Dusel R&D: A Multiplicity Meter for Benchmarking Cosmogenic Neutron Backgrounds for Underground Experiments

The nature of dark matter is one of the most important outstanding issues in particle physics, cosmology and astrophysics. A leading hypothesis is that Weakly Interacting Massive Particles, or WIMPs, were produced in the early universe and make up the dark matter. So far this matter has only been observed through its gravitational effects. WIMPs cannot be Standard Model particles and so their discovery would hail a new form of matter. A detection would also help solve a long-standing riddle in cosmology that even questions our understanding of gravity. Dark matter is concentrated in the halos of galaxies, including the Milky Way. If WIMPs make up these halos they can be detected via scattering from atomic nuclei in a terrestrial detector. Experiments that search for WIMPs are one of the critical science drivers for a Deep Underground Science and Engineering Laboratory in the United States. WIMP searches must be performed underground to shield from cosmic rays, which produce secondary particles that could fake a WIMP signal. Nuclear recoils from fast neutrons in underground laboratories are one of the most challenging backgrounds to WIMP detection. The rate of this background is at present poorly quantified, and here we propose a straightforward, portable experiment that will pin down their rate to about 10% at a depth of 2000 meters of water equivalent (mwe). These fast neutrons, with energies above about 60 MeV, result from penetrating cosmic ray muons that interact with the rock overburden and produce neutrons through direct muon spallation, and in subsequent electromagnetic and hadronic showers. Neutrons from these processes penetrate and interact with shielding material in dark matter experiments to produce slower neutrons that cause nuclear recoils. Dark matter experiments, among others, rely on numerical Monte Carlo simulations to predict the background rate due to neutrons. At depths of 2000 mwe and below, the rate of neutron-induced backgrounds is correlated with the production of fast neutrons by muons and subsequent hadronic showers. The simulation of these processes is uncertain because of the lack of appropriate data for direct comparison. In the work we propose here we will measure in a purpose-built detector the rate of fast neutrons in an underground laboratory through the rate of events they induce that have multiple lower-energy neutrons. We will use the long-used technique of gadolinium-loaded liquid scintillator to implement a neutron multiplicity meter adjacent to a lead " radiator " similar to the …