Introduction

Probably the most radical advance in X-ray instrumentation in the past five years has been the development of the single photon calorimeter, in which X-rays are detected via the temperature pulses they induce in a small (< 1 mm3) absorber, cooled to a fraction of a degree kelvin.

The detection of individual 5.9 keV X-rays (fig. 6.1) was first demonstrated by groups at NASA's Goddard Space Flight Center (GSFC) and the University of Wisconsin in 1984, using a silicon microcalorimeter operating at 0.3 K (McCammon et al, 1984). This work was specifically directed towards the production of a high-efficiency, non-dispersive focal plane spectrometer with energy resolution comparable to that of a Bragg crystal. It can, however, still be seen as the culmination of several decades’ research in fields other than X-ray astronomy, originally in nuclear physics and latterly in infrared astronomy. Andersen (1986) and Coron et al (1985a) trace calorimeter development back as far as 1903, and the radioactivity studies of Pierre Curie. They record how, by the mid-1970s, the sensitivity (in detectable watts) of IR bolometers operating at liquid helium temperatures, where heat capacities are very low, had reached the point where Niinikoski and Udo (1974) could identify the extraneous spikes seen in the output of balloon-borne bolometers with local heating produced by the passage of cosmic rays. Niinikoski and Udo appear to have been the first to suggest that it might be possible to thermally detect single photons or particles, rather than continuous fluxes.