Thermal X-Ray Detectors - Microcalorimeters

One of the most innovative x-ray detector designs of the past decade is the microcalorimeter. In a microcalorimeter the energy of an x-ray is determined by measuring the temperature rise caused by the absorption of a single x-ray photon in the detector. In principle, energy resolutions as good as ~1 eV (FWHM) should be possible. The best resolution obtained to date is ~7 eV (FWHM).

In its most basic form a microcalorimeter consists of three parts: a thermal mass that absorbs the x-ray and converts the energy to heat, a thermometer to measure the temperature rise of the thermal mass, and a thermal link to a heat sink to allow the absorbed energy to escape from the thermal mass. Figure 1 is a schematic representation of a microcalorimeter.

The energy resolution of a microcalorimeter is limited by fluctuations in its thermal energy content; a simple thermodynamic analysis gives the energy resolution as

dE = (kT2C)1/2
where k is the Boltzmann constant, T is the heat sink temperature, and C is the heat capacity of the detector. In a more detailed analysis of a microcalorimeter with a resistive thermometer, the resolution given above is modified by a multiplicative factor of order one that depends primarily on the temperature sensitivity of the thermometer.

Figure 2 shows the filtered pulse height distributions for random samples of detector noise in the left panel and for Mn K-alpha x-ray pulses in the right panel. This microcalorimeter was tested at the University of Wisconsin .

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Dr. Michael Juda
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Mail Stop 70
Cambridge, MA 02138, USA
Ph.: (617) 495-7062
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E-mail: mjuda at cfa dot harvard dot edu