[en] We show that an improved sensitivity on effective neutrino mass to the atmospheric neutrino mass scale with the next generation germanium-based double-beta decay experiment together with results from cosmology survey, θ₁₃ measurements and neutrino oscillation experiments may be able to determine the absolute mass scale of the neutrino, and answer the question of the neutrino nature. To achieve such a sensitivity of 45 meV, the next generation germanium experiment must reduce background by a factor of 440 comparing to the existing results. The planned germanium experiment at the Deep Underground Science and Engineering Laboratory (DUSEL) in western South Dakota aims at achieving such a sensitivity. Sanford Lab supported by the state of South Dakota and a private donor, Mr. T. Denny Sanford, will be up and running within the next year to pave the way for the creation of DUSEL in five years.

[en] We provide a comprehensive study of the cosmic-ray muon flux and induced activity as a function of overburden along with a convenient parametrization of the salient fluxes and differential distributions for a suite of underground laboratories ranging in depth from ∼1 to 8 km.w.e.. Particular attention is given to the muon-induced fast neutron activity for the underground sites and we develop a depth-sensitivity relation to characterize the effect of such background in experiments searching for WIMP dark matter and neutrinoless double-beta decay

[en] Radiogenic particles are known as the main sources of background for all ultra-low background experiments in the detection of dark matter and neutrino properties. In particular, the radiogenic gamma rays from PMTs are a main component of the observed backgrounds in the noble liquid detectors such as XENON100 and LUX. This suggests a more accurate screening of PMTs is needed for the next generation experiments such as LUX-Zplin or Xenon1T. Hence, we propose to develop well-shaped germanium detectors for a better understanding of the radiogenic background from PMTs. A well-shaped germanium detector array and PMT (R11410MOD) have been designed in a Geant4-based Monte Carlo simulation, in which three radiogenic background isotopes from "2"3"8U, "2"3"2Th and "4"0K have been studied. In this work, we show the detector performance including the detector efficiency, energy resolution and the detector sensitivity for low-background counting in the detection of rare event physics. (paper)

[en] We report a comprehensive study of the energy response to low-energy recoils in dual-phase xenon-based dark matter experiments. A recombination model is developed to explain the recombination probability as a function of recoil energy at zero field and non-zero field. The role of e-ion recombination is discussed for both parent recombination and volume recombination. We find that the volume recombination under a non-zero field is constrained by a plasma effect, which is caused by a high density of charge carriers along the ionization track forming a plasma-like cloud of charge that shields the interior from the influence of the external electric field. Subsequently, the plasma time that determines the volume recombination probability at non-zero field is demonstrated to be different between electronic recoils (ERs) and nuclear recoils (NRs) due to the difference of ionization density between two processes. We show a weak field dependence of the plasma time for NRs and a stronger field dependence of the plasma time for ERs. As a result, the time-dependent recombination is implemented in the determination of charge and light yield with a generic model. Our model agrees well with the available experimental data from xenon-based dark matter experiments. (paper)

[en] We report a new method, which allows us to derive the temperature dependence of the average energy expended per electron-hole (e-h) pair, ε, for germanium detectors. Applying energy partition mechanism in ionization for a given energy deposition, the Fano factor and the value of ε can be determined separately. Subsequently, we illustrate the variation of ε as a function of temperature. The impact of ε on the energy threshold for germanium detectors at a given temperature is evaluated.

[en] We report a new method of using the plasma time difference, which results from the plasma effect, between the nuclear and electronic recoil events in high-purity germanium detectors to distinguish these two types of events in the search for rare physics processes. The physics mechanism of the plasma effect is discussed in detail. A numerical model is developed to calculate the plasma time for nuclear and electronic recoils at various energies in germanium detectors. It can be shown that under certain conditions the plasma time difference is large enough to be observable. The experimental aspects in realizing such a discrimination in germanium detectors is discussed.

[en] The discovery of dark matter is of fundamental importance to cosmology, astrophysics, and elementary particle physics. A broad range of observations from the rotation speed of stars in ordinary galaxies to the gravitational lensing of superclusters tell us that 80-90% of the matter in the universe is in some new form, different from ordinary particles, that does not emit or absorb light. Cosmological observations, especially the Wilkinson Microwave Anisotropy Probe of the cosmic microwave background radiation, have provided spectacular confirmation of the astrophysical evidence. The resulting picture, the so-called ''Standard Cosmology'', finds that a quarter of the energy density of the universe is dark matter and most of the remainder is dark energy. A basic foundation of the model, Big Bang Nucleonsynthesis (BBN), tells us that at most about 5% is made of ordinary matter, or baryons. The solution to this ''dark matter problem'' may therefore lie in the existence of some new form of non-baryonic matter. With ideas on these new forms coming from elementary particle physics, the solution is likely to have broad and profound implications for cosmology, astrophysics, and fundamental interactions. While non-baryonic dark matter is a key component of the cosmos and the most abundant form of matter in the Universe, so far it has revealed itself only through gravitational effects--determining its nature is one of the greatest scientific issues of our time. Many potential new forms of matter that lie beyond the Standard Model of strong and electroweak interactions have been suggested as dark matter candidates, but none has yet been produced in the laboratory. One possibility is that the dark matter is comprised of Weakly Interacting Massive Particles, or WIMPs, that were produced moments after the Big Bang from collisions of ordinary matter. WIMPs refer to a general class of particles characterized primarily by a mass and annihilation cross section that would allow them to fall out of chemical and thermal equilibrium in the early universe at the dark matter density. Several extensions to the Standard Model lead to WIMP candidates. One that has received much attention is Supersymmetry (SUSY), which extends the Standard Model to include a new set of particles and interactions that solves the gauge hierarchy problem, leads to a unification of the coupling constants, and is required by string theory. The lightest neutral SUSY particle, or neutralino, is thought to be stable and is a natural dark matter candidate. Intriguingly, when SUSY was first developed it was in no way motivated by the existence of dark matter. This connection could be a mere coincidence--or a crucial hint that SUSY is responsible for dark matter

[en] The impact of neutral impurity scattering of electrons on the charge drift mobility in high purity n-type germanium crystals at 77 Kelvin is investigated. We calculated the contributions from ionized impurity scattering, lattice scattering, and neutral impurity scattering to the total charge drift mobility using theoretical models. The experimental data such as charge carrier concentration, mobility and resistivity are measured by Hall Effect system at 77 Kelvin. The neutral impurity concentration is derived from the Matthiessen's rule using the measured Hall mobility and ionized impurity concentration. The radial distribution of the neutral impurity concentration in the self-grown crystals is determined. Consequently, we demonstrated that neutral impurity scattering is a significant contribution to the charge drift mobility, which has a dependence on the concentration of neutral impurities in high purity n-type germanium crystal.

[en] We reported a new result of the neutral impurity scattering of holes that has impact on the charge drift mobility in high purity p-type germanium crystals at 77 Kelvin. The charge carrier concentration, mobility and resistivity are measured by Hall Effect system at 77 Kelvin. We investigated the contribution to the total charge drift mobility from ionized impurity scattering, lattice scattering, and neutral impurity scattering with the best theoretical models and experimental data. Several samples with measured Hall mobility from the grown crystals are used for this investigation. With the measured Hall mobility and ionized impurity concentration as well as the theoretical models, we calculated the neutral impurity concentration by the Matthiessen's rule. As a result, the distributions of the neutral impurity concentrations with respect to the radius of the crystals are obtained. Consequently, we demonstrated that neutral impurity scattering is a significant contribution to the charge drift mobility, which has dependence on the concentration of neutral impurities in a given germanium crystal.

[en] Muon-induced background can limit the sensitivity of next generation experiments searching for neutrinoless double beta decay and WIMP dark matter. We have established the DSR based upon the muon and muon-induced fast neutron fluxes and spectra for a number of underground laboratories. Our results indicate that the muon-induced neutron elastic and inelastic scattering processes create a significant background for germanium-based dark matter and double beta decay experiments, unless such experiments are staged deep underground. We use the measured neutron elastic and inelastic scattering processes with a segmented CLOVER (germanium) detector on the surface to test our model predictions