Nuclear Physics Backgrounds for Liquid Argon Neutrino Detectors

We present an analytic calculation of the radioactive backgrounds in neutrino detectors using liquid argon as their active medium, using the proposed LBNE far detector as a concrete example. These radioactive backgrounds are one of the main factors that will set the low-energy threshold of such a detector and will play an important role in its overall data rate. The presence or absence of these backgrounds will also determine the ultimate sensitivity of a liquid argon based neutrino detector to astrophysical neutrino signals, most importantly, those from galactic supernova bursts. The radioactive backgrounds in this study fall into two categories: intrinsic radioactivity from detector construction materials ($^{232}$Th, $^{238}$U, $^{60}$Co, $^{40}$K, $^{85}$Kr, and $^{39}$Ar), and cosmogenic backgrounds originating from {\it in situ} activation of the argon. We include neutron and proton flux as a function of detector overburden, both in terms of direct energy depositions and for cosmogenic activation. We then use this depth dependent background model to study the sensitivity of a large liquid argon detector to supernova burst neutrinos using both analytic and Monte Carlo techniques.