Transition Edge Sensors




One techniques used for measuring energy is calorimetry. The energy absorbed by a physical system with heat capacity C (the calorimeter) causes an increase in temperature according to the following equation

E_CTCalorimeter

Since the energies of interest in the Dark Matter and x-ray detection fields are on the order of 10 keV (~ 1fJ), using of calorimeters with C as low as ~ 1 pJ/K, results in a temperature change of ~ 1 mK. This illustrates the need for highly sensitive thermometers.

An accurate method of measuring temperature involves using resistive thermometer to transform the temperature change into a voltage signal, based on the following circuit.
IV-Cricuit
By applying a voltage bias across a temperature sensitive resistor and measuring the current flowing through the circuit we can detecto any temperature changes. In order to be sensitive to truly minute temperature variation, however, the dependence of the resistance on temperature, R(T) must be quite large. This dependence is quantified by the variable alpha, which is defined as Alpha_def


Superconductors are well known for having zero resistance below a certain temperature (Tc), while above the temperature they have a normal state resistance that is characteristic of the metals of which they're made.
RvsT_chk
Within the span of ~ a mK around Tc the resistance rises from zero to its normal value, resulting in a large value of alpha, and thus a strong dependence of R on T, as shown in this figure. When some energy is absorbed in the device the temperature and resistance quickly increase,
Pulse
eventally the device cools back down to the ambient temperature, accompanied by a decrease in resistance. This produced a pulse whose amplitude is a good measure of the absorbed energy.