Simulation of gamma background in the PICO bubble chambers for dark matter search

Abstract

The PICO Collaboration uses bubble chambers to search for answers to fundamental physics problems by attempting to directly identify interactions between dark matter particles and the ordinary matter of a laboratory particle detector. In order to identify these very rare interactions, it is necessary to reduce or eliminate all other event types which may mimic the appearance of dark matter interactions and act as a "background" to this search. Understanding these backgrounds requires detailed simulations of several kinds. The first of these is a simulation of gammas that are external to the detector. Using experimentally measured spectra of emission of rocks that are surrounding several PICO detectors and Geant4 Monte Carlo simulations the number of background events was calculated. The second kind of simulation was done using Co-60 and Ba-133 radioactive sources to pursue gamma background calibration for PICO-60. Electron recoils were studied inside the freon target material, both with and without iodine contamination. There was a special focus on simulating Auger cascades and low-energy electromagnetic physics in general, including secondary X-rays and