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The Curious Case of the Extra X-Rays
By Jeffrey Marlow

A few sols ago, the CheMin instrument made its long awaited debut with its negative control, blank sample cell measurement. "It was a dry run of the whole system," says instrument co-investigator Allan Treiman, "we turned on all of the parts and it worked."

But there were a few unique blips on the detector, signs that energetic particles were leaking into the CheMin chamber. Instrument chief David Blake pulls up an image in the CheMin room, a collegial, if a bit cozy, operation buzzing with activity. The instrument's scientists, after all, have been eagerly anticipating data for weeks; don't tell them it's just a control measurement.

Blake is pointing at a dark rectangle - the cross section of the detector - accented by a few dozen bright pixels, each of which represents the detection of a high-energy particle. "Our X-ray source is located like this, right above the detector," he says, bringing his palm parallel to the screen, "so the single pixel excitations make sense," as they indicate a source-to-detector perpendicular impact. But there are other features in the image, little paths of bright pixels snaking across the image. The smears reveal that the culprit particles came in at an oblique angle, and couldn't have originated from the X-ray source.

"So where are these coming from?" Blake asks, robbing me of my line. "It turns out that they're secondary X-rays produced as a by-product of the power source." The generator spews protons and neutrons, which smash into other molecules, innocent atomic victims on the spacecraft, on the ground, or in the air. These collisions release energy, occasionally in the form of X-rays that wriggle their way into CheMin.

Fortunately, the seepage of rogue X-rays is little more than a curiosity, as the team's software is able to subtract out the resulting hits, removing the blemishes to leave a pristine reading of the sample.