Cosmogenic backgrounds to 0νββ in EXO-200

As neutrinoless double-beta decay experiments become more sensitive and intrinsic radioactivity in detector materials is reduced, previously minor contributions to the background must be understood and eliminated. With this in mind, cosmogenic backgrounds have been studied with the EXO-200 experiment. Using the EXO-200 TPC, the muon flux (through a flat horizontal surface) underground at the Waste Isolation Pilot Plant (WIPP) has been measured to be Φ = 4.07 ± 0.14 (sys) ± 0.03 (stat) × 10^-7 cm^-2 s^-1, with a vertical intensity of Iv = 2.97^+0.14-0.13 (sys) ± 0.02 (stat) × 10^-7 cm^-2 s^-1 sr^-1. Simulations of muon-induced backgrounds identified several potential cosmogenic radionuclides, though only ¹³⁷Xe is a significant background for the ¹³⁶Xe 0νββ search with EXO-200. Muon-induced neutron backgrounds were measured using γ-rays from neutron capture on the detector materials. This provided a measurement of ¹³⁷Xe yield, and a test of the accuracy of the neutron production and transport simulations. The independently measured rates of ¹³⁶Xe neutron capture and of ¹³⁷Xe decay agree within uncertainties. Geant4 and FLUKA simulations were performed to estimate neutron capture rates, and these estimates agreed to within ∼ 40% or better with measurements. The ability to identify ¹³⁶Xe(n,γ) events will allow for rejection of ¹³⁷Xe backgrounds in future 0νββ analyses.

Duke Scholars

Author Jens Dilling Physics

Author Phillip S. Barbeau Physics