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Development of a low background environment for the cryogenic dark matter search

University of British Columbia

A major problem currently facing astrophysics and cosmology is the question of dark matter. Although there is little doubt about the existence of dark matter, there is considerable uncertainty about the abundance and nature of this matter. One possibility is that dark matter consists of weakly interacting massive particles (WIMPs), such as the lightest stable particle in supersymmetry models. Direct detection experiments look for nuclear recoils from WIMPs scattering in a detector. The first generation of direct detection experiments were ultimately limited by radioactive backgrounds. The Cryogenic Dark Matter Search (CDMS) is a direct detection experiment based on novel particle detectors operated at millikelvin temperatures that provide intrinsic background rejection. This capability, however, is not 100% effective. Therefore a low background environment is essential to the experiment. To create such an environment, all possible background sources have been extensively studied both by measuring the background contribution from muons, photons and neutrons and by performing detailed Monte Carlo simulations of the photon and neutron backgrounds. The results of this investigation, as discussed in this thesis, have influenced all aspects of the CDMS experiment. The initial site for the CDMS experiment is the Stanford Underground Facility. The relatively high muon flux at this site due to its shallow depth was balanced against the convenience of a local site with the unlimited access necessary for operating a complicated cryogenic system and developing new detector technology. The cryostat used to house the detectors was designed to accommodate the extensive shielding necessary to reduce the ambient backgrounds to acceptable levels and to minimize the amount of radioactive contamination near the detectors. Simulations and measurements of the local backgrounds led to a layered shield design that consists primarily of plastic scintillators to veto muons, lead and copper to attenuate photons and polyethylene to moderate neutrons. With the background rejection capabilities of the cryogenic detectors and the low background environment created in the Stanford Underground Facility, we expect to extend the current limit on WIMP dark matter by more than an order of magnitude and begin testing models for the lightest supersymmetric particle.

Thesis/Dissertation

application/pdf

2009-03-19

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http://hdl.handle.net/2429/6243

Physics

University of British Columbia

UBCV

DSpace