Alpha Background Study In Dark Matter Detection

Date of Award

December 2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor(s)

Richard Schnee

Keywords

Alpha Background, BetaCage, Dark Matter Detection, MiniCLEAN, Simulation

Subject Categories

Physical Sciences and Mathematics

Abstract

Long-life isotopes 238U and 232Th exist in almost any material. 222Rn, as a daughter of 238U, widely exists in the atmosphere. It has a half-life of 3.8 days, which is long enough for it to travel and diffuse through material surfaces. The alphas and betas from radon daughters are a dominant background for essentially all rare-event searches, including dark matter searches.

Dark matter detectors require very low background because WIMPs have very small cross-sections with nuclei. Alpha decay on the inner surface of the liquid argon sphere is one of the dominant backgrounds. This dissertation has described the simulation and discrimination of this surface alpha decay background in the MiniCLEAN experiment. By a combination of analysis specifically to reduce these events(energy, pulse shape discrimination and position reconstruction), and by hardware changes (the adding of baffles), the surface alpha background is reduced sufficiently.

Reduction of these surface backgrounds may also be achieved using the BetaCage, which is planned to be the most sensitive low-energy electron and alpha screener ever. This dissertation described the simulation of photon and radon backgrounds of BetaCage. The simulation shows that the sensitivity goal is achieved, 1000 times better than current detectors. This dissertation has described the simple model and method of particle track reconstruction.

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