Clinton Schmidt with Dan Akerib

Monte Carlo Simulation of a Neutron Rejection Technique in Dark Matter Detection

The Cryogenic Dark Matter Search (CDMS) attempts to detect dark matter in the form of Weakly Interacting Massive Particles (WIMPs).  The detectors used for the project are cooled to low temperatures to allow for the detection of scattering from nuclei by measuring the energy transferred to a nucleus.  Most of the particles that interact with the detector are charged particles that interact with the electrons in the detectors.  Such interactions are characteristically different from nuclear scatterings and can be independently identified and eliminated from the collected data.  Neutrons, however, do not interact with the electrons, but rather scatter off the nuclei on the detector in the same way that WIMPs would. 

       The second Phase of the CDMS will take place underground in a mine, so that the surrounding rock will shield the detectors from the majority of cosmic rays that create most of the background events that need to be rejected.  Neutrons produced by high-energy muons in the rock walls produce neutrons that are particularly hard to reject.  The detectors can be shielded from low energy neutrons, but these higher energy neutrons pass through that shielding and interact with the detector.  However, the production of the higher energy neutrons is accompanied by production of numerous other charged particles in the rock wall.  The charged particles do not travel far in the rock, but the neutrons can travel a significant distance.  It is proposed that streamer tubes placed in the rock wall to detect these charged particles will allow a useful veto signal to be created.  Any nuclear recoil in the detector that coincides with a charged particle interaction in the streamer tube will be rejected as a neutron event and not a WIMP interaction.  Before this method is put to use, particle-transport Monte Carlo simulations will be used to determine its effectiveness.