Tim Peshek with Daniel S. Akerib
Characterization of a Particle Detector for Use in the Cryogenic Dark Matter Search
The best evidence for dark matter comes from the observation that the luminous mass in galaxies is too small to account for the measured galactic rotational velocity. Big Bang Nucleosynthesis suggests that the fraction of critical density contributed by baryons, WB, is equivalent to 0.05 ±0.005. Comparing measurements of the potential energy of galactic clusters to predictions obtained using the virial theorem and other methods provide values of WM = 0.35 ± 0.1, where WM is the ratio of the total matter density to the critical density. Non-baryonic dark matter must make up the difference. One candidate for dark matter provided by particle physics is the lightest supersymmetric partner, a particle belonging to a class known as Weakly Interacting Massive Particles or WIMPs.
The Cryogenic Dark Matter Search (CDMS) is one experiment which is designed to look for WIMPs. The detectors employ a thin film tungsten layer sputtered onto germanium or silicon crystals. This allows the simultaneous measurement of ionization and phonon energy deposited in an event, which aids in distinguishing between nuclear and electron recoils. WIMPs preferentially scatter directly off of an atomic nucleus, whereas background electromagnetic radiation typically scatters off electrons. To improve shielding, CDMS will begin to use these detectors in a deep underground facility; therefore it is imperative that each detector’s performance be fully tested before being incorporated in the WIMP search. CWRU has been made a test-site for the CDMS collaboration and will begin to fully characterize CDMS detectors.
One aspect to consider in running these detectors is their response to electrons. Past experiments have shown that electrons interacting with the detector may contaminate the surface and make the ionization measurement difficult. New detectors should have had this problem resolved and this thesis will focus on exposing the detectors to an electron source and a neutron source as a means to characterize surface versus bulk events for discrimination.