The Cryogenic Dark Matter Search: First 5-Tower Data and Improved Understanding of Ionization Collection

Thesis/Dissertation · Fri Jan 01 00:00:00 EST 2010

DOI:https://doi.org/10.2172/980809· OSTI ID:980809

Bailey, Catherine N. ^[1]

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1. Case Western Reserve Univ., Cleveland, OH (United States)

The Cryogenic Dark Matter Search (CDMS) is searching for Weakly Interacting Massive Particles (WIMPs) with cryogenic particle detectors. These detectors have the ability to discriminate between nuclear recoil candidate and electron recoil background events by collecting both phonon and ionization energy from recoils in the detector crystals. The CDMS-II experiment has completed analysis of the first data runs with 30 semiconductor detectors at the Soudan Underground Laboratory, resulting in a world leading WIMP-nucleon spin-independent cross section limit for WIMP masses above 44 GeV/c². As CDMS aims to achieve greater WIMP sensitivity, it is necessary to increase the detector mass and discrimination between signal and background events. Incomplete ionization collection results in the largest background in the CDMS detectors as this causes electron recoil background interactions to appear as false candidate events. Two primary causes of incomplete ionization collection are surface and bulk trapping. Recent work has been focused on reducing surface trapping through the modification of fabrication methods for future detectors. Analyzing data taken with test devices has shown that hydrogen passivation of the amorphous silicon blocking layer worsens surface trapping. Additional data has shown that the iron-ion implantation used to lower the critical temperature of the tungsten transition-edge sensors causes a degradation of the ionization collection. Using selective implantation on future detectors may improve ionization collection for events near the phonon side detector surface. Bulk trapping is minimized by neutralizing ionized lattice impurities. Detector investigations at testing facilities and in situ at the experimental site have provided methods to optimize the neutralization process and monitor running conditions to maintain full ionization collection. This work details my contribution to the 5-tower data taking, monitoring, and analysis effort as well as the SuperCDMS detector development with the focus on monitoring and improving ionization collection in the detectors.

• Research Organization: Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-07CH11359
• OSTI ID: 980809
• Report Number(s): FERMILAB-THESIS-2010-17; TRN: US201012%%935
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
CRITICAL TEMPERATURE
CROSS SECTIONS
CRYOGENICS
DEPLETION LAYER
ELECTRONS
FABRICATION
HYDROGEN
IMPURITIES
IONIZATION
MODIFICATIONS
MONITORING
MONITORS
NONLUMINOUS MATTER
PASSIVATION
PHONONS
SEMICONDUCTOR DETECTORS
SENSITIVITY
SILICON
TRAPPING
TUNGSTEN
Astrophysics
Instrumentation