Introduction
Physics Today paper
History
Calorimetry has been used since the early days of nuclear physics to measure the integrated energy of various radioactivities. By the mid 1930's, the sensitivity of cryogenic operation had been recognized, and small calorimeters were operated at temperatures as low as 50 mK. These devices must have been very nearly capable of detecting individual particle or gamma ray events, but the earliest published reference that we have found to using them in this mode is the 1974 account by Tapio Niinikoski* of spurious pulses on a carbon resistance thermometer readout, which he associated with local heating due to the passage of individual cosmic rays.
The first experimental development coincidentally began simultaneously on both sides of the Atlantic in 1982. In Milan, Ettore Fiorini had been working on detecting neutrinoless double beta decay and, intrigued by a suggestion in a preprint by Guenakh Mitselmakher that the betas might be detected thermally, went to Niinikoski to investigate the practicality of this idea. They devised an approach that was developed into the first successful physics experiment using thermal spectrometers, obtaining a new lower limit for the lifetime for double beta decay in 130Te measured with 340 g TeO2 ingots in a dilution refrigerator in the Gran Sasso laboratory.
Meanwhile, at NASA's Goddard Space Flight Center, the x-ray astronomy group was looking for ways to improve upon the resolution of the silicon solid state spectrometer they had flown on the Einstein satellite. X-ray astronomer Richard Mushotzky approached Harvey Moseley, a Goddard astronomer he knew worked with InSb detectors for the infrared, hoping to take advantage of the small bandgap of this material to improve the resolution. Moseley also used infrared bolometers, however, and the previous year had calculated their sensitivity for detecting very weak laser pulses. He was able to show that thermal detection potentially offered a far larger improvement. In 1984, a team led by Stephen Holt at Goddard proposed a microcalorimeter x-ray spectrometer (XRS) for inclusion on NASA's Advanced X-ray Astrophysics Facility (AXAF). Despite its technical immaturity at the time, its tremendous potential was recognized and the proposal was accepted. Though the XRS was later removed from the AXAF design after a change in the scope of the mission, this successful instrument evolved into the XRS in the Japanese/U.S ASTRO-E mission described elsewhere in this article.
