  Astronomical observations of cosmic sources in the far-infrared and X-ray
bands require extreme sensitivity. The most sensitive detectors are cryogenic
bolometers and calorimeters operating typically at about 100 mK. The last stage
of cooling (from 300 mK to 100 mK) often poses significant difficulties in
space-borne experiments, both in system complexity and reliability. We address
the possibility of using refrigeration based on normal
metal/insulator/superconductor (NIS) tunnel junctions as the last stage cooler
for cryogenic thermal detectors. We compare two possible schemes: the direct
cooling of the electron gas of the detector with the aid of NIS tunnel
junctions and the indirect cooling method, when the detector lattice is cooled
by the refrigerating system, while the electron gas temperature is decreased by
electron-phonon interaction. The latter method is found to allow at least an
order of magnitude improvement in detector noise equivalent power, when
compared to the direct electron cooling.
