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A cryogenic calorimeter consists of an absorber and a temperature sensor in
thermal contact, weakly linked to a heat bath.
In an extremely simplified model the detector can be characterized
as an absorber with a heat capacity C. Then an energy deposition in the absorber δE
leads to a temperature rise δT of the detector
given by δT=δE/C. This relaxes back to its
equilibrium value via the thermal coupling to the heat
bath. The temperature rise is therefore a direct measurement of the
deposited energy.
In dielectric and semiconductor materials the heat capacity at low
temperatures is dominated by the phonon system in which C ∝ T3. At
millikelvin temperatures, due to the T3
dependence of the heat capacity, the energy deposition following a particle
interaction results in a measurable temperature rise.
The temperature sensors developed for CRESST are tungsten superconducting
phase transition thermometers (SPT) consisting of thin tungsten films
evaporated onto a surface of the absorbers. The thermometers are stabilized in the
transition from the normal conducting to the superconducting phase where a
small temperature rise leads to a relatively large increase in resistance,
making them extremely sensitive thermometers.
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Schema drawing of a CRESST-II calorimeter element
A typical transition curve. Since it is very steep,
a small change in temperature results in a measurable change of resistance.
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