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The X-ray Spectrometer

The X-ray Spectrometer (XRS) aboard Suzaku consists of an array of X-ray microcalorimeters. The X-ray microcalorimeter determines the energy of an X-ray by measuring the temperature change in a piece of silicon when an X-ray hits it. Just as the hood of an automobile warms up when left in the Sun, the silicon heats up when it absorbs an X-ray. The amount that it heats up is proportional to the energy of the incident X-ray.

The X-ray Microcalorimeter Array
The X-ray Microcalorimeter Array. Click on the image for a video clip of Dr. Caroline Kilbourne explaining the array.

The X-ray microcalorimeter consists of a 6 x 6 array of silicon detectors. Silicon itself is not very good at absorbing X-rays. So each detector has a tiny absorber made of mercury telluride. The X-rays heat up the absorber, which heats up the silicon detector. The entire array measures about 3.5 mm on a side.

The temperature increase due to a single X-ray is very small, about 0.001 degrees. To detect this small change, the detector must be kept cold in order to reduce its own natural thermal energy. So the detector must be kept at very cold temperatures - 0.06 K, i.e 0.06 degrees above absolute zero. This is accomplished using a 3-stage cooling process, consisting of solid neon (at 17 K), liquid helium (at 1.2 K) and an Adiabatic Demagnetization Refrigerator (0.06 K). The ADR uses the magnetic properties of certain types of salts to cool the microcalorimeter to its operating temperature.

The result is that the XRS can determine the energy of an X-ray to 0.1%. If police had radar as accurate as the XRS, you could get a ticket when driving at 100.1 km/hour when the posted speed limit is 100 km/hour. The XRS is sensitve to X-rays ranging in energy from 0.3 to 12 keV.

Publication Date: June 2005

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