Charge and heat collection in a 70 g heat/ionization cryogenic detector for dark matter search

We present detailed studies of the physical mechanisms underlying the collection of the charge carriers and of the thermal energy in a heat/ionization germanium particle detector operated at 20-28 mK, together with a presentation of its main performances. This detector is devoted to the search of dark matter particles, taking advantage of the double signal to reject background induced events. In what concerns the ionization channel, the current-voltage characteristics, energy resolution, time stability, and pulse height bias dependence are presented. A mechanism is proposed to explain the fact that the p-i-n diode maintains some rectifying properties in spite of the very low temperature. The energy resolution is 1 keV full width at half maximum (FWHM) at 86.5 keV, and the stability time is several days. A calculation of the carrier trapping contribution to the energy resolution and bias dependence is performed and its results compared to the data. The bias dependence is interpreted within a "hot" carrier model in which the shape of the trapping cross section as a function of the electric field is investigated. The time stability behavior is interpreted in terms of a space charge evolution due to traps ionization. The reasons why the densities of ionized levels are minimized are discussed. It is shown that one of them is related to the low currents obtained at rather large biases because of the choice of a p-i-n scheme. In what concerns the heat channel, an analysis of the heat flow in the thermal circuit is performed to explain the shape and amplitude of the heat pulses. The risetimes are well accounted for, allowing a determination of the NTD electronic specific heat, (1.80 +/- 0.45) x 10(-6) T J K-1 cm(-3). The analysis of the two decay times leads to an interpretation in terms of partial thermalization of the ballistic phonons in the metallic parts of the detector. This mechanism allows to estimate the experimental responsivities and energy resolutions [6 and 20 nV/keV; 1.25 and 2.8 keV (FWHM) for sensor temperatures of respectively 20.5 and 28 mK]. Both the ionization and heat channel results are used to draw guidelines for detector performance improvements and mass increase. (C) 2000 American Institute of Physics. [S0021-8979(00)02303-3].