Recent progress in calorimeters with transition edge thermometers

Tests of two calorimeters, each composed of a 19 g silicon crystal and a superconducting phase transition thermometer monitored with a squid are discussed. The first calorimeter has an iridium thermometer and is operated near 225 mK. The relaxation of alpha particle signals is characterized by two time constants of approximately 1.4 ms and 280 ms. The amplitude corresponding to the fast relaxation time when interpreted as a temperature rise, is much larger than expected, whereas the amplitude of the slow component agrees to within 15 percent with the temperature rise expected from the Debye model heat capacity of the silicon crystal. The second calorimeter uses a gold iridium proximity thermometer with a very wide transition curve (approximately 100 mK), which allows the detector response in a wide temperature range to be studied. The height of signals induced by 122 keV photons, measured from 17 to 60 mK, does not agree with the expectations from the heat capacity. The signals are larger than expected for T greater than or equal to 50 mK and smaller for T less than or equal to 25 mK. An energy resolution of 1.2 keV Full Width Half Maximum (FWHM) was obtained for 60 keV photons detected at an operating temperature of 17 mK.