Engineering topics

Varnell, L. S.

Publications and source records attributed to Varnell, L. S..

9 records

NASA NTRS1996

Radiation Effects in CdZnTe Gamma-Ray Detectors Produced by 199 MeV Protons

Many future space missions will use Cadmium Sinc Telluride (CdZnTe) gamma-ray detectors because their operation at room temperature makes compact, lightweight detector systems possible. Even though instruments for space using CdZnTe detectors have already been built, the effect of the high-energy particle space environment on these detectors has not been measured. To determine the effect of energetic charged particles on these detectors, we have bombarded seveal CdZnTe detectors with 199 MeV protons at the Indiana University Cyclotron Facility.

radiation↗

NASA NTRS1993

Gamma-ray imaging with germanium detectors

Externally segmented germanium detectors promise a breakthrough in gamma-ray imaging capabilities while retaining the superb energy resolution of germanium spectrometers. By combining existing position-sensitive detectors with an appropriate code aperture, two-dimensional imaging with 0.2-deg angular resolution becomes practical for a typical balloon experiment. Much finer resolutions are possible with larger separations between detectors and the coded aperture as would be applicable for space-based or lunar-based observatories. Two coaxial germanium detectors divided into five external segments have been fabricated and have undergone extensive performance evaluation and imaging testing in our laboratory. These tests together with detailed Monte Carlo modeling calculations have demonstrated the great promise of this sensor technology for future gamma-ray missions.

Mahoney, W. A.↗

NASA NTRS1993

High-resolution imaging gamma-ray spectroscopy with externally segmented germanium detectors

Externally segmented germanium detectors promise a breakthrough in gamma-ray imaging capabilities while retaining the superb energy resolution of germanium spectrometers. An angular resolution of 0.2 deg becomes practical by combining position-sensitive germanium detectors having a segment thickness of a few millimeters with a one-dimensional coded aperture located about a meter from the detectors. Correspondingly higher angular resolutions are possible with larger separations between the detectors and the coded aperture. Two-dimensional images can be obtained by rotating the instrument. Although the basic concept is similar to optical or X-ray coded-aperture imaging techniques, several complicating effects arise because of the penetrating nature of gamma rays. The complications include partial transmission through the coded aperture elements, Compton scattering in the germanium detectors, and high background count rates. Extensive electron-photon Monte Carlo modeling of a realistic detector/coded-aperture/collimator system has been performed. Results show that these complicating effects can be characterized and accounted for with no significant loss in instrument sensitivity.

Callas, J. L.↗

NASA NTRS1991

Segmented Ge detector rejection of internal beta activity produced by neutron irradiation

Future Ge spectrometers flown in space to observe cosmic gamma-ray sources will incorporate segmented detectors to reduce the background from radioactivity produced by energetic particle reactions. To demonstrate the effectiveness of a segmented Ge detector in rejecting background events due to the beta decay of internal radioactivity, a laboratory experiment has been carried out in which radioactivity was produced in the detector by neutron irradiation. A Cf-252 source of neutrons was used to produce, by neutron capture on Ge-74 (36.5 percent of natural Ge) in the detector itself, Ge-75 (t sub 1/2 = 82.78 min), which decays by beta emission with a maximum electron kinetic energy of 1188 keV. By requiring that an ionizing event deposit energy in two or more of the five segments of the detector, each about 1-cm thick, the beta particles, which have a range of about 1-mm, are rejected, while most external gamma rays incident on the detector are counted. Analysis of this experiment indicates that over 85 percent of the beta events from the decay of Ge-75 are rejected, which is in good agreement with Monte Carlo calculations.

Varnell, L. S.↗

NASA NTRS1989

The HEAO 3 background - Spectrum observed by a large germanium spectrometer in low earth orbit

Gamma-ray background spectra, obtained in the energy range from 45 keV to 10 MeV with the cooled high-resolution Ge spectrometer on the HEAO-3 satellite over a 50-day period in 1979-1980 are reported and analyzed. The instrument design and performance characteristics are reviewed; the data-analysis procedures are outlined; about 130 background lines are listed in a table with energies, count rates, and tentative identifications; and the spectra are presented graphically and discussed in detail.

Wheaton, W. A.↗

NASA NTRS1988

Gamma-ray observations of Co-56 in SN 1987A

During observations of SN 1987A 286 days after the supernova explosion using a JPL high-resolution gamma-ray spectrometer, a line feature was found in the net supernova spectrum at an energy of 1240.8 + or - 1.7 keV, with an intrinsic width of 8.2 + or - 3.4 keV FWHM, and a net flux of 0.0021 + or - 0.0007 photons/sq cm per s. This feature is interpreted as the 1238 keV line from the decay of Co-56 in the supernova remnant. The blueshift and intrinsic broadening of the 1238 keV line are consistent with models incorporating mixing of the radioactive cobalt in the expanding eject.

Mahoney, W. A.↗

NASA NTRS1985

Segmented Detector for Cosmic Gamma Rays

Segmented structure distinguishes gamma rays of cosmic origin from internal radioactivity. Preamplifier and discriminator channel connects each electrically independent segment of gamma-ray detector to logic circuitry. Logic compares and analyzes events in segments to determine sources. According to Monte Carlo calculations, sensitivity of detector two to three times better than conventional detectors because background interactions discerned and eliminated.

Varnell, L. S.↗

NASA NTRS1984

A position-sensitive germanium detector for gamma-ray astronomy

The critical problem in high-resolution cosmic gamma-ray spectroscopy in the energy range from 0.02 to 10 MeV is the limited spectral sensitivity of the detectors used. This results from the small effective area of the detectors and the high background noise due to induced radioactivity and scattering in the detectors' high-energy particle environment. The effective area can be increased by increasing the number of detectors, but this becomes prohibitive because of the size and expense of the resulting instrument. We have taken a new approach: a segmented large-volume germanium gamma-ray detector which can effectively discriminate against internal background yet maintain the high spectral resolution and efficiency of conventional coaxial Ge detectors. To verify this concept, a planar detector divided into two segments has been fabricated and laboratory measurements agree well with Monte Carlo calculations. A large coaxial detector which will be divided into five segments is being built using the techniques developed for the planar detector. Monte Carlo calculations show that the sensitivity (minimum detectable flux) of the segmented coaxial detector is a factor of 2-3 better than conventional detectors because of the reduction in the internal background.

Varnell, L. S.↗

NASA NTRS1978

High-energy proton radiation damage of high-purity germanium detectors

Quantitative studies of radiation damage in high-purity germanium gamma-ray detectors due to high-energy charged particles have been carried out; two 1.0 cm thick planar detectors were irradiated by 6 GeV/c protons. Under proton bombardment, degradation in the energy resolution was found to begin below 7 x 10 to the 7th protons/sq cm and increased proportionately in both detectors until the experiment was terminated at a total flux of 5.7 x 10 to the 8th protons/sq cm, equivalent to about a six year exposure to cosmic-ray protons in space. At the end of the irradiation, the FWHM resolution measured at 1332 keV stood at 8.5 and 13.6 keV, with both detectors of only marginal utility as a spectrometer due to the severe tailing caused by charge trapping. Annealing these detectors after proton damage was found to be much easier than after neutron damage.

Pehl, R. H.↗