New libraries for simulating neutron scattering in dark matter detector calibrations

Abstract: Dark matter detectors require calibrations of their energy scale and efficiency to detect nuclear recoils in the $1-50$ keV range. Most calibrations use neutron scattering and require MCNP or Geant4 simulations of neutron propagation through the detector. For most nuclei heavier than $^{16}$O, these simulations' libraries ignore the contribution of resolved resonances to the neutron elastic differential cross-section. For many isotopes and neutron energies of importance to dark matter detection, this invalid assumption can severely distort simulated nuclear recoil spectra. The correct angular distributions can be calculated from the resonance parameters using R-matrix formalism. A set of neutron scattering libraries with high resolution angular distributions for MCNP and Geant4 of $^{19}$F, $^{40}$Ar, $^{50,52}$Cr, $^{56}$Fe, $^{136}$Xe, and $^{206,207,208}$Pb is presented. An MCNPX library for simulating the production of low-energy neutrons in the $^9$Be$(\gamma,n)^8$Be reaction is also presented. Example dark matter detector calibrations are simulated with the new libraries showing how detector sensitivity could be overestimated by factors of two by relying on existing MCNP and Geant4 libraries.