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 t o measure the neutron flux inside various shielding configurations req uired for a dark matter search at this site. These measurements demons trate that lead shielding, used for photon attenuation, acts as a stro ng neutron source, with the neutrons being produced primarily by cosmi c ray muon interactions within the lead. We have determined the produc tion rate of cosmic ray induced neutrons in lead at this depth. Fortun ately, these measurements also demonstrate that it is possible to mode rate 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 ina ctive mass near the detector.