Rock neutron backgrounds from FNAL neutrino beamlines in the ?BDX-DRIFT detector

The vBDX-DRIFT Collaboration seeks to detect low-energy nuclear recoils from CEvNS or BSM interactions at FNAL. Backgrounds due to rock neutrons are an important concern. We present a GENIE and GEANT4 based model to estimate backgrounds from rock neutrons produced in neutrino-nucleus interactions within the rock walls surrounding the underground halls. This model was bench-marked against the 2009 COUPP experiment performed in the MINOS hall in the NuMI neutrino beam, and agreement is found between experimental results and the modeled result to within 30%. Working from this validated model, a similar two-stage simulation was performed to estimate recoil backgrounds in the vBDX-DRIFT detector across several beamlines. In the first stage utilizing GEANT4, neutrons were tallied exiting the walls of a rectangular underground hall utilizing four different neutrino beam configurations. These results are presented for use by other underground experiments requiring estimations of their rock neutron backgrounds. For vBDX-DRIFT, the second stage propagated neutrons from the walls and recorded energy deposited within a scintillator veto surrounding the detector and nuclear recoils within the detector's fiducial volume. The directional signal from the vBDX-DRIFT detector allows additional background subtraction. A sample calculation of a 10 m3 center dot yr exposure to the NuMI low-energy beam configuration shows a CEvNS signal-to-noise ratio of similar to 2.5.