<title>CdZnTe background measurement at balloon altitudes with an active BGO shield</title>

Abstract

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.

Background measurements from balloon-born imaging CZT detectors

“…CZT2 was designed to measure the background of a wide field-of-view pixel-array CZT detector under highaltitude, near-space conditions. A previous planar-detector CZT experiment (CZT1, see Bloser et al 1998, 2 Bloser et al 2002 1 ) explored the effectiveness of an active rear BGO shield on a wide-FOV detector; CZT2 relies on a graded Pb-Sn-Cu shield and an active plastic scintillator shield for charged particle rejection. Both of these approaches (BGO and plastic active shielding) remain in consideration for a survey mission.…”

Laboratory tests on neutron shields for gamma-ray detectors in space

“…EXIST is a coded-aperture, wide "eld of view survey mission, with a wide energy range (&5}600 keV). Several recent experiments on the background of CdZnTe detectors in space environments revealed higher background than expected [2,21,22]. In general, with active shields, the neutron-induced background is expected to be one of the dominant background sources for CdZnTe detectors at high energies (9200 keV) [2].…”

<title>Balloon flight background measurement with actively-shielded planar and imaging CZT detectors</title>

“…At high energies the simulations fail to reproduce the recorded counts by a factor of 2-3, much the same as in the 1997 flight. 9 Given the large uncertainties in the input atmospheric gamma-ray spectrum, this is not too surprising. Energy (keV) Simulated CZT1 good background Good events (BGO = 50 keV) Figure 7.…”

“…CZT is also being considered for the medium gamma-ray band (0.5-50 MeV) as the calorimeter material in advanced Compton telescopes. A major contributor to the maturity of CZT detector technology has been an active campaign of balloon flight tests over the past five years, both by our group [9][10][11] and by others. [12][13][14][15][16] These experiments are necessary to test detector design, readout, and shielding methods under flight conditions, and to measure background levels in the space environment.…”

“…[12][13][14][15][16] These experiments are necessary to test detector design, readout, and shielding methods under flight conditions, and to measure background levels in the space environment. The experiments have progressed from simple one-element detectors with passive shielding 12,13 to single-element detectors with various combinations of active and passive shielding 9,12 to complex multi-anode detectors, either strips [14][15][16] or pixels, 10, 11 again using both passive and active shielding techniques. The general conclusions that may be drawn from this experience are that CZT detector technology is indeed compatible with the needs of balloon and space-flight systems, and that background levels can be made acceptable for sensitive astronomical observations.…”