N. Gehrels

Bright X-ray Flares in Gamma-Ray Burst Afterglows

Gamma-ray burst (GRB) afterglows have provided important clues to the nature of these massive explosive events, providing direct information on the nearby environment and indirect information on the central engine that powers the burst. We report the discovery of two bright x-ray flares in GRB afterglows, including a giant flare comparable in total energy to the burst itself, each peaking minutes after the burst. These strong, rapid x-ray flares imply that the central engines of the bursts have long periods of activity, with strong internal shocks continuing for hundreds of seconds after the gamma-ray emission has ended.

Instrumental background in balloon-borne gamma-ray spectrometers and techniques for its reduction

Abstract A study of the instrumental background in balloon-borne gamma-ray spectrometers is presented. The calculations are based on newly available interaction cross sections and new analytic techniques, and are the most detailed and accurate published to date. Results compare well with measurements made in the 20 keV to 10 MeV energy range by the Goddard Low Energy Gamma-ray Spectrometer (LEGS). The principal components of the continuum background in spectrometers with Ge detectors and thick active shields are (1) elastic neutron scattering of atmospheric neutrons on the Ge nuclei, (2) aperture flux of atmospheric and cosmic gamma rays, (3) β − decays of unstable nuclides produced by nuclear interactions of atmospheric protons and neutrons with Ge nuclei, and (4) shield leakage of atmospheric gamma rays. The improved understanding of these components leads to several recommended techniques for reducing the background. These include minimizing the passive material inside the shield and reducing the level of the shield threshold. A new type of coaxial n-type Ge detector with its outer contact segmented into horizontal rings can be used in various modes to reduce background in the 20 keV to 1 MeV energy range. The resulting improvement in instrument sensitivity to spectral lines is a factor of ∼ 2 in this energy range.

CdZnTe and CdTe detector arrays for hard X-ray and gamma-ray astronomy

Abstract A variety of CdZnTe and CdTe detector arrays were fabricated at NASA/GSFC for use in hard X-ray and gamma-ray astronomy. Mosaic, pixel, and 3-D position-sensitive detector arrays were built to demonstrate the capabilities for high-resolution imaging and spectroscopy for 10 to 2xa0MeV. This paper will summarize the different arrays and their applications for instruments being developed at NASA/GSFC. Specific topics to be addressed include materials characterization, fabrication of detectors, ASIC readout electronics, and imaging and spectroscopy tests.

Isotopically enriched germanium detectors for astrophysical gamma-ray spectroscopy

Abstract A study is presented of the instrumental background in astrophysical gamma-ray spectrometers using isotopically enriched germanium detectors. Calculations show that the beta-decay background, which is the largest component between ∼ 0.1 and 1.0 MeV in balloon-borne and satellite spectrometers, is dominated by the activation of 74 Ge. This component can be reduced by an order of magnitude using detectors enriched to > 80% in 70 Ge. The predicted reduction in the total background for current balloon-borne instruments is more than a factor of 1.7 between 0.2 and 1.0 MeV. For future satellite instruments, the reduction in this energy range is by more than a factor of 5.

Performance of a Compton telescope using position-sensitive germanium detectors

We describe the use of position sensitive planar germanium detectors in a Compton telescope/camera. This Compton telescope achieves the very good energy resolution (/spl Delta/Etotal<4 keV) associated with germanium detectors and good position resolution (2 mm). By combining a 25/spl times/25 strip (2 mm pitch) detector with a 5/spl times/5 strip detector (9 mm pitch), we created a telescope with 625/spl times/25 pixel combinations. Using this detector pair, we have reconstructed positions with an angular resolution of 1 degree FWHM and 15 arcminute centroiding. Point sources are identified with less than 100 full energy events with simple image reconstruction. The angular resolution is currently limited by the uncertainty in the absolute position of the detectors and the size of the second detector pixels. We show the expected angular resolution when the pixel size no longer dominates the angular resolution and discuss proposed applications.

Spectroscopy, imaging and Compton-scatter polarimetry with a germanium strip detector

Germanium strip detectors combine the excellent energy resolution possible with germanium detectors with fine two-dimensional spatial resolution determined by orthogonal strip electrodes. We are testing a detector with an active volume of 5/spl times/5/spl times/1 cm/sup 3/ and a total of 50 electrodes, 25 on each side with a 2mm pitch. Potential astrophysics applications include use as a focal plane for a coded-aperture telescope in the energy range /spl sim/5-500 keV, as the focal plane of a grazing-incidence telescope in the /spl sim/5-100 keV band, or in a Compton scatter telescope. The utilization of germanium strip detectors for these applications requires proof of their performance capabilities in terms of energy and spatial resolution, and efficiencies for single and multi-pixel interactions. In this paper, we demonstrate the excellent spectroscopy and imaging performance of a 2mm pitch strip detector, and report on the results of Monte-Carlo simulations of photopeak efficiencies for single and multi-pixel interactions. These simulations show that 2-pixel interactions are a significant fraction of the photopeak efficiency in the 80-500 keV range. We demonstrate that these interactions can be used to measure linear polarization of a normally incident beam.

XA readout chip characteristics and CdZnTe spectral measurements

The authors report on the performance of a CdZnTe (CZT) array readout by an XA (X-ray imaging chip produced at the AMS foundry) application specific readout chip (ASIC). The array was designed and fabricated at NASA/Goddard Space Flight Center (GSFC) as a prototype for the Burst Arc-Second Imaging and Spectroscopy gamma-ray instrument. The XA ASIC was obtained from Integrated Detector and Electronics (IDE), in Norway. Performance characteristics and spectral data for /sup 241/Am are presented both at room temperature and at -20/spl deg/C. The measured noise (/spl sigma/) was 2.5 keV at 60 keV at room temperature. This paper represents a progress report on work with the XA ASIC and CZT detectors. Work is continuing and in particular, larger arrays are planned for future NASA missions.

CZT strip detectors for imaging and spectroscopy: collimated beam and ASIC readout experiments

We report the status of ongoing investigations into Cadmium Zinc Telluride (CZT) strip detectors for application in hard X-ray astronomy. We have instrumented a nine strip by nine strip region of a two sided strip detector made in our detector fabrication facility. In order to measure the position resolution of our detectors, we have implemented a collimated beam that concentrates radiation to a spot size less than the strip width of our detector. We have also performed charge collection studies as a function of incident photon energy and bias voltage with a single sided, 100 /spl mu/m pitch CZT strip detector wire bonded to an SVX ASIC charge amplifier. The detectors exhibited excellent strip uniformity in terms of photon count rate and spectroscopic information.

Charge division readout of a two-dimensional germanium strip detector

Abstract Germanium strip detectors combine superior spectroscopy typical of germanium detectors with good spatial resolution. Our work has been to develop a 2-dimensional position readout of a 5 × 5 strip X–Y detector. The prototype device uses two capacitive charge division strings. Each string is read out by two 13 bit ADCs, one on each end of the string. The four data channels are stored as an event list for subsequent processing. We form a response map over the detector surface in order to locate the position of each interaction with the spatial resolution of the strip pitch, in our case 9 mm. Cross-talk and non-linearities are removed using the response map. Energy resolution of 5–6 keV FWHM is achieved between 60 and 662 keV across the full surface of the detector.

Capacitive charge division readout of a double-sided germanium strip detector

We have implemented a capacitive charge-division readout for a germanium 5/spl times/5 orthogonal strip detector. We present measurements of non-linearity, and of energy and position resolution as a function of energy (60 and 662 keV) and of network capacitance. Energy and position non-linearities, which are worse for low capacitance networks, appear in the response along the charge division network, as well as along the length of each detector strip. The nonlinearities can be corrected by using an independent gain calibration for each pixel, allowing for use of a lower capacitance charge division network which provides better position resolution. >