A low-temperature proton detector for a neutron lifetime experiment

PENeLOPE is a new high-precision experiment with stored ultra-cold neutrons (UCN) to measure the neutron lifetime τnτn planned at the Technische Universität München. A key element to reach the goal of 0.1 s precision is the in situ detection of the decay protons during storage. For this reason we plan to cover the top of the storage volume by a large-area scintillation counter working at cryogenic temperatures: a 1μm thin layer of CsI scintillator, evaporated on a light-guide structure and read from the side by large-area avalanche photo diodes (LAAPDs) will be used. Simulations showed that the layer thickness of 1μm is sufficient to stop the n-decay protons after additional post-acceleration by 40-keV from our PENeLOPE setup. A small scale test detector with a 1.5μm thick evaporated CsI layer was successfully operated at cryogenic temperatures in the range of 50 K. The produced scintillation light was measured from the side window of the light guide by an LAAPD from RMD. The signal-to-noise ratio of a 40-keV proton beam from the paff accelerator showed a signal-to-noise ratio of 17:1. Electron–proton separation was successfully demonstrated by shooting 40-keV protons and 15-keV electrons simultaneously onto the detector.