Radioactive background in a cryogenic dark matter experiment

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A measurement is presented of the neutron production rate in lead by high energy cosmic-ray muons at a depth of 2850 m water equivalent (w.e.) and a mean muon energy of 260 GeV. The measurement exploits the delayed coincidences between muons and the radiative capture of induced neutrons in a highly segmented tonne scale plastic scintillator detector. Detailed Monte Carlo simulations reproduce well the measured capture times and multiplicities and, within the dynamic range of the instrumentation, the spectrum of energy deposits. By comparing measurements with simulations of neutron capture rates a neutron yield in lead of (5.78) has been obtained. Absolute agreement between simulation and data is of order 25%. Consequences for deep underground rare event searches are discussed.

Section: Resultssupporting

confidence: 68%

Measurement and simulation of the muon-induced neutron yield in lead

Reichhart¹,

Lindote²,

Akimov³

et al. 2013

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“…The soft- Table 6: Calculated contaminations for a sample of resistors screened using both Chaloner and Lunehead. A high level of agreement is seen across the 238 U chain with the exception of 210 Pb which has a measured specific activity 58× higher than those isotopes above it in the chain (determined using the combined Lunehead/Chaloner measurement of 226 ware has been modified to extend the energies of α-particles from the original energy cut of 6.5 MeV to 10 MeV [31], and to improve the cross-section library for a large number of materials [32,33,34], with newly added cross-sections calculated using the EMPIRE-2.19 code [35]. In the worst case scenario, we assume that all the measured contamination in the surface mount resistors is confined to the ceramic.…”

Section: Significant Disequilibrium At 210 Pbmentioning

confidence: 99%

Low-background gamma spectroscopy at the Boulby Underground Laboratory

Scovell

Meehan²,

Araújo

et al. 2018

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The Boulby Underground Germanium Suite (BUGS) comprises three low-background, high-purity germanium detectors operating in the Boulby Underground Laboratory, located 1.1 km underground in the north-east of England, UK. BUGS utilises three types of detector to facilitate a high-sensitivity, high-throughput radio-assay programme to support the development of rare-event search experiments. A Broad Energy Germanium (BEGe) detector delivers sensitivity to low-energy gamma-rays such as those emitted by 210 Pb and 234 Th. A Small Anode Germanium (SAGe) well-type detector is employed for efficient screening of small samples. Finally, a standard p-type coaxial detector provides fast screening of standard samples. This paper presents the steps used to characterise the performance of these detectors for a variety of sample geometries, including the corrections applied to account for cascade summing effects. For low-density materials, BUGS is able to radio-assay to specific activities down to 3.6 mBq kg −1 for 234 Th and 6.6 mBq kg −1 for 210 Pb both of which have uncovered some significant equilibrium breaks in the 238 U chain. In denser materials, where gamma-ray self-absorption increases, sensitivity is demonstrated to specific activities of 0.9 mBq kg −1 for 226 Ra, 1.1 mBq kg −1 for 228 Ra, 0.3 mBq kg −1 for 224 Ra, and 8.6 mBq kg −1 for 40 K with all upper limits at a 90% confidence level. These meet the requirements of most screening campaigns presently under way for rare-event search experiments, such as the LUX-ZEPLIN (LZ) dark matter experiment. We also highlight the ability of the BEGe detector to probe the X-ray fluorescence region which can be important to identify the presence of radioisotopes associated with neutron production; this is of particular relevance in experiments sensitive to nuclear recoils.

“…Due to the extremely low rate of expected events, the sensitivity of the experiments is strongly affected by the radioactive background in and around the detector. Stringent requirements are imposed on the levels of intrinsic radioactivity of components used in the experimental setup [58]. Considerable effort is usually necessary to achieve the research aims regarding material selection [59], [60] purification [61] and production of detector components [62], [63] [64], aiming to reduce their intrinsic radioactivity levels.…”

Section: Intrinsic Radioactivitymentioning

confidence: 99%

Cryogenic phonon–scintillation detectors with PMT readout for rare event search experiments

Zhang

Lin

Mikhailik

et al. 2016

a b s t r a c tCryogenic phonon-scintillation detectors (CPSD) for rare event search experiments require reliable, efficient and robust photon detectors that can resolve individual photons in a scintillation event. We report on a cryogenic detector containing a scintillating crystal, equipped with an NTD-Ge phonon sensor and a photon detector based on a low-temperature photomultiplier tube (PMT) that is powered by a CockcroftWalton generator. Here we present results from the characterisation of two detector modules, one with CaWO 4 , the other with CaMoO 4 as scintillator. The energy resolutions (FWHM) at 122.1 keV for the scintillation/PMT channel are 19.9% and 29.7% respectively for CaWO 4 and CaMoO 4 while the energy resolutions (FWHM) for the phonon channels are 2.17 keV (1.8%) and 0.97 keV (0.79%). These characteristics compare favourably with other CPSDs currently used in cryogenic rare-event search experiments. The detection module with PMT readout benefits from the implementation of a well-understood, reliable, and commercially available component and improved time resolution, while retaining the major advantages of conventional CPSD, such as high sensitivity, resolving power and discrimination ability.