Electron Recoils In the PICO Direct Dark Matter Detection Experiment
The PICO-40L bubble chamber for direct dark matter detection is finishing the commissioning phase at SNOLAB. The detector will operate for 1 live year before being replaced by the next-generation detector, PICO-500. The elastic scattering of dark matter with the nuclei in the superheated liquid will nucleate bubbles if the energy deposited in the detector is above the threshold energy. Particle identification is achieved using the acoustic and imaging data collected. Other signals may mimic the signature of dark matter. Understanding and eliminating these background signals are critical in the future discovery of dark matter. Electron recoil induced bubbles have similar signatures as dark matter. However, they are extremely inefficient in bubble nucleation. The electron recoil background levels for PICO-40L and PICO-500 are studied in detail. In order to operate at a threshold that simultaneously maximizes the nuclear recoil acceptance and the electron recoil rejection, limiting the number of expected electron recoil events to $<$1 per live year, both detectors should operate at 13.1$^o$C and 25 psi with a threshold of 2.95 keV. At this threshold, a 141 Bq Cf-252 neutron source and a 33 MBq Co-60 gamma source can be used to calibrate the nuclear and electron recoil response of PICO-40L. Existing sources have sufficient strengths to calibrate PICO-500 to the statistical accuracy required, provided that the calibrations start by the end of 2021.