Phonon signal in cryogenic detector discriminates single electron-hole pair

Cryogenic detectors have traditionally utilized charge amplifiers to detect ionization signals in a crystal. Work described in Applied Physics Letters presents an optimized phonon-based low-threshold sensor which converts ionization signals to phonon signals with sufficient resolution to detect individual electron-hole pairs in a silicon crystal.

Advancements in superconducting phonon sensor circuits, and the availability of high purity silicon crystals, helped realize this single charge-scale signal detection using phonons by improving on a mechanism first proposed in the 1980s. Notably, phonon amplitude is proportional to the voltage applied across the crystal, so the phonon signal emitted by an electron-hole pair increases with increasing voltage. Based on this linear relationship, researchers expect that future experiments with larger voltage ranges and phonon sensors on both sides of the crystals will have improved signal-to-noise ratios.

This cryogenic detector features two quasiparticle-trap-assisted electro-thermal-feedback transition edge sensor (QET) arrays, a bias grid, and an optical fiber that illuminates the crystal with 650 nanometer photons to create electron-hole pairs. The phonon sensors are photolithographically patterned on ultrapure silicon crystals and consist of superconducting Al electrodes and W transition edge sensors (TESs).

When a laser excites electron-hole pairs, the charges excite phonons in the crystal that diffuse into the superconducting electrodes. This energy causes Cooper pair breakage, producing quasiparticles that increase resistance in the TESs, detected via amplified current decreases.

Resolving individual electron-hole pairs by the excitations of phonon sensor measurements would also enable discrimination between energy absorbed by a nucleus versus by an electron. This is significant because gamma rays, the primary background radiation in the search for dark matter, produce electron recoils. “This type of particle that we’re looking for, called the WIMP [weakly interacting massive particle], would primarily interact with nuclei,” said Blas Cabrera, an author on the paper. “These detectors would also be sensitive to very low mass dark matter particles, often called dark photons and open a new window on other possible dark matter candidates.”

Source: “Thermal detection of single e-h pairs in a biased silicon crystal detector,” by R. K. Romani, P. L. Brink, B. Cabrera, M. Cherry, T. Howarth, N. Kurinsky, R. A. Moffatt, R. Partridge, F. Ponce, M. Pyle, A. Tomada, S. Yellin, J. J. Yen, and B. A. Young, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5010699 .