Excited states of 122Xe populated via 8+and electron capture (EC) decay of 122Cs have been studied with gamma -ray spectroscopy using the 8n spectrometer at the TRIUMF Isotope Separator and Accelerator facility. Two sets of data were collected to optimize the signal-to-background ratio for decays of the ground (t1/2 = 21.2 s) and isomeric (t1/2 = 3.7 min) states of 122Cs. The data collected have enabled the observation of 520 transitions and 191 levels, including eight new excited 0+ states in 122Xe. The gamma -gamma angular correlations have been analyzed that permitted the assignment or restriction of the spins for 66 levels. The E2/M1 multipolarity mixing ratios for 38 gamma -ray transitions were also extracted from these analyses. Low-spin members of the 0+2 , 0+3 , 0+4 , and Kn= 4+ bands are assigned, as well as suggested Kn= 2+2 and Kn= 2+3 bandheads. Combined with previous results for high-spin states, rotational bands built on a newly assigned Kn= 3-1 state and its 4-1 signature partner, the 5-2 and its 6-2 signature partner, are suggested. The results for the positive-parity bands are compared with a model using a general Bohr Hamiltonian derived from a mean-field theory based on the UNEDF0 energy-density functional that displays good overall agreement.

Cross sections for production of the medical isotope 225Ac by the 226Ra(p,2n) reaction have not previously been measured in fine steps over the relevant energy region, and no measurements are presently available in the literature for the actinium contaminant isotopes created by the adjacent 226Ra(p,n)226Ac and 226Ra(p,3n)224Ac reactions. We report thin-target cross-section measurements for production of 224Ac and 225Ac by protons of 15.1-16.8 MeV incident on radium. An upper limit for the 226Ac cross section is also reported.

The specific activity of the β decay of ^39 Ar in atmospheric argon is measured using the DEAP-3600 detector. DEAP-3600, located 2 km underground at SNOLAB, uses a total of (3269 ± 24) kg of liquid argon distilled from the atmosphere to search for dark matter. This detector is well-suited to measure the decay of ^39 Ar owing to its very low background levels. This is achieved in two ways: it uses low background construction materials; and it uses pulse-shape discrimination to differentiate between nuclear recoils and electron recoils. With 167 live-days of data, the measured specific activity at the time of atmospheric extraction is (0.964 ± 0.001 _stat ± 0.024 _sys ) Bq/kg _atmAr , which is consistent with results from other experiments. A cross-check analysis using different event selection criteria and a different statistical method confirms the result.

We used the $^{138}\mathrm{Ba}(d,\ensuremath{\alpha})$ reaction to carry out an in-depth study of states in $^{136}\mathrm{Cs}$, up to around 2.5 MeV. In this Letter, we place emphasis on hitherto unobserved states below the first ${1}^{+}$ level, which are important in the context of solar neutrino and fermionic dark matter (FDM) detection in large-scale xenon-based experiments. We identify for the first time candidate metastable states in $^{136}\mathrm{Cs}$, which would allow a real-time detection of solar neutrino and FDM events in xenon detectors, with high background suppression. Our results are also compared with shell-model calculations performed with three Hamiltonians that were previously used to evaluate the nuclear matrix element (NME) for $^{136}\mathrm{Xe}$ neutrinoless double beta decay. We find that one of these Hamiltonians, which also systematically underestimates the NME compared with the others, dramatically fails to describe the observed low-energy $^{136}\mathrm{Cs}$ spectrum, while the other two show reasonably good agreement.

Received 24 December 2021DOI:https://doi.org/10.1103/PhysRevD.105.029901© 2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCosmologyDark matterParticle astrophysicsParticle dark matterPhysical SystemsGalactic halosWeakly interacting massive particlesTechniquesCosmic ray & astroparticle detectorsDark matter detectorsParticles & FieldsGravitation, Cosmology & Astrophysics

In this work, an experimental assessment is made of the performance of an integrated system for the detection of shielded nuclear material that features muon tomography and polyvinyl toluene bi-panel detector systems. Experimental setups consisting of a barrel of sand containing a lead flask and/or depleted uranium were defined, and were measured in turn by each detection system. For each system, detection parameters for the presence of depleted uranium were put forward that can be used to integrate the two detection systems in a quantitative manner. In the integrated system, the detectability of differences between setups using a given detection parameter is quantitatively assessed through the use of relative operating characteristic (ROC) curves. Furthermore, in an effort to enhance detection sensitivity, pairs of parameters were combined, through sorting on either maximum or average values, to produce "data fusion" ROC curves. The efficacy of this approach and the complementarity of the detection techniques employed are discussed.

Dark matter with Planck-scale mass (≃10^{19}  GeV/c^{2}) arises in well-motivated theories and could be produced by several cosmological mechanisms. A search for multiscatter signals from supermassive dark matter was performed with a blind analysis of data collected over a 813 d live time with DEAP-3600, a 3.3 t single-phase liquid argon-based detector at SNOLAB. No candidate signals were observed, leading to the first direct detection constraints on Planck-scale mass dark matter. Leading limits constrain dark matter masses between 8.3×10^{6} and 1.2×10^{19}  GeV/c^{2}, and ^{40}Ar-scattering cross sections between 1.0×10^{-23} and 2.4×10^{-18}  cm^{2}. These results are interpreted as constraints on composite dark matter models with two different nucleon-to-nuclear cross section scalings.

The DEAP-3600 detector searches for the scintillation signal from dark matter particles scattering on a 3.3 tonne liquid argon target. The largest background comes from 39 Ar beta decays and is suppressed using pulse-shape discrimination (PSD). We use two types of PSD estimator: the prompt-fraction, which considers the fraction of the scintillation signal in a narrow and a wide time window around the event peak, and the log-likelihood-ratio, which compares the observed photon arrival times to a signal and a background model. We furthermore use two algorithms to determine the number of photons detected at a given time: (1) simply dividing the charge of each PMT pulse by the mean single-photoelectron charge, and (2) a likelihood analysis that considers the probability to detect a certain number of photons at a given time, based on a model for the scintillation pulse shape and for afterpulsing in the light detectors. The prompt-fraction performs approximately as well as the log-likelihood-ratio PSD algorithm if the photon detection times are not biased by detector effects. We explain this result using a model for the information carried by scintillation photons as a function of the time when they are detected.

Detailed spectroscopy employing the β +/EC decay of 110,112In and the β − decay of 112Ag has been used to study the excited states of 110,112Cd. Low-energy decay branches from highly excited states have been observed and, combined with level lifetimes from the ( n,n’γ ) reaction, permit B(E2) values to be determined thus revealing rotational-like bands built on excited 0+ states and γ bands built on the ground and the shape-coexisting intruder states. The excitation energies of the 04+ states appear incompatible with that expected for a pure π(4p6h) configuration. The experimental results for the 0+ excited states are compared with beyond-mean-field calculations that suggest they possess different shapes, including prolate, oblate, and triaxial.

AbstractDEAP-3600 is a liquid-argon scintillation detector looking for dark matter. Scintillation events in the liquid argon (LAr) are registered by 255 photomultiplier tubes (PMTs), and pulseshape discrimination (PSD) is used to suppress electromagnetic background events. The excellent PSD performance of LAr makes it a viable target for dark matter searches, and the LAr scintillation pulseshape discussed here is the basis of PSD. The observed pulseshape is a combination of LAr scintillation physics with detector effects. We present a model for the pulseshape of electromagnetic background events in the energy region of interest for dark matter searches. The model is composed of (a) LAr scintillation physics, including the so-called intermediate component, (b) the time response of the TPB wavelength shifter, including delayed TPB emission at $${\mathcal {O}}$$O(ms) time-scales, and c) PMT response. TPB is the wavelength shifter of choice in most LAr detectors. We find that approximately 10% of the intensity of the wavelength-shifted light is in a long-lived state of TPB. This causes light from an event to spill into subsequent events to an extent not usually accounted for in the design and data analysis of LAr-based detectors.

Low-lying states of Zr-94 are populated following beta(-) decay of Y-94, and the emitted. rays from Zr-94 are detected using the 8. spectrometer composed of 20 Compton-suppressed HPGe detectors. High- statistics coincidence data have been used for the placement of very weak decay branches in the level scheme. Combining the results of level lifetimes from a previous experiment and the precisely measured branching ratio values of the weak decay branches from the present experiment, it is possible to extract the B(E2) values for all the possible decay branches from a given level. These values are helpful for proper identification of the collective and non-collective states of Zr-94. The experimental findings have been compared with predictions from shell-model calculations with a limited valence space; however, these calculations are inadequate in reproducing all of the measured spectroscopic quantities.

The beta(-) decay of K-47 to Ca-47 is an appropriate mechanism for benchmarking interactions spanning the sd and pf shells, but current knowledge of the beta(-) -decay scheme is limited. We have performed a high-resolution, high-efficiency study of the beta(-) decay of K-47 with the GRIFFIN spectrometer at TRIUMF-ISAC. The study revealed 48 new transitions, a more precise value for the K-47 half-life [17.38(3) s], and new spin and parity assignments for 8 excited states. Levels placed for the first time here raise the highest state observed in beta(-) decay to within 568(3) keV of the Q value and confirm the previously measured large beta(-) decay branching ratios to the low-lying states. Previously unobserved beta(-) feeding to 3/2(+) states between 4.5 and 6.1 MeV excitation energy was identified with a total beta(-) feeding intensity of 1.29(2)%. The sum of the B(GT) values for these states indicates that the 1s(1/2) proton hole strength near this excitation energy is comparable to the previously known 1s(1/2) proton and neutron hole strengths near 2.6 MeV.

DEAP-3600 is a single-phase liquid argon detector aiming to directly detect weakly interacting massive particles (WIMPs), located at SNOLAB (Sudbury, Canada). After analyzing data taken during the first year of operation, a null result was used to place an upper bound on the WIMP-nucleon, spin-independent, isoscalar cross section. This study reinterprets this result within a nonrelativistic effective field theory framework and further examines how various possible substructures in the local dark matter halo may affect these constraints. Such substructures are hinted at by kinematic structures in the local stellar distribution observed by the Gaia satellite and other recent astronomical surveys. These include the Gaia Sausage (or Enceladus), as well as a number of distinct streams identified in recent studies. Limits are presented for the coupling strength of the effective contact interaction operators O-1, O-3, O-5, O-8, and O-11, considering isoscalar, isovector, and xenonphobic scenarios, as well as the specific operators corresponding to millicharge, magnetic dipole, electric dipole, and anapole interactions. The effects of halo substructures on each of these operators are explored as well, showing that the O-5 and O-8 operators are particularly sensitive to the velocity distribution, even at dark matter masses above 100 GeV/c(2).

From detailed spectroscopy of Cd-110 and Cd-112 following the beta(+)/EC decay of In-110,In-112 and the beta(-) decay of Ag-112, the presence of very weak decay branches from nonyrast states is revealed. In Cd-112, 2(5)(+) -> 0(4)(+) and 4(6)(+) -> 2(5)(+) transitions are observed that yield B(E2; 2(5)(+) -> 0(4)(+)) = 34 +/- 15 W.u. and B(E2; 4(6)(+) -> 2(5)(+)) = 77 +/- 30 W.u., respectively, clearly indicating a collective structure. In 110Cd, a weak decay branch from the 4(6)(+) level to the 2(5)(+) level is observed, and from a lifetime measurement following the (n, n gamma' reaction, B(E2; 4(6)(+) -> 2(5)(+)) = 55 +/- 14 W.u. is determined. A new branch is also observed for the decay of the 6(4)(+) level to the 4(6)(+) state, indicating that the sequence 2(5)(+), 4(6)(+), and 6(4)(+) forms part of a collective structure. The presence of 3(3)(+) and 5(2)(+) levels spaced between the previous sequence is highly suggestive of a gamma band built on the 0(2)(+) shape-coexisting intruder state. The 0(4)(+) levels in Cd-110,Cd-112,Cd-114 have preferred decays to the lowest 2(+) members of the intruder bands, and for 114Cd a previous measurement had established an enhanced B(E2; 0(4)(+) -> 2(3)(+)). The energy systematics of the 0(2)(+), 0(3)(+), and 0(4)(+) levels all display the characteristic parabolic-shaped pattern, suggesting that they are built on multiparticle-multihole proton excitations. The results are compared with beyond-mean-field calculations that reproduce qualitatively the observed levels and their decays and suggest that the 0(1)(+), 0(2)(+), 0(3)(+), and 0(4)(+) levels and the excited states built on them possess different deformations.

The Hamamatsu R5912-HQE photomultiplier-tube (PMT) is a novel high-quantum efficiency PMT. It is currently used in the DEAP-3600 dark matter detector and is of significant interest for future dark matter and neutrino experiments where high signal yields are needed. We report on the methods developed for in-situ characterization and monitoring of DEAP's 255 R5912-HQE PMTs. This includes a detailed discussion of typical measured single-photoelectron charge distributions, correlated noise (afterpulsing), dark noise, double, and late pulsing characteristics. The characterization is performed during the detector commissioning phase using laser light injected through a light diffusing sphere and during normal detector operation using LED light injected through optical fibres.

Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new high-efficiency γ-ray spectrometer designed for use in decay spectroscopy experiments with low-energy radioactive ion beams provided by TRIUMF’s Isotope Separator and Accelerator (ISAC-I) facility. GRIFFIN is composed of sixteen Compton-suppressed large-volume clover-type high-purity germanium (HPGe) γ-ray detectors combined with a suite of ancillary detection systems and coupled to a custom digital data acquisition system. The infrastructure and detectors of the spectrometer as well as the performance characteristics and the analysis techniques applied to the experimental data are described.

Lifetimes of $2^+_1$ and $4^+_1$ states, as well as some negative-parity and non-yrast states, in $^{188-200}$Hg were measured using $\gamma-\gamma$ electronic fast timing techniques with the LaBr$_3$(Ce) detector array of the GRIFFIN spectrometer. The excited states were populated in the $\epsilon/\beta^+$-decay of $J^\pi =7^+/2^-$ $^{188-200}$Tl produced at the TRIUMF-ISAC facility. The deduced B(E2) values are compared to different interacting boson model predictions. The precision achieved in this work over previous ones allows for a meaningful comparison with the different theoretical models of these transitional Hg isotopes, which confirms the onset of state mixing in $^{190}$Hg.

DEAP-3600 is a single-phase liquid argon (LAr) direct-detection dark matter experiment, operating 2 km underground at SNOLAB (Sudbury, Canada). The detector consists of 3279 kg of LAr contained in a spherical acrylic vessel. This paper reports on the analysis of a 758 tonne . day exposure taken over a period of 231 live-days during the first year of operation. No candidate signal events are observed in the WIMP-search region of interest, which results in the leading limit on the WIMP-nucleon spin-independent cross section on a LAr target of 3.9 x 10(-45) cm(2) (1.5 x 10(-44) cm(2)) for a 100 GeV/c(2) (1 TeV/c(2)) WIMP mass at 90% C.L. In addition to a detailed background model, this analysis demonstrates the best pulse-shape discrimination in LAr at threshold, employs a Bayesian photoelectron-counting technique to improve the energy resolution and discrimination efficiency, and utilizes two position reconstruction algorithms based on the charge and photon detection time distributions observed in each photomultiplier tube.

The Dark matter Experiment using Argon Pulse-shape discrimination (DEAP) has been designed for a direct detection search for particle dark matter using a single-phase liquid argon target. The projected cross section sensitivity for DEAP-3600 to the spin-independent scattering of Weakly Interacting Massive Particles (WIMPs) on nucleons is 10(-46) cm(2) for a 100 GeV/c(2) WIMP mass with a fiducial exposure of 3 tonne-years. This paper describes the physical properties and construction of the DEAP-3600 detector. (C) 2018 Elsevier B.V. All rights reserved.