AAS Meeting #193 - Austin, Texas, January 1999
Session 66. X-Ray/Gamma-Ray Detectors
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[66.05] Background Studies in CZT Detectors at Balloon Altitudes

K. R. Slavis, P. F. Dowkontt, J. W. Epstein, P. L. Hink (Washington University), J. L. Matteson, F. Duttweiler, G. L. Huszar, P. C. Leblanc, R. T. Skelton, E. A. Stephan (University of California-San Diego)

Cadmium Zinc Telluride (CZT) is a room temperature semiconductor detector well suited for high energy X-ray astronomy. We have developed a CZT detector with crossed strip readout, 500 micron resolution, and an advanced electrode design that greatly improves energy resolution. The latter varies from 3 keV to 6 keV FWHM over the range from 14-184 keV. We have conducted two balloon flights using this cross-strip detector and a standard planar detector sensitive in the energy range of 20-350 keV. These flights utilized a total of seven shielding schemes: 3 passive (7, 2, and 0 mm thick Pb/Sn/Cu), 2 active (NaI-CsI with 2 opening angles) and 2 hybrid passive-active. In the active shielding modes, the shield pulse heights were telemetered for each CZT event, allowing us to study the effect of shield energy-loss threshold on the background. The flights were launched from Fort Sumner, NM in October 1997 and May 1998, and had float altitudes of 109,000 and 105,000 feet respectively. Periodic energy calibrations showed the detector performance to be identical to that in the laboratory. The long duration of the May flight, 22 hours, enables us to study activation effects in the background. We present results on the effectiveness of each of the shielding schemes, activation effects and two new background reduction techniques for the strip detector. These reduction techniques employ the depth of interaction, as indicated by the ratio of cathode to anode pulse height, and multiple-site signatures to reject events that are unlikely to be X-rays incident on the detector's face. The depth of interaction technique reduces the background by a factor of 4 in the 20-40 keV energy range with passive shielding. Our preliminary results indicate a background level of 8.6x10-3 cts/cm2-s-keV using passive shielding and 6x10-4 cts/cm2-s-keV using active shielding in the 2040 keV range.

The author(s) of this abstract have provided an email address for comments about the abstract: slavis@hbar.wustl.edu

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