Gamma Ray Spectrometer - 2001 Mars Odyssey Lunar and Planetary Lab The University of Arizona

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GRS Instrumentation

Gamma Sensor Head

The gamma ray detector is a large (1.2 kg) high-purity Germanium (Ge) crystal. The crystal is held at a voltage of approximately 3000 volts. Little or no current flows (less than one nanoAmp) unless a high-energy ionizing photon or charged particle strikes it. The electric charge from such a strike is amplified, measured and digitally converted into one of 16,384 (214) channels, or bins. After a specified number of seconds, a histogram is produced, which shows the distribution of events (number of strikes) as a function of energy (channel number). This histogram is one gamma ray spectrum.

At left (top) is a model of the gamma sensor head of the GRS. It is mounted on the end of a long (6-meter) boom attached to the orbiter. The boom is necessary to reduce interference from gamma rays generated by the spacecraft. (All elements in space radiate gamma-rays by the same processes which generate them on the surface of Mars).

The gamma sensor head (left, bottom) contains the detector, a radiative cooler, low temperature pre-amplifier , a thermal shield with door, and a bracket to mount it to the end of the boom.

Predicted Count Rate

Gamma Sensor Head Components

The cooler has a door which opens in flight, exposing a radiator, allowing the sensor to cool to below 90 Kelvin for science data collection. The thermal shield and door are needed to allow us to periodically warm up the sensor head to 100 Celsius to anneal radiation damage to the crystal.

The predicted gamma ray count rate at Mars is shown at right. As can be seen, there is a broad, sloping continuum interspersed with distinct lines. The lines will identify elements in the surface layer of Mars. The continuum under the lines comes from gamma-rays which lose a portion of their energy by being scattered, mostly by surface materials overlying the point of emission and the atmosphere. Other lines from less-sensitive elements are two small to be shown on this scale.

The advantage of the high-purity Germanium sensor is that the lines are very sharp. The count rate is very low, but long integration times permit most elements to be determined with a precision of about 10%.

Predicted spectra for 6 hours (detail).

The GRS spectra are typically only 30 seconds in duration, but longer accumulation times are achieved by summing spectra over a particular region of the planet.

Predicted spectra for 231 days (detail).

A detail of the expected Mars spectrum for two different accumulation times is shown at left. At top, 6 hours of accumulated counts is shown. On the bottom is the predicted output for 231 days of integration. Some lines show as sharp peaks, others are buried in the continuum. None of these weak peaks are the analytical peaks used in to determine the element's concentration. The spectrum with the long integration time shows strong peaks for potassium and silicon.

The spatial resolution of the instrument is about 300 km and a region this large receives about 6 hours of accumulation time near the equator at the end of the mission and much longer accumulation times near the poles (about a factor of 5 more). Elements which need longer accumulation times can be determined with degraded spatial resolution by summing spectra over larger regions of the planet. For example, Oxygen, Silicon, Chlorine, Potassium, and Iron can be determined in a 300 km spot, but Nickel and Chromium can only be determined by summing the data over very large regions, e.g. all of the highlands or all of the lowlands.

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