Neganov–Luke Phonon Amplification in P-type Point Contact Detectors

Abstract

The Cryogenic Dark Matter Search (CDMS) detectors measure ionization and athermal phonons in high purity germanium crystals to discriminate between nuclear recoils from dark matter candidates and radioactive backgrounds. In order to reach lower energy detection thresholds, the CDMSlite experiment operates the CDMS detectors with a larger voltage bias to increase the signal-to-noise ratio using the Neganov–Luke effect. Breakdown in those detectors was observed at fields of order 30 V/cm, but the reason for the breakdown is unknown. It is unclear if the breakdowns are due to surface leakage current, impact ionization in the bulk of the crystals, or some other effect due to the very low operating temperatures of the detectors. Germanium detectors used in gamma spectroscopy at 77 K are regularly operated with fields in excess of 1,000 V/cm. In order to understand the origin of breakdown in the CDMS detectors, a P-type Point Contact detector was equipped with transition edge phonon thermistors and operated at a base temperature of \(\sim \)30 mK. The linearity of the Neganov–Luke phonon amplification was studied and no sign of breakdown for biases up to 400 V was observed. This excludes impact ionization on neutral impurity states as the primary cause of the breakdown observed in the CDMSLite detectors. This demonstrates that the Neganov–Luke phonon amplification is a viable method for lowering the energy threshold in germanium detectors of masses of order 1 kg.