Benjamin Kreis with Dan Akerib and Richard Schnee

Development of a Neutron Multiplicity Detection, Shielding, and Veto System for the Cryogenic Dark Matter Search

The Cryogenic Dark Matter Search (CDMS) experiment is designed to detect Weakly Interacting Massive Particles (WIMPs), a leading dark matter candidate. Neutrons that collide with the nuclei of the CDMS detectors can mimic WIMP signals by producing similar ionization and phonon energy, and therefore, to optimize the experiment’s sensitivity, the neutron background must be minimized. This is done by placing the detectors deep underground and implementing both active and passive shielding. The number of background neutrons is estimated and then used to determine the expected number of neutron-caused WIMP-like signals present in a given data set. Currently, with five kilograms of detector mass, CDMS can achieve zero neutron background for an exposure of two years at the Soudan Underground Laboratory. As the exposure of the experiment increases, neutrons will become a significant background for the experiment, ultimately limiting CDMS’s sensitivity.

At Case, we are studying a potential improvement to the CDMS shield that would have the ability to detect the presence of unwanted neutrons. With this improvement, the detectors and modified shielding would be surrounded by a photomultiplier-tube-monitored tank of gadolinium-loaded liquid scintillator that would allow CDMS to veto WIMP-like signals caused by neutrons. In this approach, the neutron background is measured, rather than estimated, thereby lowering the expected number of unvetoed WIMP-like signals and ultimately increasing the experiment’s sensitivity. Such a measurement would also provide an important cross-check of the Monte Carlo background estimations used by underground experiments worldwide.

This project will consist of simulating and characterizing the performance of the proposed shield and veto system, both as a shield and as a neutron multiplicity detector, when exposed to CDMS’s neutron background.