An evolutionary algorithm (ES) for automated deconvolution of γ-ray spectra is described that fits peak shape
morphologies typical of spectra acquired from variably radiation damaged γ-ray detectors. Space radiation effects
significantly impair semi-conductor γ-ray detector efficiency and induce variable degrees of nuclide peak broadening,
distortion in spectra. Mars Odyssey Gamma-ray spectrometer data are used to demonstrate applicability of described
algorithms for three degrees of radiation damage. ES methods accurately identify and quantify the discrete set of
nuclide peaks in an arbitrary spectrum using a nuclide library. A novel method of constraining peak low energy tails,
broadened by detector radiation damage, reduces the peak shape model from six parameters to four yielding a
significant minimization of model complexity. Benefits of this approach include the simple implementation of highly
specific parameter constraints that appropriately define feasible solution spaces. Methods describe peak low energy
tailing descriptors as a continuum of low energy peak tailing curves representing increasing degrees of radiation
damage. Curves are addressable by a single real valued parameter. Results illustrate the use of methods to simply
describe relative radiation dosimetry using this parameter. Analysis of degraded spectra indicates method sensitivity
to low and high levels of space radiation damage prior to and post MO-GRS detector annealings.
This study investigates the effectiveness of chemical etchants to remove surface damage caused by mechanical polishing during the fabrication of Cd0.9Zn0.1Te (CZT) nuclear radiation detectors. We evaluate different planar CZT devices fabricated from the same CZT crystals. All detectors used electroless Au for the metal contacts. Different polishing particle sizes ranging from 22.1-μm SiC to 0.05-μm alumina were used, which caused different degrees of surface roughness. Current-voltage measurements and detector testing were used to characterize the effects of surface roughness and etching on the material and detector properties.
The Mars Odyssey spacecraft was launched on 7 April 2001 and went into orbit around Mars on 24 October 2001. One of the primary scientific instruments carried on the spacecraft is a germanium gamma-ray spectrometer that will measure the elemental composition of Mars. Cruise measurements taken during July and August 2001 are used to characterize the energy resolution of the detector and to measure and identify the background gamma rays. These gamma rays originate in the detector, in material surrounding the detector, and from the spacecraft. More than 110 gamma rays were observed in the background spectrum. The sources of most of these gamma rays were tentatively identified. Understanding the sources of the background gamma rays is important for the future when using the orbital data to determine the composition of Mars.
The literature of radiation damage measurements on cadmium zinc telluride (CZT), cadmium telluride (CT), and mercuric iodide is reviewed and supplemented in the case of CZT by new alpha particle data. CZT strip detectors exposed to intermediate energy proton fluences exhibit increased interstrip leakage after 1010 p/cm2 and significant bulk leakage after 1012 p/cm2. CZT exposed to 200 MeV protons shows a two-fold loss in energy resolution after a fluence of 5 X 109 p/cm2 in thick planar devices but little effect in 2 mm devices. No energy resolution effects were noted from moderated fission spectrum neutrons after fluences up to 1010 n/cm2, although activation was evident. Exposures of CZT to 5 MeV alpha particle at fluences up to 1.5 X 1010 (alpha) /cm2 produced a near linear decrease in peak position with fluence and increases in FWHM beginning at about 7.5 X 109 (alpha) /cm2.
Timothy McClanahan, Irina Mikheeva, Jacob Trombka, Samuel Floyd, William Boynton, H. Bailey, Jasbir Bhangoo, Richard Starr, Pamela Clark, Larry Evans, Steven Squyres, Elaina McCartney, E. Noe, Ralph McNutt, Johannes Brueckner
An x-ray and gamma-ray spectrometer (XGRS) is onboard the Near Earth Asteroid Rendezvous (NEAR) spacecraft to determine the elemental composition of the surface of the asteroid 433 Eros. The Eros asteroid is highly non-spherical in physical shape and the development of data management and analysis methodologies are in several areas a divergence from traditional remotely sensed geographical information systems techniques. Field of view and asteroid divergence from traditional remotely sensed geographical information system techniques. Field of view and asteroid surface geometry must be derived virtually and then combined with real measurements of solar, spectral and instrument calibration information to derive meaningful scientific results. Spatial resolution of planned geochemical maps will be improved from the initial conditions of low statistical significance per integration by repeated surface flyovers and regional spectral accumulation. This paper describes the results of a collaborative effort of design and development of the NEAR XGRS instrument ground system undertaken by participants at the Goddard Space Flight Center, University of Arizona, Cornell University, Applied Physics Laboratory, and Max Planck Institute.
The on-board flight software for the Near Earth Asteroid Rendezvous (NEAR) spacecraft was modified to produce continuous 1-sec sampled rate information from the shield of the x-ray and gamma ray spectrometer (XGRS) instrument. Since the XGRS shield can also detect gamma ray bursts (GRB), this rate information can be used in combination with the GRB detections by the Ulysses and near-Earth GRB instruments as part of the interplanetary network (IPN) to triangulate the source direction of GRBs. It is the long baseline of NEAR combined with the Ulysses baseline that makes small error box locations possible. We have developed an automated system to analyze the periodic telemetry dumps from the NEAR spacecraft. It extracts this new data type, scans the ate information for increases which are plausibly of GRB origin, and combines these with the GRB detections from the others spacecraft. Because the processing is automated, the time delay to produce the triangulated positions is kept to a minimum, up to 48 hours. This automated processing and distribution of the GRB locations is done within the GRB Coordinates Network system. About 60 locations per year with errors ranging from a few to tens of arcminutes are expected. These rapid precise localizations may provide about 10 times the rate currently provided by the WFC and NFI instruments on BeppoSAX.
Two new small satellites will be launched in late 1996, each carrying x-ray and gamma-ray detectors capable of high spectral resolution while operating at or near room temperature. The Argentinean Satellite de Aplicaciones Cientificas B (SAC-B) and the Small Spacecraft Technology Institute Clark mission will each carry several arrays of x- ray detectors primarily to study solar flares and gamma ray bursts. Arrays of small (8 mm X 8 mm X 2 mm thick) cadmium zinc telluride detectors will provide x-ray measurements in the 5 - 60 keV range with an energy resolution of 2 - 4 keV. Arrays of both silicon avalanche photodiodes (APDs) and P-intrinsic-N photodiodes will provide energy coverage from 2 to 20 keV with approximately 1 keV resolution. High energy data from 30 to 300 keV will be obtained on SAC-B using CsI(TI) scintillators coupled to silicon APDs, resulting in significant savings in weight, power, and volume relative to conventional CsI- photomultiplier systems.
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