Neutron background for a dark matter experiment at a shallow depth site

The neutron flux in the Stanford Underground Facility, a shallow depth site with an overburden of 17 ± 1 m.w.e., has been measured using a moderated BF3 proportional counter. This counter has also been used to measure the neutron flux inside various shielding configurations required for a dark matter search at this site. These measurements demonstrate that lead shielding, used for photon attenuation, acts as a strong neutron source, with the neutrons being produced primarily by cosmic ray muon interactions within the lead. We have determined the production rate of cosmic ray induced neutrons in lead at this depth. Fortunately, these measurements also demonstrate that it is possible to moderate and capture these neutrons so that the resulting neutron flux is no longer sensitive to the presence of the lead shielding. For a dark matter search, however, care still has to be taken to minimize the inactive mass near the detector.