[Triumf-seminars] TRIUMF Seminar today at 14:00

[TRIUMF Seminars]

Date/Time: Thu 2017-05-18 at 14:00

Location:  Auditorium          

Speaker:   Shawn Westerdale (Carleton University)

Title:     A Study of Nuclear Recoil Backgrounds in Liquid Argon Dark Matter Detectors

Abstract: Despite the great success of the Standard Model of particle physics, a preponderance of astrophysical evidence suggests that it cannot explain most of the matter in the universe. This so-called dark matter has eluded direct detection, though many theoretical extensions to the Standard Model predict the existence of particles with a mass on the 1–1000 GeV scale that interacts only via the weak nuclear force. Particles in this class are referred to as Weakly Interacting Massive Particles (WIMPs), and their high masses and low scattering cross sections make them viable dark matter candidates. The rarity of WIMP-nucleus interactions makes them challenging to detect: any background can mask the signal they produce. Background rejection is therefore a major problem in dark matter detection. The most dangerous class of backgrounds is nuclear recoils, which can produce a signal that mimics what we expect from WIMPs. One source of nuclear recoils results from surface radioactivity sending nuclei recoiling into the detector. While these backgrounds are normally rejected using position cuts, we will discuss a novel technique for rejecting them based on signals they make in the wavelength shifter coating on the inner surfaces of some detectors.The other major source of nuclear recoil backgrounds results from neutrons scattering off of nuclei in the detector. These backgrounds may produce a signal identical to what we expect from WIMPs and are extensively discussed here. We will present a new tool for calculating (alpha,n) yields in various materials. In this seminar, we will discuss these backgrounds in the context of DarkSide-50, a liquid argon-based dark matter detector. We introduce the concept of a neutron veto system designed to shield against, measure, and provide an anti-coincidence veto signal for background neutrons. We discuss the research and development that informed the design of the DarkSide-50 boron-loaded liquid scintillator neutron veto. We describe the specific implementation of this veto system 
in DarkSide-50, including a description of its performance, and show that it can reject neutrons with a high enough efficiency to allow DarkSide-50 to run background-free for three years. These technologies can be applied to larger-scale detectors in the future to further advance the search for dark matter.



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