Nuclear-recoil energy scale in CDMS II silicon dark-matter detectors
The Cryogenic Dark Matter Search (CDMS II) experiment aims to detect dark matter particles that elastically scatter from nuclei in semiconductor detectors. The resulting nuclear-recoil energy depositions are detected by ionization and phonon sensors. Neutrons produce a similar spectrum of low-energy nuclear recoils in such detectors, while most other backgrounds produce electron recoils. The absolute energy scale for nuclear recoils is necessary to interpret results correctly. The energy scale can be determined in CDMS II silicon detectors using neutrons incident from a broad-spectrum ^(252)Cf source, taking advantage of a prominent resonance in the neutron elastic scattering cross section of silicon at a recoil (neutron) energy near 20 (182) keV. Results indicate that the phonon collection efficiency for nuclear recoils is 4.8_(−0.9)^(+0.7)% lower than for electron recoils of the same energy. Comparisons of the ionization signals for nuclear recoils to those measured previously by other groups at higher electric fields indicate that the ionization collection efficiency for CDMS II silicon detectors operated at ∼4 V/cm is consistent with 100% for nuclear recoils below 20 keV and gradually decreases for larger energies to ∼75% at 100 keV. The impact of these measurements on previously published CDMS II silicon results is small.
© 2018 Published by Elsevier B.V. Available online 18 July 2018. The CDMS Collaboration gratefully acknowledges the contributions of numerous engineers and technicians; we would like to especially thank Dennis Seitz, Jim Beaty, Bruce Hines, Larry Novak, Richard Schmitt, and Astrid Tomada. In addition, we gratefully acknowledge assistance from the staff of the Soudan Underground Laboratory and the Minnesota Department of Natural Resources. This work is supported in part by the National Science Foundation, by the U.S. Department of Energy, by the Swiss National Foundation, by NSERC Canada, and by MULTIDARK. This document was prepared by the SuperCDMS collaboration using the resources of the Fermi National Accelerator Laboratory (Fermilab), a U.S. Department of Energy, Office of Science, HEP User Facility. Fermilab is managed by Fermi Research Alliance, LLC (FRA), acting under Contract No. DE-AC02-07CH11359. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute under Contract No. DE-AC05-76RL01830 for the U.S. Department of Energy. SLAC is operated under Contract No. DE-AC02-76SF00515 with the U.S. Department of Energy.
Submitted - 1803.02903.pdf