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 an underground detector.
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. In this proposal, funds are requested for a portable experiment that will pin down their rate to about 10% by operating it in the Soudan Mine for a period of at least one year at a depth of 2000 meters of water equivalent. The long-used technique of gadolinium-loaded liquid scintillator will be employed to implement a neutron multiplicity meter adjacent to a lead radiator," similar to the shielding in an actual dark matter experiment.
All dark matter experiments, as well as many other underground experiments, rely on knowledge of the neutron background environment. Being able to predict this background with greater reliability will serve well this community, and provide improved extrapolations for experiments aiming to operate in deeper sites in the future.
