Low-background Germanium Radioassay for the Majorana Collaboration

The focus of the Majorana Collaboration is the search for nuclear neutrinoless double beta decay. If discovered, this process would prove that the neutrino is its own anti- particle, or a Majorana particle. Being constructed at the Sanford Underground Research Facility, the Majorana Demonstrator aims to show that a background rate of 3 counts per region of interest (ROI) per tonne per year in the 4 keV ROI surrounding the 2039-keV Q- value energy of 76Ge is achievable and to demonstrate the technological feasibility of building a tonne-scale Ge-based experiment. Because of the rare nature of this process, detectors in the system must be isolated from ionizing radiation backgrounds as much as possible. This involved building the system with materials containing very low levels of naturally- occurring and anthropogenic radioactive isotopes at a deep underground site. In order to measure the levels of radioactive contamination in some components, the Majorana Demonstrator uses a low background counting facility managed by the Experimental Nuclear and Astroparticle Physics (ENAP) group at UNC. The UNC low background counting (LBC) facility is located at the Kimballton Under- ground Research Facility (KURF) located in Ripplemead, VA. The facility was used for a neutron activation analysis of samples of polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP) tubing intended for use in the Demonstrator. Calculated ini- tial activity limits (90% C.L.) of 238U and 232Th in the 0.002-in PTFE samples were 7.6 ppt and 5.1 ppt, respectively. The same limits in the FEP tubing sample were 150 ppt and 45 ppt, respectively. The UNC LBC was also used to γ-assay a modified stainless steel flange to be used as a vacuum feedthrough. Trace activities of both 238U and 232Th were found in the sample, but all were orders of magnitude below the acceptable threshold for the Majorana experiment. Also discussed is a proposed next generation ultra-low background system designed to utilize technology designed for the Majorana Demonstrator. Fi- nally, a discussion is presented on the design and construction of an azimuthal scanner used by the Majorana collaboration.Doctor of Philosoph