution Compton telescope constructed from Ge planar strip detectors for the 0.3 - 10 MeV energy region with a coded-aperture system for the 10 - 200 keV domain. The Ge Compton telescope provides a broad field of view with exceptional spectral and imaging resolution. The requirements, capabilities and simulations of ATHENA are discussed.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In conventional mammography, x rays transmitted through the breast are converted to light in a phosphor screen, and the light exposes a film emulsion. The information in the image is degraded in this detector due to limitations in the screen and film. Photodiode arrays can convert the x rays directly into charge and overcome these problems. A preliminary investigation of a thick crystalline silicon photodiode array as a solid state digital detector was performed. The prototype device consists of a 300 micrometers thick, 256 X 256 photodiode array of 30 X 30 micrometers 2 pixels. The array was hybridized to two different readout structures for evaluation purposes, one structure being used for imaging and the other for single pixel experiments. Imaging performance, such as linearity, resolution, and noise were measured and used to predict the performance of a proposed clinical version of the prototype. Results show the detector response to be linear over the range of exposures required for mammography, the modulation transfer function (MTF) to be superior to that of screen-film detectors, and the noise to be dominated by x-ray quantum fluctuation. Based on results from the prototype devices, we predict that the detective quantum efficiency (DQE) of the clinical design will be significantly higher than that of a screen-film detector for all spatial frequencies of interest.

A portable hand held hidden substance detector has been developed and manufactured. Neutrons from a californium-252 source are emitted through the front face of the Compact Integrated Narcotics Detection Instrument (CINDI) and penetrate dense compartment materials with little change in energy, but are backscattered by hydrogen rich materials such as drugs. These backscattered neutrons can be readily detected. CINDI incorporates a highly sensitive detection scheme which permits the use of weak radioactive sources for safety without compromising detectability. CINDI is able to detect hydrogen-dense materials most effectively directly behind panels made of steel, wood, fiberglass, or even lead-lined materials. This makes it useful for inspecting marine vessels, ship bulkheads, automobiles, structure walls, or small sealed containers. The present CINDI version selectively detects hydrogen rich substances only. The new technique will detect both neutrons and gamma rays simultaneously. The backscatter mechanism of gamma rays and neutrons are sufficiently different that they complement each other and lead to a higher likelihood of identifying the concealed material.

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.