Performance of a large area photon detector for rare event search applications

We present the design and characterization of a large-area Cryogenic PhotoDetector designed for active particle identification in rare event searches, such as neutrinoless double beta decay and dark matter experiments. The detector consists of a 45.6 ^{cm 2} surface area by a 1-mm-thick 10.6 g Si wafer. It is instrumented with a distributed network of Quasiparticle-trap-assisted Electrothermal feedback Transition-edge sensors with superconducting critical temperature _{T c} = 41.5 mK to measure athermal phonons released from interactions with photons. The detector is characterized and calibrated with a collimated ⁵⁵Fe x-ray source incident on the center of the detector. The noise equivalent power is measured to be 1 × ^{10 - 17} W / √{ Hz } in a bandwidth of 2.7 kHz . The baseline energy resolution is measured to be _{σ E} = 3.86 ± 0.04 ₍^{stat . ) - 0.00 + 0.19} ( syst . ) eV . The detector also has an expected timing resolution of _{σ t} = 2.3 μ s for 5 _{σ E} events.