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Detector Makes Unearthly Impression

Photonics Spectra
Nov 1997
Michael D. Wheeler

The highly touted Mars Pathfinder voyage in August made headlines not just because it afforded the world its first up-close images of the Red Planet, but also because it marked NASA's successful use of inexpensive, commercially available technology.


On Sojourner's wanderings, the spectrometer recorded large amounts of iron along with lesser amounts of magnesium, aluminum, calcium and potassium in Mars' topsoil.

This included photonics technology -- notably Amptek Inc.'s XR-100T x-ray detector and the S1223 PIN photodiode from Hamamatsu. Both devices were incorporated into an alpha proton x-ray spectrometer aboard Pathfinder's rover, Sojourner.

A derivative of instruments flown on the Russian Vega and Phobos satellites, and identical to the spectrometer slated for use on the failed Russian Mars '96 voyage, the device works by bombarding rock or soil samples with alpha particle radiation. When the alpha proton particles hit the test sample, certain interactions such as backscattering and proton production occur. These interactions yield x-rays that are then deflected back to the detector. The photodiode inside the rover's sensor head converts the x-rays to electrical charges, which scientists use to determine exactly what is present in the sample.

On Sojourner's wanderings, the spectrometer recorded large amounts of iron along with lesser amounts of magnesium, aluminum, calcium and potassium in Mars' topsoil. Following tests of the soil, scientists sent Sojourner to test Martian rocks. The first, playfully named Barnacle Bill, contained large amounts of silicate, suggesting the probability of large deposits of mineral quartz.

A near miss

While the reverberations of the Martian discoveries continue, John Pantazis, Amptek president and co-developer of the x-ray detector, said his device almost didn't make it aboard the rover. Pantazis recalled he had originally contacted NASA years ago with an idea for a thermoelectrically cooled detector, rather than one cooled by liquid nitrogen. After NASA turned down the proposal, he set about developing a device for use in common medical applications. Not until several years later did he hear from NASA again -- although indirectly. A group from the University of Chicago was developing a spectrometer for NASA and needed a replacement for the mercuric iodine detector.


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