Kimballton Underground Research Facility Research Articles

Measurement of muon-induced high-energy neutrons from rock in an underground Gd-doped water detector

We present a measurement of the rate of correlated neutron captures in the WATCHBOY detector, deployed at a depth of approximately 390 meters water equivalent (m.w.e.) in the Kimballton Underground Research Facility (KURF). WATCHBOY consists of a cylindrical 2 ton water target doped with 0.1% gadolinium, surrounded by a 40 ton undoped water hermetic shield. We present a comparison of our results with the expected rate of correlated neutron captures arising from high-energy neutrons incident on the outside of the WATCHBOY shield, predicted by a hybrid FLUKA/GEANT4-based simulation. The incident neutron energy distribution used in the simulation was measured by a fast neutron spectrometer, the 1.8-ton Multiplicity and Recoil Spectrometer (MARS) detector, at the same depth. We find that the measured detection rate of two correlated neutrons is consistent with that predicted by simulation. The result lends additional confidence in the detection technique used by MARS, and therefore in the MARS spectra as measured at three different depths. Confirmation of the fast neutron flux and spectrum is important as it helps validate the scaling models used to predict the fast neutron fluxes at different overburdens.

Search for Pauli exclusion principle violating atomic transitions and electron decay with a p-type point contact germanium detector

A search for Pauli-exclusion-principle-violating K-alpha electron transitions was performed using 89.5 kg-d of data collected with a p-type point contact high-purity germanium detector operated at the Kimballton Underground Research Facility. A lower limit on the transition lifetime of 5.8x10^30 seconds at 90% C.L. was set by looking for a peak at 10.6 keV resulting from the x-ray and Auger electrons present following the transition. A similar analysis was done to look for the decay of atomic K-shell electrons into neutrinos, resulting in a lower limit of 6.8x10^30 seconds at 90 C.L. It is estimated that the MAJORANA DEMONSTRATOR, a 44 kg array of p-type point contact detectors that will search for the neutrinoless double-beta decay of 76-Ge, could improve upon these exclusion limits by an order of magnitude after three years of operation.

Search for neutrinoless double-electron capture ofDy156

Background: Multiple large collaborations are currently searching for neutrinoless double-$\ensuremath{\beta}$ decay, with the ultimate goal of differentiating the Majorana-Dirac nature of the neutrino.Purpose: Investigate the feasibility of resonant neutrinoless double-electron capture, an experimental alternative to neutrinoless double-$\ensuremath{\beta}$ decay.Method: Two clover germanium detectors were operated underground in coincidence to search for the de-excitation $\ensuremath{\gamma}$ rays of $^{156}\mathrm{Gd}$ following the neutrinoless double-electron capture of $^{156}\mathrm{Dy}$. 231.95 d of data were collected at the Kimballton underground research facility with a 231.57 mg enriched $^{156}\mathrm{Dy}$ sample.Results: No counts were seen above background and half-life limits are set at $O({10}^{16}\text{--}{10}^{18})$ yr for the various decay modes of $^{156}\mathrm{Dy}$.Conclusion: Low background spectra were efficiently collected in the search for neutrinoless double-electron capture of $^{156}\mathrm{Dy}$, although the low natural abundance and associated lack of large quantities of enriched samples hinders the experimental reach.

Low energy neutron background in deep underground laboratories

The natural neutron background influences the maximum achievable sensitivity in most deep underground nuclear, astroparticle and double-beta decay physics experiments. Reliable neutron flux numbers are an important ingredient in the design of the shielding of new large-scale experiments as well as in the analysis of experimental data. Using a portable setup of 3He counters we measured the thermal neutron flux at the Kimballton Underground Research Facility, the Soudan Underground Laboratory, on the 4100ft and the 4850ft levels of the Sanford Underground Research Facility, at the Waste Isolation Pilot Plant and at the Gran Sasso National Laboratory. Absolute neutron fluxes at these laboratories are presented.

Search for the β decay of 96Zr

96Zr and 48Ca are unique among double-β decay candidate nuclides in that they may also undergo single-β decay. In the case of 96Zr, the single-β decay mode is dominated by the fourth-forbidden β decay with a 119keV Q value. A search was conducted for the β decay of 96Zr by observing the decay of the daughter 96Nb nucleus. Two coaxial high-purity germanium detectors were used in coincidence to detect the γ-ray cascade produced by the daughter nucleus as it de-excited to the ground state. The experiment was carried out at the Kimballton Underground Research Facility and produced 685.7 days of data with a 17.91g enriched sample. No counts were seen above background, producing a limit of T1/2>2.4×1019year. This is the first experimental search that is able to discern between the β decay and the double-β decay to an excited state of 96Zr.

Search for two-neutrino double-βdecay ofZr96to excited states ofMo96

Background: Double-$\ensuremath{\beta}$ decay is a rare second-order nuclear decay. The importance of this decay stems from the possibility of neutrinoless double-$\ensuremath{\beta}$ decay and its applications to neutrino physics.Purpose: A search was conducted for the $2\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ decay of $^{96}\mathrm{Zr}$ to excited final states of the daughter nucleus, $^{96}\mathrm{Mo}$. Measurements of this decay are important to test nuclear matrix element calculations, which are necessary to extract the neutrino mass from a measurement of the neutrinoless double-$\ensuremath{\beta}$ decay half-life.Method: Two coaxial high-purity germanium detectors were used in coincidence to detect $\ensuremath{\gamma}$ rays produced by the daughter nucleus as it de-excited to the ground state. The experiment was carried out at the Kimballton Underground Research Facility and produced 685.7 d of data with a 17.91 g enriched sample.Results: No counts were seen above background. For the decay to the first excited ${0}^{+}$ state, a limit of ${T}_{1/2}g3.1\ifmmode\times\else\texttimes\fi{}{10}^{20}$ yr was produced. Limits to higher excited states are also reported.Conclusion: The new limits on double-$\ensuremath{\beta}$ decay are an improvement over previous experiments by a factor of 2 to 5 for the various excited states. The nuclear matrix element for the double-$\ensuremath{\beta}$ decay to the first excited ${0}^{+}$ state is found to be $l0.13$.

A Dark Matter Search with MALBEK

The Majorana Demonstrator is an array of natural and enriched high purity germanium detectors that will search for the neutrinoless double-beta decay of 76Ge and perform a search for weakly interacting massive particles (WIMPs) with masses below 10GeV. As part of the Majorana research and development efforts, we have deployed a modified, low-background broad energy germanium detector at the Kimballton Underground Research Facility. With its sub-keV energy threshold, this detector is sensitive to potential non-Standard Model physics, including interactions with WIMPs. We discuss the backgrounds present in the WIMP region of interest and explore the impact of slow surface event contamination when searching for a WIMP signal.

Two-neutrino double-βdecay ofNd150to excited final states inSm150

Double-beta decay is a rare nuclear process in which two neutrons in the nucleus are converted to two protons with the emission of two electrons and two electron anti-neutrinos. We measured the half life of the two-neutrino double-beta decay of $^{150}$Nd to excited final states of $^{150}$Sm by detecting the de-excitation gamma rays of the daughter nucleus. This study yields the first detection of the coincidence gamma rays from the 0$^+_1$ excited state of $^{150}$Sm. These gamma rays have energies of 333.97 keV and 406.52 keV, and are emitted in coincidence through a 0$^+_1\rightarrow$2$^+_1\rightarrow$0$^+_{gs}$ transition. The enriched Nd$_2$O$_3$ sample consisted of 40.13 g $^{150}$Nd and was observed for 642.8 days at the Kimballton Underground Research Facility, producing 21.6 net events in the region of interest. This count rate gives a half life of $T_{1/2}=(1.07^{+0.45}_{-0.25}(stat)\pm0.07(syst.))\times 10^{20}$ years. The effective nuclear matrix element was found to be 0.0465$^{+0.0098}_{-0.0054}$. Finally, lower limits were obtained for decays to higher excited final states. Our half-life measurement agrees within uncertainties with another recent measurement in which no coincidence was employed. Our nuclear matrix element calculation may have an impact on a recent neutrinoless double-beta decay nuclear matrix element calculation which implies the decay to the first excited state in $^{150}$Sm is favored over that to the ground state.

Cosmic muon flux measurements at the Kimballton Underground Research Facility

In this article, the results from a series of muon flux measurements conducted at the Kimballton Underground Research Facility (KURF), Virginia, United States, are presented. The detector employed for these investigations, is made of plastic scintillator bars readout by wavelength shifting fibers and multianode photomultiplier tubes. Data was taken at several locations inside KURF, spanning rock overburden values from ∼ 200 to 1450 m.w.e. From the extracted muon rates an empirical formula was devised, that estimates the muon flux inside the mine as a function of the overburden. The results are in good agreement with muon flux calculations based on analytical models and MUSIC.

Measurement of cosmogenic radioisotope production on water at the Kimballton Underground Research Facility

Measurement of cosmogenic radioisotope production on water at the Kimballton Underground Research Facility

Measurement of airborne fission products in Chapel Hill, NC, USA from the Fukushima Dai-ichi reactor accident

We present measurement results of airborne fission products in Chapel Hill, NC, USA, from 62 d following the March 11, 2011, accident at the Fukushima Dai-ichi nuclear power plant. Airborne particle samples were collected daily in air filters and radio-assayed with two high-purity germanium (HPGe) detectors. The fission products 131I and 137Cs were measured with maximum activity concentrations of 4.2 ± 0.6 mBq/m3 and 0.42 ± 0.07 mBq/m3 respectively. Additional activity from 131,132I, 134,136,137Cs and 132Te were measured in the same air filters using a low-background HPGe detector at the Kimballton Underground Research Facility (KURF).

Kimballton Underground Research Facility

Kimballton Underground Research Facility

Low-background gamma counting at the Kimballton Underground Research Facility

The next generation of low-background physics experiments will require the use of materials with unprecedented radio-purity. A gamma-counting facility at the Kimballton Underground Research Facility (KURF) has been commissioned to perform initial screening of materials for radioactivity primarily from nuclides in the 238U and 232Th decay chains, 40K and cosmic-ray induced isotopes. The facility consists of two commercial low-background high purity germanium (HPGe) detectors. A continuum background reduction better than a factor of 10 was achieved by going underground. This paper describes the facility, detector systems, analysis techniques and selected assay results.

Low-background gamma counting at the Kimballton Underground Research Facility

Low-background gamma counting at the Kimballton Underground Research Facility

Astroparticle physics with a customized low-background broad energy Germanium detector

The MAJORANA Collaboration is building the MAJORANA DEMONSTRATOR, a 60 kg array of high purity germanium detectors housed in an ultra-low background shield at the Sanford Underground Laboratory in Lead, SD. The MAJORANA DEMONSTRATOR will search for neutrinoless double-beta decay of 76Ge while demonstrating the feasibility of a tonne-scale experiment. It may also carry out a dark matter search in the 1-10 GeV/c^2 mass range. We have found that customized Broad Energy Germanium (BEGe) detectors produced by Canberra have several desirable features for a neutrinoless double-beta decay experiment, including low electronic noise, excellent pulse shape analysis capabilities, and simple fabrication. We have deployed a customized BEGe, the MAJORANA Low-Background BEGe at Kimballton (MALBEK), in a low-background cryostat and shield at the Kimballton Underground Research Facility in Virginia. This paper will focus on the detector characteristics and measurements that can be performed with such a radiation detector in a low-background environment.