Gadolinium-Loaded Plastic Scintillators for Dark Matter Sea

Despite a decade of astounding experimental progress in direct searches for dark matter in the GeV-TeV mass scale, there have been no compelling signals to date. Recent dark-sector theories argue that we may have been looking at the wrong mass scale and that dark matter resides in the MeV-GeV mass range. This low-mass regime has, up to now, not been accessible with direct searches, which we propose to do. To quote from the US Cosmic Visions, New Ideas in Dark Matter 2017, Community Report: “New, small-scale projects present an opportunity for the US DM program to play the leading role in light DM and dark sector physics during the next decade. By leveraging existing technologies and facilities, a high-impact program could be quickly deployed to achieve significant science in the next few years.” In response to this call, a large effort is underway to explore this new mass range. Physicists at Canisius College, Occidental College, and Lamar University (the COLa collaboration) are actively involved in these investigations by developing the ‘Beam Dump eXperiment Directional Recoil Identification From Tracks’ (BDX-DRIFT) detector, to be positioned 23 m directly downstream of the electron beam dump at the DarkMESA facility in Mainz, Germany. How the Problem is Addressed: RMD, in partnership with the COLa collaboration, is proposing to build a veto detector for BDX-DRIFT based on gadolinium-loaded plastic scintillators. Gadolinium has the highest thermal neutron absorption cross section of any naturally occurring element, producing low energy conversion electrons as well as a cascade of associated Auger electrons, X-rays and gamma rays ranging in energy from a few eV to several MeV. Our small test samples of gadolinium-loaded plastic scintillators have shown that these radiations are readily absorbed in the plastic matrix and produce significant light output and high neutron detection efficiency. Plans for Phase I: In the Phase I project, we will explore different organometallic gadolinium compounds to optimize the composition in terms of uniformity, light yield, , attenuation length and neutron efficiency. Parameters such as gadolinium concentration and detector thickness will be adjusted based on the neutron detection efficiency. At the end of Phase-I, we will down-select the best gadolinium compound and fabricate optimized gadolinium-loaded plastic scintillator for evaluation by the COLa and the DarkMESA collaborations. Commercial and Scientific Potential: The potential applications for the proposed plastic scintillators include nuclear physics, nuclear non-proliferation - large portal monitors, high energy particle physics research, nuclear waste characterization, industrial non-destructive evaluation, biological and materials research, astronomy, and health physics.