Future DUSEL experiments will need to know the external sources of radioactivity including muon-induced neutrons at different levels of the DUSEL site. These external backgrounds, particularly fast neutrons and cosmogenic radioactivity from muon-induced processes, must be eliminated for underground experiments in pursuit of double beta decay, searches for dark matter candidates, and oscillations of low-energy neutrinos.

This award will provide funds to measure such backgrounds with liquid scintillator, sodium iodide and germanium detectors. The results from such measurements will enable the experiments to be placed at different levels according to their designed sensitivity and shielding requirements. Measuring muon-induced neutron flux at the 300-ft level will enable one to benchmark Monte Carlo simulation codes for all incoming experiments. Measuring and understanding high energy neutron excitation of the shielding and detector materials for neutrinoless double-beta decay experiments are crucial for interpreting results and establishing shielding requirements. On the other hand, measuring partial gamma-ray cross sections provides useful data for benchmarking Monte Carlo simulation. These funds will provide the support needed to analyze existing data sets and to take more data.

Among the broader impacts, it is noted that characterizing the DUSEL site is also useful to other DUSEL experiments, such as the underground nuclear astrophysics accelerator program and proton decay experiments. Knowledge gained from the proposed measurements can have wide-ranging application. The proposed research activities have impact on education and outreach by involving two undergraduate students from two institutions in two states.

Project Report

The Homestake Mine in Lead, South Dakota has been converted to serve as the Sanford Underground Research Facility (SURF) and now hosts the Large Underground Xenon (LUX) and Majorana Demonstrator (MJD) experiments, which have been built to explore dark matter and neutrinoless double-beta decay. A possible long baseline neutrino experiment (LBNE) is being planned with the goal of building a 35 kiloton liquid argon detector at SURF. Because the Davis Campus where LUX and MJD are located was under water for several years due to the cessation of the water pumps in 2001, the radiogenic background including radon emission is very different from the Davis Campus of twenty years ago when the Mine was in operation. It is well known that the fluxes of muons, neutrons, and the concentration of radon modulate annually and could mimic the experimental signature for dark matter, while external radioactivity can generate events in the region of interest for neutrinoless double-beta decay. Long baseline neutrino experiment needs to understand the level of background from cosmogenic processes. Thus, measuring radiogenic and cosmogenic backgrounds at SURF is a key to success in neutrino and dark matter (WIMP searches) experiments. Characterization of the environmental background components at the 4850-ft level at the Homestake Mine is an important first step in opening the facility to physics experiments after the flooding since 2003. Existing information is being collected, coordinated, and analyzed. Subsequently, a campaign of new relevant measurements at the different levels has been undertaken with the aim of covering missing data, and resolving possible inconsistencies. This is particularly important for the neutron and gamma ray background components. Different techniques and detectors are being employed, and results are being compared. This will allow development of a consistent database of the relevant background components at different levels. Such a database is very important to the planned SURF experiments in terms of the design of the shield and the understanding of the background. The following team members have contributed significantly to the project. Fred Gray (Regis University), Keenan Thomas (USD), Chao Zhang (USD), Alyssa Day (USD), Jaret Heise (Sanford Lab), Dan Durben (BHSU), Patrick Davis (USD), Brian Woltman (USD), Dana Byram (USD), Jason Goon (USD), Angela Chiller (USD), Chris, Chiller (USD), Iseley Marshall (USD), Wenzhao Wei (USD), D'Ann Barker (USD), and Dongming Mei (the PI of the project). During the grant period, we have: 1) developed a large liquid scintillation detector for measuring fast neutrons for ultralow background experiments; 2) developed a Gd-doped water Cherenkov detector for measuring neutron multiplicity and thermal neutrons; 3) built the first low background counting facility at SURF; and 4) created a database to calculate (α,n) neutron yield and energy spectrum for different materials. The infrastructure has played and will continue to play an important role in promoting the research and education capabilities in the state of South Dakota. In over three years of current grant support, we have published 18 papers and made 26 presentations in various conferences. These publications and presentations cover the measured muon fluxes, gamma-ray fluxes, and neutron fluxes at the levels of the surface, 800 ft, 2000 ft, and 4550 ft. The radon concentrations are being measured for the levels of the surface, 800 ft, 1250 ft, 2000 ft, 4550 ft, and 4850ft. The measured results are being fed to the SURF design team and the planned SURF experiments for their consideration of the radon mitigation and the design of external shielding. The online database of (α, n) neutron yield and energy spectrum for different materials that are used to build detectors has been visited more than 2000 times by the community members internationally. All publications and presentations have been cited by many low background experiments. It is worth mentioning that several students and postdocs were involved in the publications, and that the majority of presentations were made by students and postdocs. The natures of neutrinos and dark matter are of considerable interest to the public. A rich program of outreach based upon neutrinos and dark matter is already established at SURF, which our collaborators already participated in, and will continue to do so. Measuring radiogenic and cosmogenic backgrounds at SURF has contributed to a broad scientific effort, which is led by AARM, to builds a global database for all underground laboratories. Characterizing the SURF site will be also useful to other SURF experiments, such as the underground nuclear astrophysics accelerator program and proton decay experiments. The development of research infrastructure including a large neutron detector array and low background counting facility will afford many research opportunities for physics students from the University of South Dakota, which just started a Ph.D. in Physics. The PI has been strongly supporting efforts that have utilized the developed detector system in outreach programs aimed at recruiting underrepresented groups into science and engineering.

National Science Foundation (NSF)
Division of Physics (PHY)
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James J. Whitmore
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University of South Dakota Main Campus
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