We report on ASCA observations of the low-mass X-ray binaries GX 354[0 and KS 1731[260. The spectrum of GX 354[0 is best described as a power law or a Comptonized spectrum with q D 5 and kT D 8 keV and a residual at D6.5 keV. The residual may be a disk reÑection or a Compton-broadened Gaussian line from the hot inner advection-dominated accretion ÑowÈlike coronal region. The absorption column density to the source is 2.9 ] 1022 cm~2. No soft thermal component was detected. The spectrum from KS 1731[260 is softer, and it is best Ðt with a two-component model with a column density of 1.1 ] 1022 cm~2. The likely interpretation is emission from a Comptonizing cloud with an optical depth q [ 12 and either a neutron star or a disk blackbody emission. We discuss the likely location of the Comptonizing cloud for both sources within the context of several proposed emission models.

We report progress in the study of thick CZT strip detectors for 3-d imaging and spectroscopy and discuss two approaches to device design. We present the spectroscopic, imaging, detection efficiency and response uniformity performance of prototype devices. Unlike double-sided strip detectors, these devices feature both row and column contacts implemented on the anode surface. This electron-only approach circumvents problems associated with poor hole transport in CZT that normally limit the thickness and energy range of double-sided strip detectors. These devices can achieve similar performance to pixel detectors. The work includes laboratory and simulation studies aimed at developing compact, efficient, detector modules for 0.05 to 1 MeV gamma radiation measurements. The low channel count strip detector approach can significantly reduce the complexity and power requirements of the readout electronics. This is particularly important in space-based coded aperture or Compton telescope instruments requiring large area, large volume detector arrays. Such arrays will be required for NASA's Black Hole Finder Probe (BHFP) and Advanced Compton Telescope (ACT).

This paper reviews the development status of GRAPE (the Gamma-Ray Polarimeter Experiment), a hard X-ray Compton Polarimeter. The purpose of GRAPE is to measure the polarization of hard X-rays in the 50-300 keV energy range. We are particularly interested in X-rays that are emitted from solar flares and gamma-ray bursts (GRBs), although GRAPE could also be employed in the study of other astrophysical sources. Accurately measuring the polarization of the emitted radiation will lead to a better understating of both emission mechanisms and source geometries. The GRAPE design consists of an array of plastic scintillators surrounding a central high-Z crystal scintillator. The azimuthal distribution of photon scatters from the plastic array into the central calorimeter provides a measure of the polarization fraction and polarization angle of the incident radiation. The design of the detector provides sensitivity over a large field-of-view (> π steradian). The design facilitates the fabrication of large area arrays with minimal deadspace. This paper presents the latest design concept and the most recent results from laboratory tests of a GRAPE science model.

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We report results of an experiment conducted in May 1997 to measure CdZnTe background and background reduction schemes in space flight conditions similar to those of proposed hard X-ray astrophysics missions. A 1 cm 2 CdZnTe detector was placed adjacent to a thick BGO anticoincidence shield and flown piggybacked onto the EXITE2 scientific balloon payload. The planar shield was designed to veto background counts produced by local gamma-ray production in passive material and neutron interactions in the detector. The CdZnTe and BGO were partially surrounded by a Pb-Sn-Cu shield to approximate the grammage of an X-ray collimator, although the field of view was still ∼ 2π sr. At an altitude of 127000 feet we find a reduction in background by a factor of 6 at 100 keV. The non-vetoed background is 9 × 10 −4 cts cm −2 s −1 keV −1 at 100 keV, about a factor of 2 higher than that of the collimated (4.5 • FWHM) EXITE2 phoswich detector. We compare our recorded spectrum with that expected from simulations using GEANT and find agreement within a factor of 2 between 30 and 300 keV. We also compare our results with those of previous experiments using passive lead and active NaI shields, and discuss possible active shielding schemes in future astronomy missions employing large arrays of CdZnTe detectors.