UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Calibration of SuperCDMS dark matter detectors for low-mass WIMPs MacDonell, Danika


Observational evidence suggests that the majority of mass in the universe takes the form of non-luminous "dark matter". The Super Cryogenic Dark Matter Search (SuperCDMS) is a direct-detection dark matter experiment that searches primarily for a well-motivated dark matter candidate known as the weakly-interacting massive particle (WIMP). The experiment looks for an above-background excess of nuclear recoil events in cryogenic solid-state detectors that could be attributed to WIMP-nucleon collisions. The most recent SuperCDMS run at the Soudan underground laboratory set a world-leading limit on the spin-independent WIMP-nucleon cross section for a WIMP mass as low as ~3 GeV/c², and the next installation of the experiment at SNOLAB aims to be sensitive to WIMP masses below 1 GeV/c². To better understand the response of solid-state germanium detectors to low-mass WIMPs, "photoneutron" calibration data was taken at the Soudan laboratory in Minnesota by passing quasi-monoenergetic neutrons through SuperCDMS detectors. Gamma rays used in the photoneutron production process create an overwhelmingly dominant background of electron recoil events in the detector. This gamma background is measured directly with regular "neutron-off" data-taking periods during which the neutron production mechanism is removed. We compare the observed electron and nuclear recoil spectra with Geant4-simulated spectra to obtain a model-dependent calibration of the nuclear recoil energy scale of the detectors. The calibration is performed using a negative log likelihood fit to a parameterized Lindhard ionization yield model. The fit includes a semi-analytical model of the gamma background component obtained from the neutron-off data.

Item Citations and Data


Attribution-NonCommercial-NoDerivatives 4.0 International