Cryogenic particle detectors operate at very low temperature, typically only a few degrees above absolute zero.
However, at very low temperature, certain material properties become very sensitive to energy deposited by particles in their passage through the sensor, and the gain from these changes may be even more than that from reduction in thermal noise.
Originally, astronomy pushed the development of cryogenic detectors for optical and infrared radiation.
Most pure superconductors have a very sharp transition from normal resistivity to superconductivity at some low temperature.
A photon absorbed on one side of a STJ breaks Cooper pairs and creates quasiparticles.
The STJ can also be used as a heterodyne detector by exploiting the change in the nonlinear current–voltage characteristic that results from photon-assisted tunneling.
If it is held slightly below the transition temperature, the superconductivity vanishes on heating by particle radiation, and the field suddenly penetrates the interior.
A good deal more information can be found by measuring the elementary excitations of the crystal lattice, or phonons, caused by the interacting particle.
[4][5] This resistive non-superconducting section then leads to a detectable voltage pulse of a duration of about 1 nanosecond.
A particle striking an electron or nucleus in this superfluid can produce rotons, which may be detected bolometrically or by the evaporation of helium atoms when they reach a free surface.
4He is intrinsically very pure so the rotons travel ballistically and are stable, so that large volumes of fluid can be used.