A microcalorimeter is a device for measuring the energy of X-rays. works by measuring the heat of a single photon. is basically a thermal device. The energy of an X-ray photon is deposited in an absorber and detected with a thermistor. An X-ray which hits the absorber knocks an electron loose from an atom of the absorber material. This photoelectron then rattles around in the absorber, ultimately raising the temperature of the absorber by a few milliKelvin. The thermistor is partially isolated from the absorber, to give the absorber time to come into equilibrium before the thermistor begins to see the temperature rise.
After a few milliseconds, the thermistor comes to the same temperature as the absorber, a few mK warmer than the heatsink. Then the thermistor begins to cool as the heat flows out the weak link (the "legs" of the detector) to the heatsink. After a few tens of milliseconds, the thermistor has returned to its normal operating temperature.
The temperature rise DT measured by the thermistor is approximately proportional to the energy of the X-ray photon:
DT ~ E/C
where E is the energy of the X-ray and C is the heat capacity of the absorber. So by measuring how much the temperature changes, we can determine the energy of the X-ray.