We report on several features in the energy spectrum from an ultralow-noise germanium detector operated deep underground. By implementing a new technique able to reject surface events, a number of ...cosmogenic peaks can be observed for the first time. We discuss an irreducible excess of bulklike events below 3 keV in ionization energy. These could be caused by unknown backgrounds, but also dark matter interactions consistent with DAMA/LIBRA. It is not yet possible to determine their origin. Improved constraints are placed on a cosmological origin for the DAMA/LIBRA effect.
Fifteen months of cumulative CoGeNT data are examined for indications of an annual modulation, a predicted signature of weakly interacting massive particle (WIMP) interactions. Presently available ...data support the presence of a modulated component of unknown origin, with parameters prima facie compatible with a galactic halo composed of light-mass WIMPs. Unoptimized estimators yield a statistical significance for a modulation of ∼2.8σ, limited by the short exposure.
A claim for evidence of dark matter interactions in the DAMA experiment has been recently reinforced. We employ a new type of germanium detector to conclusively rule out a standard isothermal ...galactic halo of weakly interacting massive particles as the explanation for the annual modulation effect leading to the claim. Bounds are similarly imposed on a suggestion that dark pseudoscalars might lead to the effect. We describe the sensitivity to light dark matter particles achievable with our device, in particular, to next-to-minimal supersymmetric model candidates.
Pacific Northwest National Laboratory has accumulated years of data with ultra-low-background proportional counters collected in an on-site shallow underground laboratory. This large dataset of ...events is exploited to study the impact of using neural networks for data analysis compared to simple pulse shape discrimination (PSD). The PSD method can introduce false positives for overlapping event distributions; however, a neural network can separate and correctly classify these events. This paper describes the training, testing, and validation of a neural network, analysis of challenge datasets, and a comparison between the standard PSD approach and a dense, fully-connected neural network.
The Majorana Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-β decay in 76Ge. The Majorana Demonstrator comprises 44.1 kg of Ge detectors (29.7 kg ...enriched in 76Ge) split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. Here we present results from data taken during construction, commissioning, and the start of full operations. We achieve unprecedented energy resolution of 2.5 keV FWHM at Qββ and a very low background with no observed candidate events in 9.95 kg yr of enriched Ge exposure, resulting in a lower limit on the half-life of 1.9×1025 yr (90% C.L.). This result constrains the effective Majorana neutrino mass to below 240-520 meV, depending on the matrix elements used. In our experimental configuration with the lowest background, the background is 4.0−2.5+3.1 counts/(FWHM t yr).
The Majorana Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-β decay in 76Ge. The Majorana Demonstrator comprises 44.1 kg of Ge detectors (29.7 kg ...enriched in 76Ge) split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. Here we present results from data taken during construction, commissioning, and the start of full operations. We achieve unprecedented energy resolution of 2.5 keV FWHM at Qββ and a very low background with no observed candidate events in 9.95 kg yr of enriched Ge exposure, resulting in a lower limit on the half-life of 1.9 × 1025 yr (90% C.L.). This result constrains the effective Majorana neutrino mass to below 240–520 meV, depending on the matrix elements used. In our experimental configuration with the lowest background, the background is 4.0+3.1−2.5 counts / (FWHM t yr).
Development of ultra low background gas proportional counters has made the contribution from naturally occurring radioactive isotopes – primarily α and β activity in the uranium and thorium decay ...chains – inconsequential to instrumental sensitivity levels when measurements are performed in above ground surface laboratories. Simple lead shielding is enough to mitigate against gamma rays as gas proportional counters are already relatively insensitive to naturally occurring gamma radiation. The dominant background in these surface laboratory measurements using ultra low background gas proportional counters is due to cosmic ray generated muons, neutrons, and protons. Studies of measurements with ultra low background gas proportional counters in surface and underground laboratories as well as radiation transport Monte Carlo simulations suggest a preferred conceptual design to achieve the highest possible sensitivity from an array of low background gas proportional counters when operated in a surface laboratory. The basis for a low background gas proportional counter array and the preferred shielding configuration is reported, especially in relation to measurements of radioactive gases having low energy decays such as 37Ar.
•Optimization of a cosmic ray shield to measure environmental radioactive gases.•Proportional counter sensitivity may reach ambient environmental 37Ar concentrations.•Variable length counter method evaluated for low concentration absolute gas counting.•Feasibility of several hour duration preparation of gas samples for 37Ar measurement.
Pacific Northwest National Laboratory has recently opened a shallow underground laboratory intended for measurement of low-concentration levels of radioactive isotopes in samples collected from the ...environment. The development of a low-background liquid scintillation counter is currently underway to further augment the measurement capabilities within this underground laboratory. Liquid scintillation counting is especially useful for measuring charged particle (e.g., β and α) emitting isotopes with no (or very weak) gamma-ray yields. The combination of high-efficiency detection of charged particle emission in a liquid scintillation cocktail coupled with the low-background environment of an appropriately designed shield located in a clean underground laboratory provides the opportunity for increased-sensitivity measurements of a range of isotopes. To take advantage of the 35m-water-equivalent overburden of the underground laboratory, a series of simulations have evaluated the scintillation counter's shield design requirements to assess the possible background rate achievable. This report presents the design and background evaluation for a shallow underground, low background liquid scintillation counter design for sample measurements.
•Graded-shielding can produce an ultra-low-background liquid scintillation counter.•Location in a shallow underground cleanroom further enhances background reduction.•A novel light collection design and selected low background materials are utilized.•The background is predicted to be 10–100 times below typical commercial systems.•Simulations tentatively predict a background rate of order 10 counts per day.
The Ultra-Low Background Liquid Scintillation Counter developed by Pacific Northwest National Laboratory will expand the application of liquid scintillation counting by enabling lower detection ...limits and smaller sample volumes. By reducing the overall count rate of the background environment approximately 2 orders of magnitude below that of commercially available systems, backgrounds on the order of tens of counts per day over an energy range of ~3–3600keV can be realized. Initial test results of the ULB LSC show promising results for ultra-low background detection with liquid scintillation counting.
•Observed background is within a factor of 2 of the predictions from simulation.•Background achieved is 2 orders of magnitude below commercially available systems.•Backgrounds are in tens of counts per day over an energy range of ~3– 3600keV.•Initial results show promise for ultra-low background detection with the ULB LSC.
A new ultra-low-background proportional counter was recently developed with an internal volume of 100cm3 and has been characterized at pressures from 1–10atm with P-10 (90% Ar, 10% methane) gas. This ...design, along with a counting system providing event digitization and passive and active shielding, has been developed to complement a new shallow underground laboratory (30m water-equivalent). Backgrounds and low-level reference materials have been measured, and system sensitivity for 37Ar has been calculated.
•A new PNNL shallow underground laboratory is operational.•A low-background gas proportional counting system for argon has been prepared.•First background data has been collected relevant to an Ar-37 signature.•First calibration measurements of a low-level standard have been made.•Detector response to Ar-37 has been calculated and Ar-37 sensitivity projected.
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