Aaron Manalaysay 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, or WIMPs.  The project uses low-temperature devices to detect these particles via scattering from nuclei.  Such experiments involve considerable background events, and much effort is devoted to developing ways to discriminate WIMP scatters from other sources of particles.  Most subatomic particles that reach the detectors are electrically charged, and thus interact with the electrons of the detectors.  Electron recoils are easily identified and rejected.  Charged particles can interact with nuclei as well, but the amount of energy deposited is below the threshold of the detectors.  The most problematic background events come from neutron scatters.  Both WIMPs and neutrons are electrically neutral, making them invisible to the electronic structure of the detectors; they both scatter primarily from atomic nuclei.  Various techniques are used to identify neutrons, and reject them as background.  However, before a method is put to use, particle-transport Monte Carlo simulations are utilized to determine the method’s effectiveness.

             The second phase of CDMS will be conducted in a mine deep underground.  The advantage of this is that the surrounding rock shields much of the cosmic rays that would otherwise shower the detectors with background events.  However, there is a process of neutron production that is particularly difficult to reject.  In this process, residual high-energy muons interact with the surrounding rock of the mine.  The resulting “cascade” produces many different subatomic particles, including neutrons.  The other subatomic particles produced carry electric charges, and rarely travel through the rock into the mine.  A proposed method to reject these neutrons is to embed the surrounding rock with streamer tubes that can detect charged particles.  If a nuclear recoil is seen in the detector that coincides with a signal in the streamer tubes, that event can be rejected.  A simulation of this method must be performed to determine what percentage of these neutron cascades could be rejected.