Cosmogenic backgrounds to 0 nu beta beta 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 Phi = 4.07 +/- 0.14 (sys) +/- 0.03 (stat) x 10(-7) cm(-2) s(-1), with a vertical intensity of I-v = 2.97(-0.13)(+0.14) (sys) +/- 0.02 (stat) x 10(-7) cm(-2) s(-1) sr(-1). Simulations of muon-induced backgrounds identified several potential cosmogenic radionuclides, though only Xe-137 is a significant background for the Xe-136 0v beta beta search with EXO-200. Muon-induced neutron backgrounds were measured using gamma-rays from neutron capture on the detector materials. This provided a measurement of Xe-137 yield, and a test of the accuracy of the neutron p roduction and transport simulations. The independently measured rates of Xe-137 neutron capture and of Xe-137 decay agree within uncertainties. Geant4 and FLUKA simulations were performed to estimate neutron capture rates, and these estimates agreed to within similar to 40% or better with measurements. The ability to identify Xe-136(n, gamma) events will allow for rejection of Xe-137 backgrounds in future 0v beta beta analyses.