Gamma Ray Spectrometer (GRS)

Description

The Gamma Ray Spectrometer (GRS) experiment was designed to record the spectrum of gamma rays and neutrons emitted by the radioactive decay of elements contained in the Martian surface. The data from the GRS were to be used to: (1) determine the composition of the Martian crust; (2) correlate the composition of the fine-grained dust composing much of the Martian surface with parent material to yield insight into weathering processes; (3) determine the distribution and depth of volatiles (primarily H2O and CO2) in the Martian regolith; and, (4) determine a variety of atmospheric processes (especially atmospheric thickness and the deposition and sublimation of the polar ice caps) through the attenuation of gamma rays. In addition to detecting gamma rays and neutrons emitted from the Martian surface and near sub-surface, the GRS could be used to determine characteristics of galactic, extragalactic, and solar gamma rays, and would function as a vertex of the Interplanetary Network of spacecraft to locate gamma ray burst events (along with the Compton Observatory, Ulysses, and Granat).

The sensor head, excluding the sun shield, was in the form of a right-rectangular pyramid, with a base 75.4 cm on a side and with a vertex height of 43.5 cm. The sensor head was mounted at the end of a 6 m boom. The primary components of the sensor head were a germanium detector assembly, a cooler subsystem, and an anti-coincidence shield/neutron detector. The gamma ray detector consisted of a large, single crystal of n-type ultrahigh-purity germanium (HPGe), about 5.5 cm in diameter and 5.5 cm long. The crystal was implanted with electrodes so that it formed a diode which was operated in a reverse bias mode with a potential of 3000 V and a leakage current of less than 1 nA. The detector was capable of detecting gamma rays in a range from 200 keV to 10 MeV. The gamma ray detector was surrounded by a V-shaped anti-coincidence shield which served two purposes. The first was to assist in discriminating charged particle detections from gamma rays. The shield also served as a neutron detector. The shield was lined with a plastic scintillator, doped with boron to increase the neutron absorption cross-section, which, when struck by ionizing radiation, emitted light which was picked up by one of four photomultiplier tubes mounted in the center of the four triangular pyramid faces. In this manner, the number of incident neutrons could be calculated. This shield permitted the detection of neutrons in the range 0--2.5 MeV.

The spectral resolution of both the gamma ray and neutron detectors were between 0.61--1.22 keV. The spatial resolution of the gamma ray spectrometer at the surface of Mars was 280 km x 280 km. The HPGe crystal was mounted on a support to the vertex of the pyramid with the crystal positioned close to the base. In order to permit the HPGe detector to be run as cold as possible, the sensor head contained a set of V-shaped vanes, made of vapor-deposited gold on graphite, which were designed to radiate heat away from the detector assembly. In addition, a sunshade was mounted above the sensor head which permitted a 180 x 139 degree view of space to the radiator assembly. Although the detector should optimally have been run cooler (close to the boiling point of liquid nitrogen, 77 K), the radiative cooling system permitted normal operation of the detector at about 130 K. Prolonged exposure to radiation (approximately equivalent to 1 year exposure in space) at this temperature degrades the resolution of such a detector. However, the germanium crystal could have the radiation damage removed by heating it to temperatures on the order of 370 K for several days. Such an annealing process was intended to be conducted on an annual basis with little degradation of the resolution.

Contact with Mars Observer was lost for unknown reasons on August 21, 1993, three days before scheduled orbit insertion, so only cruise mode data were returned for this investigation.