Compound Semiconductor Radiation Detectors
We discuss the potential benefits of using compound semiconductors for the detection of X- and γ-ray radiation. While Si and Ge have become detection standards for energy dispersive spectroscopy in the laboratory, their use for an increasing range of applications is becoming marginalized by one or more of their physical limitations; namely the need for ancillary cooling systems or bulky cryogenics, their modest stopping powers and radiation intolerance. Compound semiconductors encompass such a wide range of physical properties that it is technically feasible to engineer a material to any application. Wide band-gap compounds offer the ability to operate in a wide range of thermal and radiation environments, whilst still maintaining sub-keV spectral resolution at hard X-ray wavelengths. Narrow band-gap materials, on the other hand, offer the potential of exceeding the spectral resolution of both Si and Ge, by as much as a factor of 3. Assuming that the total system noise can be reduced to a level commensurate with Fano noise, spectroscopic detectors could work in the XUV, effectively bridging the gap between the ultraviolet and soft X-ray wavebands. Thus, in principle, compound semiconductor detectors can provide continuous spectroscopic coverage from the far infrared through to γ-ray wavelengths. However, while they are routinely used at infrared and optical wavelengths, in other bands, their development has been plagued by material and fabrication problems. This is particularly true at hard X- and γ-ray wavelengths, where only a few compounds (e.g., GaAs, CdZnTe and HgI 2 ) have evolved sufficiently to produce working detection systems. In this paper, we examine the current status of research in compound semiconductors and by a careful examination of material properties and future requirements, recommend a number of compounds for further development. In the longer term, when material problems are sufficiently under control, we believe the future lies in the development of heterostructures and inserted interface layers to overcome contacting problems and quantum heterostructures and superlattices to facilitate low-noise readout.

title: A BeppoSAX observation of the massive x-ray binary 4U1700-37
abstract: A 0.5-200 keV BeppoSAX spectrum of the non- pulsating high-mass X-ray binary 4U 1700-37 is presented. The spectrum is well characterized by the standard accret- ing pulsar model of an absorbed power-law with a photon in- dex of 1:07 0:02 0:03 , sharply modified by an exponential cutoff above 5:9 0:2 keV. The e-folding energy of the cutoff is 23:9 0:5 keV. A soft bremsstrahlung component with a tem- perature of0:20:1 keV is also required, together with a narrow iron line at 6.5 keV. Both continuum components are absorbed by a column of (5:1 0:2) 10 22 atom cm 2 . There is some evidence for the presence of a broad cyclotron absorption fea- ture at 37 keV, although we cannot exclude the possibility that this is due to an incorrect modeling of the continuum, or instrumental effects. The hypothesis that the compact object is a neutron star rather than a black hole seems most likely.

title: BeppoSAX observations of GRB970402
abstract: GRB970402 is the fourth gamma-ray burst detected by BeppoSAX simultaneously in the Gamma Ray Burst Monitor (GRBM) and one of the two Wide Field Cameras (WFCs). A rapid pointing of the BeppoSAX Narrow Field Instruments (NFIs) 8 hours after the GRB led to the identification of an unknown weak X-ray source: 1SAX J1450.1-6920. Its position was approximately at the center of the 3 arcmin error circle derived from the WFC image. Both the Medium Energy (MECS, 2-10 keV) and Low Energy (LECS, 0.1-10 keV) concentrators detected the source. A follow-up observation performed 1.5 days later and lasting 54 ks showed that the source had faded almost to, but not below the detectability threshold. The flux decrease between the two observations was a factor ~= 2.5. 1SAX J1450.1-6920 was the second X-ray afterglow associated with a GRB. Searches promptly started at other wavelengths (optical, IR, radio) did not reveal any transient event within the 3 arcmin error circle.

title: SLAC Linac Preparations for FACET
abstract: The SLAC 3km linear electron accelerator has been cut at the two-thirds point to provide beams to two independent programs. The last third provides the electron beam for the Linac Coherent Light Source (LCLS), leaving the first two-thirds available for FACET, the new experimental facility for accelerator science and test beams. In this paper, we describe this separation and projects to prepare the linac for the FACET experimental program.

title: THE COMPLEX X-RAY SPECTRUM OF THE LOW-MASS X-RAY BINARY 4U 1626-67
abstract: We report on observations of the X-ray pulsar 4U 1626-67 by the LECS instrument on-board Bepposax. We confirm the recent ASCA discovery of excess emission near 1 keV (Angelini et al. 1995). The pulse period of 7.66794 +/- 0.00004 s indicates that the source continues to spin-down. The phase averaged spectrum is well fit by an absorbed power-law of photon index 0.61 +/- 0.02 and a blackbody of temperature 0.33 +/- 0.02 keV, together with an emission feature at 1.05 +/- 0.02 keV. This spectral shape is similar to that observed by ASCA during the spin-down phase, but significantly different from measurements during spin-up. This suggests that the change in spectrum observed by ASCA may be a stable feature during spin-down intervals. The source intensity is a factor ~2 lower than observed by ASCA three years earlier, confirming that 4U 1626-67 continues to become fainter with time.

title: Gamma-Ray Burst 980329 and Its X-Ray Afterglow
abstract: The gamma-ray burst (GRB) 980329 is the brightest gamma-ray burst detected so far with the Wide Field Cameras aboard BeppoSAX, in both gamma rays and X-rays. With respect to its fluence (2.6×10−5 ergs s−1 cm−2 in 50-300 keV), it would be in the top 4% of gamma-ray bursts in the 4B catalog (Meegan et al.). The time-averaged burst spectrum from 2 to 20 keV and from 70 to 650 keV can be well described by the empirical model of Band et al. The resulting photon index above the break energy is exceptionally hard at -1.32±0.03. An X-ray afterglow was detected with the narrow-field instruments aboard BeppoSAX 7 hr after the event within the error box as determined with the Wide Field Cameras. Its peak flux is (1.4±0.2)×10−12 ergs s−1 cm−2 (2-10 keV). The afterglow decayed according to a power-law function with an index of -1.35±0.03. GRB 980329 is characterized by being bright and hard and by lacking strong spectral evolution.

title: Two-component model for cosmic-ray fluctuations at neutron monitor energies