This work is supported in part by NSF grant PHY-0705055
Nuclear recoils from fast neutrons in underground laboratories are one of the most challenging backgrounds to dark matter direct detection experiments. The rate of this background is at present poorly quantified. The neutron detector pictured to the left represents 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 60MeV, 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.
The detector measures the flux of high energy neutrons via their multiplicity signature. Thanks to additional lead shielding along the sides and over the top of the water tanks, the rate of slow neutron singles may be discernable above the low background rate of environmental gamma interactions. Half-life measurements of long-lived fission isotopes may also be possible.