Improved calculations of β decay backgrounds to new physics in liquid xenon detectors
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73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
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Results for “nuclear structure and decays”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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Global calculation of two-neutrino double- β decay within the finite amplitude method in nuclear density functional theory
Two-neutrino double-beta (2νββ) decay has been used to constrain the neutron-proton part of effective interactions, which in turn is used to compute the nuclear matrix elements for neutrinoless double-beta decay, the observation of which would have important consequences for fundamental physics. We carefully examine 2νββ matrix elements within the proton-neutron quasiparticle random-phase approximation with nuclear energy density functionals. Here we work with functionals that are fit globally to single-beta-decay half-lives and charge-exchange giant-resonance energies, but not to 2νββ half-lives themselves, to evaluate the 2νββ nuclear matrix elements for all important nuclei, including those whose half-lives have not yet been measured. Such a comprehensive evaluation in large model spaces without configuration truncation requires an efficient computational scheme; we employ a double contour integration within the finite amplitude method. The results generally reproduce the nuclear matrix element extracted from half-lives well, without the use of any of those half-lives in the fitting procedure. We present predictions of the matrix elements in a total of 27 nuclei with half-lives that are still unmeasured.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Optimization of the generator coordinate method with machine-learning techniques for nuclear spectra and neutrinoless double- β decay: Ridge regression for nuclei with axial deformation
The generator coordinate method (GCM) is an important tool of choice for modeling large-amplitude collective motion in atomic nuclei. The computational complexity of the GCM increases rapidly with the number of collective coordinates. It imposes a strong restriction on the applicability of the method. In this work, we propose a subspace-reduction algorithm that employs optimal statistical ML models as surrogates for exact quantum-number projection calculations for norm and Hamiltonian kernels. The model space of the original GCM is reduced to a subspace relevant for nuclear low energy spectra and the NME of ground state to ground state 0νββ decay based on the orthogonality condition (OC) and the energy-transition-orthogonality procedure (ENTROP), respectively. For simplicity, the polynomial ridge regression (RR) algorithm is used to learn the norm and Hamiltonian kernels of axially deformed configurations. The efficiency and accuracy of this algorithm are illustrated for 76 Ge and 76 Se by comparing results obtained using the optimal RR models to direct GCM calculations. The low-lying energy spectra of 76 Ge and 76 Se, as well as the 0νββ-decay NME between their ground states, are computed. Furthermore, the results show that the performance of the GCM+OC/ENTROP+RR is more robust than that of the GCM+RR alone, and the former can reproduce the results of the original GCM calculation accurately with a significantly reduced computational cost.
59 ≤ A ≤ 89↗
Total absorption spectroscopy for the 𝛽 + decay strength distribution of 60 Ga
𝛽-decay properties play an important role in most astrophysical processes. In the absence of experimental data, astrophysical models rely on global theoretical calculations to provide the relevant properties. It is therefore important to provide strong experimental constraints when possible. In the case of 𝛽-decay, the most sensitive probe is the 𝛽-decay strength distribution. We report here on the first measurement of the latter quantity for the 𝛽 + decay of 60 Ga using the total absorption spectroscopy technique. The experimental results are compared to theoretical calculations often used in astrophysical models, namely the shell model and the quasiparticle random phase approximation (QRPA), as well as an extension of QRPA that includes higher-order nucleonic calculations. Both models are in reasonable agreement with the experimental results.
59 ≤ A ≤ 89↗
Measurement of the Spectral Shape of the $β$-Decay of 137 Xe to the Ground State of 137 Cs in EXO-200 and Comparison with Theory
Herein we report on a comparison between the theoretically predicted and experimentally measured spectra of the first-forbidden nonunique $β$-decay transition 137 Xe ( 7/2 - ) → 137 Cs ( 7/2 + ) . The experimental data were acquired by the EXO-200 experiment during a deployment of an AmBe neutron source. The ultralow background environment of EXO-200, together with dedicated source deployment and analysis procedures, allowed for collection of a pure sample of the decays, with an estimated signal to background ratio of more than 99 to 1 in the energy range from 1075 to 4175 keV. In addition to providing a rare and accurate measurement of the first-forbidden nonunique $β$-decay shape, this work constitutes a novel test of the calculated electron spectral shapes in the context of the reactor antineutrino anomaly and spectral bump.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Coupled-Cluster Calculations of Neutrinoless Double- β Decay in Ca 48
We use coupled-cluster theory and nuclear interactions from chiral effective field theory to compute the nuclear matrix element for the neutrinoless double-$\beta$ decay of $^{48}$Ca. Benchmarks with the no-core shell model in several light nuclei inform us about the accuracy of our approach. For $^{48}$Ca we find a relatively small matrix element. We also compute the nuclear matrix element for the two-neutrino double-$\beta$ decay of $^{48}$Ca with a quenching factor deduced from two-body currents in recent ab initio calculation of the Ikeda sum rule in $^{48}$Ca [Gysbers et al., Nat. Phys. 15, 428 (2019)].
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Description of the Proton-Decaying 0 2 + Resonance of the α Particle
The recent precise experimental determination of the monopole transition form factor from the ground state of 4 He to its $0$$^{+}_{2}$ resonance via electron scattering has reinvigorated discussions about the nature of this first excited state of the α particle. The $0$$^{+}_{2}$ state has been traditionally interpreted in the literature as the isoscalar monopole resonance (breathing mode) or, alternatively, as a particle-hole shell-model excitation. To better understand the nature of this state, which lies only ~410 keV above the proton emission threshold, we employ the coupled-channel representation of the no-core Gamow shell model. By considering the [ 3 H + p], [ 3 He + n], and [ 2 H + 2 H] reaction channels, we explain the excitation energy and monopole form factor of the $0$$^{+}_{2}$ state. Furthermore, we argue that the continuum coupling strongly impacts the nature of this state, which carries characteristics of the proton decay threshold.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Nuclear structure of 157 Sm via 𝛽−decay of 157 Pm
Excited states of the neutron-rich nucleus 157 Sm were populated through the 𝛽−decay of 157 Pm , which has a tentatively assigned ground-state spin and parity of 𝐽 𝜋 =(5/2 − ). Over 30 levels have been observed, 16 of which are new, and over 45 new 𝛾-ray transitions have been placed in the level scheme. An evolution in the ground-state configurations for 𝑁 = 95 nuclei from 5/2 − [523] (Er/Yb), to 5/2 + [642] (Dy), to 3/2 − [521] (Sm/Gd) can be explained based on increasing deformation from 𝑍 = 70 to 𝑍 = 62 and the fact that these three orbitals are energetically close to each other at deformations near 𝛽 2 ≈ 0.25–0.3. Finally, tentative spin and parity assignments are made for most of the states below 1500 keV based on the decay properties of the levels and using excitation-energy systematics of the various orbitals observed in 𝑁 = 95 nuclei.
beta decay↗
First observation of a high-𝐾 band structure in 162 Er and implications in the context of the identical bands phenomenon
The first ever identification of a high-𝐾 band structure in 162 Er is reported. Based on a 𝐾 𝜋 = 7 (−) isomer, it is found to be identical in nature to the corresponding 𝐾 𝜋 = 7 − sequence in 164 Er up to its highest observed spin. Furthermore, the phenomenon of identical high-K bands built on a two-quasiparticle configuration in an isotopic chain is reported here for the first time. While this is a notable addition to the systematics of known identical bands in nuclei at normal deformation, a satisfactory global understanding of the phenomenon remains elusive.
150 ≤ A ≤ 189↗
Extraction of the non-spin- and spin-transfer isovector responses via the 12 C ( 10 Be, 10 B + 𝛾) 12 B reaction
The isovector response in 12 B was investigated via the 12 C ( 10 Be, 10 B + 𝛾) 12 B* reaction at 100𝐴MeV. By utilizing the 𝛾-decay properties of the 1.74 MeV 0 + and 0.718 MeV 1 + states in 10 B, the separate extraction of the non-spin-transfer (Δ𝑆 = 0) and spin-transfer (Δ𝑆 = 1) isovector responses up to an excitation energy of 50 MeV in 12 B in a single measurement is demonstrated. The experimental setup employed the S800 spectrometer to detect and analyze the 10 B ejectiles and the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) for obtaining the Doppler-reconstructed spectrum for 𝛾 rays emitted in flight by 10 B. A 12 C foil was placed at the pivot point of the spectrograph. Here, the 12 B reaction product was not detected. Contributions from transitions associated with the transfer of different units of angular momentum in the non-spin- and spin-transfer responses were analyzed using a multipole decomposition analysis. The extracted non-spin-dipole (Δ𝑆 = 0, Δ𝐿 = 1) and spin-dipole (Δ𝑆 = 1, Δ𝐿 = 1) responses were found to be consistent with available data from other charge-exchange probes, validating the non-spin- and spin-transfer filters used. While statistical uncertainties and experimental resolutions were relatively large due to the modest intensity of the 10 Be secondary beam, the results show that, with the much higher intensities that will be available at new rare-isotope beam facilities, the ( 10 Be, 10 B + 𝛾) reaction and its Δ𝑇 𝑧 = −1 partner, the ( 10 C, 10 B + 𝛾) reaction, are powerful tools for elucidating the isovector non-spin- and spin-transfer responses in nuclei.
Charge-exchange reactions↗
Intensity of a weak 519-keV γ ray following β decay of the superallowed emitter 34 Ar determined via the 33 S(p,γ) 34 Cl reaction
The predominant branch in the β decay of 34 Ar is the superallowed 0 + → 0 + transition to the ground state of 34 Cl. To determine its important branching ratio one must first establish the ratios for the competing Gamow-Teller branches based on the measured intensities of γ rays subsequently emitted from the excited states they populate in 34 Cl. The strongest of these branches populates the 1 + state at 666 keV in 34 Cl, which has three possible γ-decay paths. Here, we report here a measurement of the decay of this state, which we populated via resonant proton capture in the reaction 33 S(p,γ) 34 Cl. We find that the intensity of the 519-keV γ-ray path is 1.46(19)% relative to that of the 666-keV path. This result is critical to new precise measurements of the superallowed decay of 34 Ar.
20 ≤ A ≤ 38↗
In-beam γ-ray spectroscopy of 62,64 Cr
The region of neutron-rich Cr isotopes has garnered much attention in recent years due to a rapid onset of collectivity near neutron number N = 40. We report here on the first γ-ray spectroscopy beyond the ($4$ $^{+}_{1}$) state in 62,64 Cr, using nucleon removal reactions from several projectiles within a rare-isotope beam cocktail. A candidate for the 6 + state in 64 Cr is presented as well as one for, possibly, the second excited 0 + state in 62 Cr. The results are discussed in comparison to the LNPS shell-model predictions that allow for neutron excitations across the N = 40 harmonic oscillator gap into the g 9/2 and d 5/2 orbitals. Here, the calculated level schemes for 62,64 Cr reveal intriguing collective structures. From the predicted neutron particle-hole character of the low-lying states in these Cr isotopes, 62 Cr emerges as a transitional system on the path to the center of the N = 40 island of inversion.
59 ≤ A ≤ 89↗
Signatures of muonic activation in the Majorana Demonstrator
Experiments searching for very rare processes such as neutrinoless double-beta decay require a detailed understanding of all sources of background. Signals from radioactive impurities present in construction and detector materials can be suppressed using a number of well-understood techniques. Background from in situ cosmogenic interactions can be reduced by siting an experiment deep underground. However, the next generation of such experiments have unprecedented sensitivity goals of 10 28 years half-life with background rates of 10 -5 cts/(keV kg yr) in the region of interest. To achieve these goals, the remaining cosmogenic background must be well understood. In the work presented here, Majorana Demonstrator data are used to search for decay signatures of metastable germanium isotopes. Contributions to the region of interest in energy and time are estimated using simulations and compared to Demonstrator data. Correlated time-delayed signals are used to identify decay signatures of isotopes produced in the germanium detectors. A good agreement between expected and measured rate is found and different simulation frameworks are used to estimate the uncertainties of the predictions. The simulation campaign is then extended to characterize the background for the LEGEND experiment, a proposed tonne-scale effort searching for neutrinoless double-beta decay in 76 Ge .
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Deciphering Nb 98 β decay with the Modular Total Absorption Spectrometer at ORNL
An assessment done under the auspices of the Organization for Economic Co-operation and Developement Nuclear Energy Agency (OECD-NEA) in 2007 suggested that the β decays of many abundantly produced fission products in nuclear reactors may be incomplete. In this assessment, the fission product 98 Nb was assigned the highest priority for study by total absorption spectroscopy due to its large cumulative fission branching fraction and because the β-decay data from several experiments are discrepant. To obtain the complete β-decay feeding pattern of 98 Nb and determine the impact on the average γ energy per 98 Nb β decay and v¯e emission calculations. The complete 98 Nb β-decay feeding pattern includes ground-state to ground-state β feeding and direct β feeding to the 0 + first-excited state (both have no associated γ rays), and the ground-state to excited-state β transitions followed by γ transitions to the ground state of the daughter nucleus, 98 Mo. The complete β-decay intensities of 98 Nb were measured with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory (ORNL). Here, the 98 Nb was produced by the β decay of mass 98 fission fragments at ORNL's On-Line Test Facility (OLTF) using proton-induced fission of 238 U. We find that changes to the current ENSDF assessment of 98 Nb β-decay intensity are required. We report improved uncertainties for the β-decay feeding values and report new β feedings to high-energy levels in 98 Mo. A more complete 98 Nb β-feeding pattern with improved accuracy and precision is offered. The impacts of the measured changes to the 98 Nb β-feeding pattern on both reactor decay heat calculations and predicted detection rates of reactor v¯e are presented. The Modular Total Absorption Spectrometer measurements of 98 Nb demonstrate the importance of reexamining and remeasuring complex β-decaying fission products with total absorption spectroscopy, including nuclei very near β stability.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
β + Gamow-Teller Strengths from Unstable 14 O via the (d, 2 He) Reaction in Inverse Kinematics
For the first time, the (d, 2 He) reaction was successfully used in inverse kinematics to extract the Gamow-Teller transition strength in the β + direction from an unstable nucleus. The new technique was made possible by the use of an active-target time-projection chamber and a magnetic spectrometer, and opens a path to addressing a range of scientific challenges, including in astrophysics and neutrino physics. Here, in this Letter, the nucleus studied was 14 O, and the Gamow-Teller transition strength to 14 N was extracted up to an excitation energy of 22 MeV. The data were compared to shell-model and state-of-the-art coupled-cluster calculations. Shell-model calculations reproduce the measured Gamow-Teller strength distribution up to about 15 MeV reasonably well, after the application of a phenomenological quenching factor. In a significant step forward to better understand this quenching, the coupled-cluster calculation reproduces the full strength distribution well without such quenching, owing to the large model space, the inclusion of strong correlations, and the coupling of the weak interaction to two nucleons through two-body currents.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Cluster structure of 3𝛼+𝑝 states in 13 N
Cluster states in 13 N are extremely difficult to measure due to the unavailability of 9 B +𝛼 elastic-scattering data. Using 𝛽-delayed charged-particle spectroscopy of 13 O, clustered states in 13 N can be populated and measured in the 3𝛼+𝑝 decay channel. One-at-a-time implantation and decay of 13 O was performed with the Texas Active Target Time Projection Chamber. 149𝛽3𝛼𝑝 decay events were observed and the excitation function in 13 N reconstructed. Four previously unknown 𝛼-decaying excited states were observed in 13 N at an excitation energy of 11.3, 12.4, 13.1, and 13.7 MeV decaying via the 3𝛼+𝑝 channel. These states are seen to have a [ 9 B (g.s) ⨂𝛼/𝑝 + 12 C (0$^+_2$)], [ 9 B ($\frac{1}{2}$ + ) ⨂𝛼], [ 9 B ($\frac{5}{2}$ + ) ⨂𝛼], and [ 9 B ($\frac{5}{2}$) ⨂𝛼] structure, respectively. A previously seen state at 11.8 MeV was also determined to have a [𝑝+ 12 C (g.s.)/𝑝+ 12 C (0$^+_2$)] structure. The overall magnitude of the clustering is not able to be extracted, however, due to the lack of a total width measurement. Clustered states in 13 N (with unknown magnitude) seem to persist from the addition of a proton to the highly 𝛼-clustered 12 C . Evidence of the $\frac{1}{2}$ + state in 9 B was also seen to be populated by decays from 13 N ★ .
Physics↗
Global properties of nuclei at finite-temperature within the covariant energy density functional theory
In stellar environments nuclei appear at finite temperatures, becoming extremely hot in core-collapse supernovae and neutron-star mergers. However, due to theoretical and computational complexity, most model calculations of nuclear properties are performed at zero temperature, while those existing at finite temperatures are limited only to selected regions of the nuclide chart. In this study we perform the global calculation of nuclear properties for even-even 8 ≤ Z ≤ 104 nuclei at temperatures in range 0 ≤ T ≤ 2 MeV. Calculations are based on the finite-temperature relativistic Hartree-Bogoliubov model supplemented by the Bonche-Levit-Vautherin vapor subtraction procedure. We find that near the neutron-drip line the continuum states have significant contribution already at moderate temperature T ≈ 1 MeV, thus emphasizing the necessity of the vapor subtraction procedure. Results include neutron emission lifetimes, quadrupole deformations, neutron-skin thickness, proton and neutron pairing gaps, entropy and excitation energy. Up to the temperature T ≈ 1 MeV, the nuclear landscape is influenced only moderately by the finite-temperature effects, mainly by reducing the pairing correlations. Here, as the temperature increases further, the effects on nuclear structures become pronounced, reducing both the deformations and the shell effects.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Impact of Clustering on the Li 8 β Decay and Recoil Form Factors
We place unprecedented constraints on recoil corrections in the β decay of 8 Li, by identifying a strong correlation between them and the 8 Li ground state quadrupole moment in large-scale ab initio calculations. The results are essential for improving the sensitivity of high-precision experiments that probe the weak interaction theory and test physics beyond the standard model. In addition, our calculations predict a 2 + state of the α + α system that is energetically accessible to β decay but has not been observed in the experimental 8 Be energy spectrum, and has an important effect on the recoil corrections and β decay for the A = 8 systems. Furthermore, this state and an associated 0 + state are notoriously difficult to model due to their cluster structure and collective correlations, but become feasible for calculations in the ab initio symmetry-adapted no-core shell-model framework.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗