Single-Photon-Sensitive Superconducting TES Sensors for EUV Photons in Superfluid Helium

Incident radiation can excite superfluid helium into a diatomic He2* excimer, which decays through the emission of a 15~eV photon. Such excimers have been used as tracers to measure the superfluid's quantum turbulence, thanks partly to the long half-life of the He2* triplet state ($\sim$13 seconds). However, the efficient detection of these excimers remains a challenge. This work presents two different detector designs capable of in-situ detection of the He2* excimers either directly, or by collecting the 15~eV emission upon decay. Both detectors are based on the superconducting transition edge sensor. One is designed to operate near 2~K, while the other is designed for $\sim$100 mK operation in a dilution refrigerator. We will discuss operating characteristics of both, and present preliminary data from the 2~K detector.