X-ray Detectors
Detecting Heat Signatures

X-ray photons have energies that are among the highest for light. When detector material absorbs the energy from an incoming photon, the material heats up just a little. The amount of the temperature difference gives a measure of the energy of the incoming X-ray.

X-ray Calorimeters

Photo of a 32 by 32 microcalorimeter array. (Credit: NASA/Goddard Space Flight Center)

Perhaps the most intriguing advance in X-ray astronomy instrumentation in the past couple decades has been the development of single photon calorimeters, spearheaded by work at NASA's Goddard Space Flight Center. These devices detect X-rays by the temperature pulses they generate in a small absorber which is cooled to a fraction of a Kelvin.

When an X-ray photon heats a solid, it gives its energy to the whole solid. On average, each atom vibrates a little bit more than before the X-ray hit. Temperature relates the total energy of a system to its state of disorder, or entropy. The "heat capacity" of a material gives a measure of how much the temperature rises for a given energy input.

To measure the temperature increase in an absorber due to an X-ray, it is necessary to have a very sensitive thermometer – something that had a physical property that changed drastically for a small change in temperature. That detector should also have a small heat capacity, so its temperature would change a lot for a small change in energy. It's also best to have the detector and thermometer work at very low temperatures, because a small temperature change will be much more pronounced in a very cold system.

Magnified image of an array of X-ray calorimeters. (Credit: NASA/Goddard Space Flight Center)

This is how an X-ray calorimeter works. It uses a silicon thermistor which has an electrical resistance which changes dramatically with small changes in temperature. It has a low heat capacity. And it operates at less than 0.1 Kelvin. (For scale, zero on the Kelvin temperature scale is an absolute zero and represents the cessation of all thermal vibrations. Water freezes at 273 K. Nitrogen liquefies at 77 K. Helium liquefies at 4 K. and we operate calorimeters at less than 0.1 K.)

Astro-H will feature an X-ray calorimeter as one of it's X-ray detectors.