The Experimental Nuclear and Astroparticle Physics group conducts research in deep underground, low background experiments. We are actively involved in several major International experiments as well as leading several local R&D experiments and activities. Our group practices experimental nuclear physics experiments, background measurements, simulations, material science and more. Several of our projects are conducted underground at the Sanford Underground Research Facility at Homestake in Lead, South Dakota.
EXO-200 and nEXO
The goal of the EXO Collaboration is to observe the, as yet only theorized, neutrinoless double-beta decay of Xe-136. Although the decay would violate a quantity that has always been observed to be conserved, it does not derive from the standard model of particle physics. Further, the occurrence of such decays would alleviate the tension between our model/cosmology and the universe in which we live; namely it would answer the question 'Why is the universe made up of matter and not equal amounts of matter and antimatter?'
The 200kg scale Entriched Xenon Observatory (EXO-200) is currently installed 655m underground at the Waster Isolation Pilot Plant in New Mexico. This experiment has already produced world leading results from a truly novel detector. Most importantly, it has demonstrated the exceptional background rejection of a single phase liquid xenon time projection chamber in the search for neutrinoless double beta decay. Looking for such a rare decay, even a small amount of backgrounds that mimic it would make such an experiment unfeasible. With the demonstration EXO-200 provides we can confidently predict the success of a multi-tonne scale detector. EXO-200 will continue to take exceptionally low background data in the coming years while we research incremental design enhancements in preparation for designing this next generation detector: next EXO or nEXO.
Neutrinoless double-beta decay searches play a major role in determining the nature of neutrinos, the existence of a lepton violating process and the effective Majorana neutrino mass. The Majorana Collaboration proposes to assemble an array of HPGe detectors to search for neutrinoless double-beta decay in 76-Ge. Our proposed method uses the well-established technique of searching for neutrinoless double-beta decay in high purity Ge-diode radiation detectors that play both roles of source and detector. The technique is augmented with recent improvements in signal processing and detector design, and advances in controlling intrinsic and external backgrounds. Initially, Majorana aims to construct a prototype module containing 60 kg of Ge detectors to demonstrate the potential of a future 1-tonne experiment. The Demonstrator Module is under construction at the Sanford Underground Laboratory at Homestake in Lead, South Dakota.
The Large Underground Xenon (LUX) Experiment presents a program for the construction and deployment of a large two-phase liquid/gas xenon dark matter detector and water shield, to be installed in the Sanford Deep Underground Laboratory at the Homestake Mine, South Dakota. Liquid Xenon both scintillates and becomes ionized when hit by particles (i.e. photons, neutrons and potentially dark matter). The ratio of scintillation over ionization energy caused by the collision provides a way of identifying the interacting particle. The leading theoretical dark matter candidate, the Weakly Interacting Massive Particle (WIMP), could be identified in this way. A large detector is required to not only set such a sensitivity limit, but also to accumulate WIMP statistics in a reasonable time frame if a signal is detected. The LUX program will also help develop the technologies required for 1–10 ton dark matter detectors. The detector is currently being assembled and tested in a surface laboratory while the underground site, the Davis Cavern on the 4850-ft. level, is being outfitted for experiments.
The DEAP/CLEAN collaboration is pursuing a staged approach to WIMP detection, focused on exploiting the unique properties of liquid argon and neon as a scintillator. All noble gases scintillate when charged particles pass through them, and when liquified, have a high enough density to make an effective WIMP target. In addition, the timing of the scintillation light allow nuclear recoils (the signal of WIMPs) to be separated from electron recoils (caused by natural radioactivity) with extremely high efficiency. Under construction to begin operation at SNOLAB in 2011 is MiniCLEAN, a 150 kg fiducial volume (360 kg total) of liquid argon or 85 kg fiducial volume (310 kg total) of liquid neon.
The SuperCDMS experiment is one of the foremost dark matter search experiments in the world and is located in the Soudan Underground Laboratory. The search for dark matter is one of the most important experiments in astrophysics today.
Faculty at USD are participating in experiments to detect weakly interacting massive particles (WIMPS), the most likely candidate for dark matter. SuperCDMS detectors "hear" the phonons (sound) and collects the electric charge produced when a WIMP hits a single nucleus in one of its germanium detectors causing the nucleus to recoil. Many people, including students, help build the detectors, analyze the data collected and troubleshoot the experiment.
The collaboration for the Assay and Acquisition of Radiopure Materials (AARM) seeks to establish a Facility for the Assay and Acquisition of Radiopure Materials (FAARM) to fulfill the assay and characterization needs of low-background experiments stationed underground at DUSEL. The goal is to provide a laboratory that can not only assist in the screening of materials used in experiments, but also provide an integrated, shielded laboratory space for R&D activities in a clean, low-background environment. Dongming Mei is a Co-PI of AARM.
Ultra-high purity germanium crystals are grown at USD for ultra-low background experiments at the Sanford Underground Research Facility (SURF) at Homestake. When completed, SURF will be the only site in the world where growth of germanium crystals of unprecedented purity in an underground environment is possible.
Currently, High quality crystals are grown and harvested at the temporary surface growth facility located on the USD campus. One third of the grown crystals will be manufactured into detectors. The remaining crystals will be fabricated into wafers that have broad applications in electro and optical devices and solar panels, with the potential to create jobs and generate revenue within the state.
Dongming Mei, Ph.D., and his research team are developing a plan to convert bulk germanium into the ultra-pure material used to create detectors. They have also developed a plan for the wafers, considered a less pure byproduct, which will include commercial applications for greater economic development.
Crystal Growth and Detector Development
The next generation neutrinoless double-beta decay experiment and dark matter direct searches require ultra-pure detectors. Our group is leading an effort to grow ultra-pure Ge crystals. The project includes efforts in purification via zone refining, crystal characterization and detector development which will all ultimately take place underground at the Sanford Underground Laboratory at Homestake to avoid cosmogenic radioisotope production in the grown crystals.
USD is evaluating thermal diffusion as a method of isotopic separation for depleting argon gas of its radioactive isotope, 39-Argon, for potential use in noble liquid dark matter experiments as it could reduce a source of intrinsic background. Thermal diffusion exploits a temperature gradient to produce a concentration gradient along the length of a vertical column, which concentrates lighter isotopes towards the top of a long narrow column. A set of test columns is currently under R&D operation to optimize the technique and develop a system for gas extraction and automation.
Homestake Background Characterization
Low-Background nuclear and particle astrophysics experiments often require underground locations to avoid cosmogenic backgrounds found on the surface. However, there are still sources of external backgrounds underground such as those presented by gamma rays and neutrons from the rock, muon-induced neutrons, muons not attenuated by the overburden and the contamination challenges associated by radon in the air. The University of South Dakota and collaborators are actively measuring these backgrounds at the Sanford Underground Laboratory at Homestake in Lead, South Dakota.
High-Purity Germanium Detectors are often used as a method of gamma spectroscopy for screening of materials used in sensitive experiments. Placing such a detector underground increases its sensitivity as it provides earthen shielding from the cosmic ray background found at the surface. The University of South Dakota is developing a screening station that will be installed in 2012 at the Sanford Underground Laboratory in a designated space on the 4850-ft. level of the Homestake Mine in Lead, South Dakota.
As an application of GEANT4, this package aims at developing an integrated simulation framework for DUSEL-related underground experiments at the Homestake Mine. It will incorporate all characterized Homestake background data including external muon, gamma, neutron radon and rock composition which will be shared with individual modular geometries. It will provide dedicated physics focusing on low energy regions for dark matter searches and neutrinoless double beta decay experiments. The final package will deliver a useful tool to support experiment design, operation and data analysis.