Quasiparticle diffusion over several mm in cryogenic detectors

The use of quasiparticle diffusion in a superconducting film has the potential to allow an increase in the size of a cryogenic detector without proportional loss of energy resolution. The quasiparticle lifetime and the diffusion constant are critical parameters which have limited this development. Using W superconducting phase transition thermometers as the sensors and a W/Al bilayer as the diffusion film, we have measured quasiparticle diffusion over a distance of 2 mm and deduced a diffusion constant of D=2.5×10−4 m2/s and a quasiparticle lifetime of τ=9.0 ms, which is, to our knowledge, by far the longest ever observed. With Ir/Au thermometers and an Ir/Au/Al diffusion film we found D=4.6×10−3 m2/s and τ=0.43 ms with diffusion over 4 mm, the longest distance observed to date.