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At least 271 records · Page 15
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DOE OSTI2022

Investigating nuclear structure near N=32 and N=34: Precision mass measurements of neutron-rich Ca, Ti, and V isotopes

Nuclear mass measurements of isotopes are key to improving our understanding of nuclear structure across the chart of nuclides, in particular, for the determination of the appearance or disappearance of nuclear shell closures. Here, we present high-precision mass measurements of neutron-rich Ca, Ti, and V isotopes performed at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN) and the Low Energy Beam and Ion Trap (LEBIT) facilities. These measurements were made using the TITAN multiple-reflection time-of-flight mass spectrometer (MR-ToF-MS) and the LEBIT 9.4T Penning trap mass spectrometer. In total, 13 masses were measured, 8 of which represent increases in precision over previous measurements. These measurements refine trends in the mass surface around N=32 and N=34, and support the disappearance of the N=32 shell closure with increasing proton number. Additionally, our data do not support the presence of a shell closure at N=34.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Identification of excited states in Bi 188 and Po 188

The neutron-deficient 188 Bi and 188 Po isotopes have been studied by γ-ray spectroscopy using the recoil-decay tagging technique with the Argonne Gas-Filled Analyzer. A new 0.25(5)-μs isomeric state and a prompt cascade formed by 319-, 366-, and 462-keV γ rays have been established on top of the (10¯) α -decaying isomer in 188 Bi. The first excited (2 + ) state in 188 Po was identified, its excitation energy of 242(2) keV continues the nearly constant trend for the first 2 + states in 190, 192,194 Po. Here, the state is most likely a member of a prolate rotational band built on the ground state, albeit mixing with other coexisting configurations cannot be excluded. The new results obtained in the present work provide new information to shape coexistence in bismuth and polonium isotopes near the neutron midshell at N = 104 . In this mass region, a reduction in the prompt γ-ray yield obtained with recoil decay tagging was observed for a few nuclides, and the possible reasons are presented.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

β decay of Ba 141

The β-decay strength function of nuclides produced in fission is important as it dictates the distribution of decay energy between electrons, neutrinos, and γ rays and so is critical for calculating decay heat in reactors and for estimating the reactor antineutrino spectrum. Several experimental techniques are available to determine this strength function, including electron spectroscopy, γ-ray calorimetry (TAGS spectroscopy), and detailed, high-resolution spectroscopy with modern large high-purity germanium arrays. This work investigates the decay of the well-known and strongly produced fission fragment 141 Ba. A beam of 141 Cs was implanted at the target position of the Gammasphere and the subsequent decay of the daughter 141 Ba was studied. Extensive decay spectroscopy was possible up to the decay Q value of 3.197(7) MeV, including a significant extension of the level scheme and detailed angular correlation measurements for all levels with greater than 0.25% β feeding. The distribution of the β-decay strength was then inferred and compared to previous calorimetric studies. In conclusion, the agreement was excellent and provides a benchmark for comparing strength function methods and data for a more detailed understanding of the structure of 141 La.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Precise $\textit{Q}$-value measurements of 112,113 Ag and 115 Cd with the Canadian Penning trap for evaluation of potential ultralow $\textit{Q}$-value $\textit{β}$ decays

Background: An ultralow $\textit{Q}$-value $\textit{β}$ decay can occur from a parent nuclide to an excited nuclear state in the daughter such that $Q_{\text{UL}}$ 1 keV. These decay processes are of interest for nuclear $\textit{β}$-decay theory and as potential candidates in neutrino mass determination experiments. To date, only one ultralow $\textit{Q}$-value $\textit{β}$ decay has been observed—that of 115 In with $Q_β$ = 147(10) eV. A number of other potential candidates exist, but improved mass measurements are necessary to determine if these decay channels are energetically allowed and, in fact, ultralow. Purpose: To perform precise $\textit{β}$-decay $\textit{Q}$-value measurements of 112,113 Ag and 115 Cd and to use them in combination with nuclear energy level data for the daughter isotopes 112,113 Cd and 115 In to determine if the potential ultralow $\textit{Q}$-value $\textit{β}$-decay branches of 112,113 Ag and 115 Cd are energetically allowed and 1 keV. Method: The Canadian Penning Trap at Argonne National Laboratory was used to measure the cyclotron frequency ratios of singly charged 112,113 Ag and 115 Cd ions with respect to their daughters 112,113 Cd and 115 In. From these measurements, the ground-state to ground-state $\textit{β}$-decay $\textit{Q}$ values were obtained. Results: The 112 Ag → 112 Cd, 113 Ag → 113 Cd, and 115 Cd → 115 In $\textit{β}$-decay $\textit{Q}$ values were measured to be $Q_β$( 112 Ag) = 3990.16(22) keV, $Q_β$( 113 Ag) = 2085.7(4.6) keV, and $Q_β$( 115 Cd) = 1451.36(34) keV. These results were compared to energies of excited states in 112 Cd at 3997.75(14) keV, 113 Cd at 2015.6(2.5) and 2080(10) keV, and 115 In at 1448.787(9) keV, resulting in precise $Q_{\text{UL}}$ values for the potential decay channels of –7.59(26) keV, 6(11) keV, and 2.57(34) keV, respectively. Conclusion: The potential ultralow $\textit{Q}$-value decays of 112 Ag and 115 Cd have been ruled out. 113 Ag is still a possible candidate until a more precise measurement of the 2080(10) keV, 1/2+ state of 113 Cd is available. In the course of this work we have found the ground state mass of 113 Ag reported in the 2020 Atomic Mass Evaluation [Wang et al., Chin. Phys. C 45, 030003 (2021)] to be lower than our measurement by 69(17) keV (a 4σ discrepancy).

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

β — -delayed fission in the coupled quasiparticle random-phase approximation plus Hauser-Feshbach approach

Beta-delayed neutron emission and β-delayed fission (βdf) probabilities were calculated for heavy, neutronrich nuclei using the Los Alamos coupled quasiparticle random-phase approximation plus Hauser-Feshbach (QRPA + HF) approach. In this model, the compound nucleus is initially populated by β decay and is followed through subsequent statistical decays taking into account competition among neutrons, γ rays, and fission. The primary output of these calculations includes branching ratios along with neutron and γ -ray spectra. Here, we find a relatively large region of heavy nuclides where the probability of βdf is near 100%. For a subset of nuclei near the neutron drip line, delayed neutron emission and the probability of fission are both large, which leads to the possibility of multichance βdf (mc-βdf). We comment on prospective neutron-rich nuclei that could be probed by future experimental campaigns and provide a full table of branching ratios in ASCII format in the Supplemental Material for use in various applications.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Production and discovery of neutron-rich isotopes by fragmentation of 198 Pt

Production cross sections were measured for fragments produced by an 85 MeV/u 198 Pt beam incident on a beryllium target. Event-by-event particle identification of A, Z, and q for the reaction products was performed by employing energy loss, time-of-flight, magnetic rigidity, and total kinetic energy measurements. Over 70 nuclei in the Hf-Pt region were identified, including three isotopes first observed in this work: 191,192 Hf and 189 Lu. Due to the existence of multiple charge states between H-like and C-like ions, a new analysis method was introduced, incorporating Monte Carlo calculations of charge state fractions for a given charge state of the projectile residue just after the reaction. For the first time, charge-state probability distribution functions after the reaction have been deduced from experimental data. Furthermore, this study provides insight into how to produce key nuclides near N = 126 and the ability of a fragmentation residue to retain electrons from the primary beam.

190 ≤ A ≤ 219↗
DOE OSTI2023

Invariant-mass spectroscopy in projectile fragmentation reactions

The fragmentation of a projectile into a number of pieces can lead to the creation of many resonances in different nuclei. We discuss application of the invariant-mass method to the products from such reactions to find some of the most exotic resonances located furthest beyond the proton drip line. We show examples from fragmentation of a fast 13 O beam including the production of the newly identified 9 N resonance. In extracting resonance parameters from invariant-mass spectra, accurate estimates of the background from nonresonant prompt protons are needed. These prompt protons are correlated with the resonances due to long-range Coulomb interactions and will suppress events when resonance decay products and the prompt protons have small invariant masses. The shape of this background is especially important in determining the widths of wide resonances typically found at the edge of the chart of nuclides. An event-mixing recipe is proposed to describe this background, where the mixed events have reduced weighting for the smaller invariant masses to account for the effect of the Coulomb final-state interactions. The weighting is based on the measured correlations of heavier hydrogen isotopes with the resonances or the projectile residues. We also show that the relative magnitude of the background can be reduced in some cases by selecting events where the resonance decay products are accompanied by a deuteron or triton cluster. Here, the deuteron and triton clusters are largely not products of resonant decay and thus events with an accompanying cluster are associated with fewer background protons than events without them. The width of the 5/2¯ state in 9 C with the new background prescription was found to be consistent with the value obtained using proton elastic scattering on 8 B. Improved values for the widths of 10 C, 10,11 N, and 12 O resonances were also obtained using the new background prescription.

6 ≤ A ≤ 19↗
DOE OSTI2024

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↗
DOE OSTI2024

Landscape of nuclear deformation softness with spherical quasiparticle random-phase approximation

We investigate the stability and softness of nuclei against quadrupole, octupole, and hexadecapole deformation. By applying the spherical Skyrme-force Hartree-Fock Bardeen-Cooper-Schrieffer quasiparticle random phase approximation, we diagnose ground-state deformation when imaginary solutions are obtained, i.e., the spherical ground state collapses. We also calculate the multipole polarizability in spherical nuclei with no collapse, as a measure of softness. This numerically light and theoretically sound method is found able to capture deformation patterns across the nuclide chart. As a result, the connection between the intrinsic shape of nuclei and the dynamics of their low-lying collective states is established and the role of shell structure is discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Decay of a microsecond seniority 3 isomeric state in Hf 155

Excited states in the neutron-deficient nuclide Hf 155 have been investigated in experiments performed at the Accelerator Laboratory of the University of Jyväskylä. The Hf 155 nuclei were produced in fusion-evaporation reactions induced by beams of 295 and 315 MeV Ni 58 ions bombarding an isotopically enriched Pd 102 target and separated using the recoil mass separator MARA. An isomeric state having a half-life of 510(30) ns was discovered and is interpreted as a seniority υ = 3 , ( π h 11 / 2 2 ⊗ ν f 7 / 2 ) 27 / 2 − configuration. The γ -ray transitions emitted in the deexcitation of the isomeric state to the ground state were identified and a level scheme was constructed, from which the excitation energy of the isomer was determined to be 2581.5(10) keV. A B ( E 2 ) value of 0.45(3) W.u. was deduced for the 105.4 keV transition depopulating the isomeric state. The deduced level scheme and B ( E 2 ) value are compared with systematics and shell-model calculations. Published by the American Physical Society 2024

Physics↗
DOE OSTI2025

Weak decays in superheavy nuclei

Superheavy nuclei represent the extreme atoms and nuclides known at the limit of mass and charge. The observed superheavy nuclei are all proton-rich; they decay primarily by emitting 𝛼 particles and by fission with a possible small electron capture (EC) branch. Here, due to the huge atomic numbers and associated relativistic effects, EC decays of superheavy systems are expected to differ from what is known in lighter nuclei. In this letter, using the quantified relativistic nuclear density functional theory and the quasiparticle random-phase approximation with the interaction optimized to experimental EC/𝛽 ± -decay half-lives, and Gamow-Teller resonance energies, we study the EC/𝛽 ± -decays in 𝑍=101–118 nuclei. Both allowed (1 + ) and first-forbidden (0 − ,1 − and 2 − ) transitions are considered. We show that the first-forbidden 1 − transitions dominate the decay rates in almost all studied nuclei. For proton-rich nuclei, EC dominates over 𝛽 + decay. Based on calculations with two relativistic energy density functionals, we identify 45 candidate nuclei in which a competition between weak decays and 𝛼 decay and spontaneous fission is expected.

A ≥ 220↗
DOE OSTI2020

Identification of the New Isotope 244 Md

In an experiment performed at Lawrence Berkeley National Laboratory’s 88-inch cyclotron, the isotope Md 244 was produced in the Bi 209 ( Ar 40 , 5 n ) reaction. Decay properties of Md 244 were measured at the focal plane of the Berkeley Gas-filled Separator, and the mass number assignment of A = 244 was confirmed with the apparatus for the identification of nuclide A . The isotope Md 244 is reported to have one, possibly two, α -decaying states with α energies of 8.66(2) and 8.31(2) MeV and half-lives of 0.4 – 0.1 + 0.4 and ~ 6 s , respectively. Furtermore, first evidence of the α decay of Bk 236 was observed and is reported.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

Solving the Puzzles of the Decay of the Heaviest Known Proton-Emitting Nucleus 185 Bi

Two long-standing puzzles in the decay of 185 Bi, the heaviest known proton-emitting nucleus are revisited. These are the non-observation of the 9/2 – state, which is the ground state of all heavier odd-A Bi isotopes, and the hindered nature of proton and α decays of its presumed 60-μs 1/2 + ground state. The 185 Bi nucleus has now been studied with the 95 Mo( 93 Nb; 3n) reaction in complementary experiments using the Fragment Mass Analyzer and Argonne Gas-Filled Analyzer at Argonne National Laboratory’s ATLAS facility. The experiments have established the existence of two states in 185 Bi; the short-lived T 1/2 = $2.8$$^{+2.3}_{–1.0}$ μs, proton- and α-decaying ground state, and a 58(2)–μs γ-decaying isomer, the half-life of which was previously attributed to the ground state. The reassignment of the ground-state lifetime results in a proton-decay spectroscopic factor close to unity and represents the only known example of a ground-state proton decay to a daughter nucleus ( 184 Pb) with a major shell closure. Furthermore, the data also demonstrate that the ordering of low- and high-spin states in 185 Bi is reversed relative to the heavier odd-A Bi isotopes, with the intruder-based 1/2 + configuration becoming the ground state, similar to the lightest At nuclides.

150 ≤ A ≤ 189↗
DOE OSTI2022

Constraining the P 30 ( p , γ ) S 31 Reaction Rate in ONe Novae via the Weak, Low-Energy, β -Delayed Proton Decay of Cl 31

The 30 P(p,γ) 31 S reaction plays an important role in understanding the nucleosynthesis of A ≥ 30 nuclides in oxygen-neon novae. The Gaseous Detector with Germanium Tagging was used to measure 31 Cl β-delayed proton decay through the key J π = 3/2 + , 260-keV resonance. The intensity $I$$^{260}_{βp}$ = $8.3$$^{+1.2}_{–0.9}$ × 10 –6 represents the weakest β-delayed, charged-particle emission ever measured below 400 keV, resulting in a proton branching ratio of Γ p /Γ = $2.5$$^{+0.4}_{–0.3}$ × 10 –4 . Here, by combining this measurement with shell-model calculations for Γγ and past work on other resonances, the total 30 P(p,γ) 31 S rate has been determined with reduced uncertainty. The new rate has been used in hydrodynamic simulations to model the composition of nova ejecta, leading to a concrete prediction of 30 Si: 28 Si excesses in presolar nova grains and the calibration of nuclear thermometers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

First Evidence of Axial Shape Asymmetry and Configuration Coexistence in Zn 74 : Suggestion for a Northern Extension of the N = 40 Island of Inversion

The excited states of N=44 74 Zn were investigated via γ-ray spectroscopy following 74 Cu β decay. By exploiting γ-γ angular correlation analysis, the 2$_2^+$, 3$_1^+$, 0$_2^+$, and 2$_3^+$ states in 74 Zn were firmly established. The γ-ray branching and E2/M1 mixing ratios for transitions deexciting the 2$_2^+$, 3$_1^+$, and 2$_3^+$ states were measured, allowing for the extraction of relative B(E2) values. In particular, the 2$_3^+$→0$_2^+$ and 2$_3^+$→4$_1^+$ transitions were observed for the first time. Here the results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the N=40 gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize 74 Zn in its ground state. Furthermore, an excited K=0 band with a significantly larger softness in its shape is identified. A shore of the N=40 "island of inversion" appears to manifest above Z=26, previously thought as its northern limit in the chart of the nuclides.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Strong Evidence for N 9 and the Limits of Existence of Atomic Nuclei

The boundaries of the Chart of Nuclides contain exotic isotopes that possess extreme proton-to-neutron asymmetries. Here we report on strong evidence of 9 N, one of the most exotic proton-rich isotopes where more than one half of its constitute nucleons are unbound. With seven protons and two neutrons, this extremely proton-rich system would represent the first-known example of a ground-state five-proton emitter. Here, the invariant-mass spectrum of its decay products can be fit with two peaks whose energies are consistent with the theoretical predictions of an open-quantum-system approach, however we cannot rule out the possibility that only a single resonance-like peak is present in the spectrum.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Machine Learning to Select Experiments Driven by Fundamental Science and Applications for Targeted Nuclear Data Improvement

This work describes a blueprint for a process that accelerates progress in science by quantitatively answering the following question: What is the optimal combination of fundamental-science and application-driven experiments to maximally reduce pertinent data uncertainties? Answering this question entails solving a high-dimensional and complex optimization problem that is best solved with advanced statistic techniques often classified as machine learning. We apply this process within the framework of nuclear data with the aim to select an experiment combination that will reduce uncertainties in 239 Pu nuclear data for neutron energies between 1 and 600 keV. In this field, fundamental-physics driven data, called differential, look at one nuclear physics observable at a time. They are contrasted to application-driven, integral, data where one or few resulting values inform a broad set of nuclear data across several nuclides and energies. The candidates for integral experiments are criticality measurements that were refined by a genetic algorithm to be maximally sensitive to 239 Pu fission cross sections in the desired energy range. Twenty-three candidate differential experiments were investigated and span multiple nuclear physics observables (e.g., total, capture cross sections) for isotopes appearing in the integral experiments. The optimal combination among these candidate experiments was investigated via generalized least squares fitting, augmented with Gaussian processes to ameliorate statistical irregularities in data, and the D-optimality criterion. The latter evaluates for each pair of candidates the joint reduction in uncertainties of all 12200 nuclear data appearing in the integral experiments compared to the knowledge we have from 168 past experiments, theory, and nuclear data. We chose as differential measurements those that investigate 63 Cu and 239 Pu total cross sections, based on D-optimality rank and feasibility constraints. Two integral (criticality) experiments were selected: An experiment with Al 2 ⁢O 3 and graphite interleaved with Pu and a thick Cu reflector explores 1–30 keV, while we target the 30–600 keV range with an experiment that swaps boron in place of graphite with a different geometry.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗