Microscopic optical potentials derived from ab initio translationally invariant nonlocal one-body densities
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Gennari, Michael↗
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Measurement of key resonances for the Al 24 ( p , γ ) Si 25 reaction rate using in-beam γ -ray spectroscopy
Energy levels and branching ratios for the rp-process nucleus 25Si were determined from the reactions 9 Be ( 26 Si, 25 Si) X and 9 Be ( 25 Al, 25 Si) X using in-beam γ-ray spectroscopy with both high-efficiency and high-resolution detector arrays. Proton-unbound states at 3695(14) and 3802(11) keV were identified and assigned tentative spins and parities based on comparison to theory and the mirror nucleus. The 24 Al (p, γ) 25 Si reaction rate was calculated using the experimental states and states from charge-dependent USDA and USDB shell-model calculations with downward shifts of the 1s 1/2 proton orbital to account for the observed Thomas-Ehrman shift, leading to a factor of 10–100 increase in rate for the temperature region of 0.22 GK as compared to a previous calculation. These shifts may be applicable to neighboring nuclei, impacting the proton capture rates in this region of the chart.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Toward complete spectroscopy using β decay: The example of Cl 32 ( β γ ) S 32
$^{32}\textbf{Cl}$ is a neutron-deficient isotope with a $β$-decay half-life of 298 ms and a spin and parity of $J^π=1^+$. Previous measurements of $^{32}\textbf{Cl}$ $β$-delayed $γ$ rays have yielded a $β$-decay scheme with twelve $β$-decay transitions, contributing to studies of nuclear structure and fundamental symmetries. Those experiments have been limited to the observation of $^{32}\textbf{S}$ states with $J^π = 0^+,1^+, 2^+$. Our goal is to search for new $β$-delayed $γ$ rays and $β$-decay transitions of $^{32}\textbf{Cl}$ to $^{32}\textbf{S}$. A measurement of $^{32}\textbf{Cl}$ $β$-delayed $γ$ decay has been performed using the Clovershare array of high-purity germanium detectors at the National Superconducting Cyclotron Laboratory. By acquiring the highest-statistics $^{32}\textbf{Cl}$ $β$-delayed $γ$-ray spectrum to date and exploiting a new sensitivity to $γ-γ$ coincidences, this experiment has enabled the observation of nine previously unobserved $β$-delayed $γ$-ray transitions, leading to the inference of five $β$-decay transitions never before observed in $^{32}\textbf{Cl}$ $β$-delayed $γ$ decay. The set of observed states includes negative-parity states for the first time. By combining the new information with data from previous work, the lifetimes and partial widths of the 8861- and 9650-keV states of $^{32}\textbf{S}$ have been determined. In addition, the $^{31}\textbf{P}$($p,α$)$^{28}\textbf{Si}$ resonance strength of the 9650-keV state has been limited to $ωγ<9.8$ meV, which is an improvement over direct measurements. An enhanced decay scheme has been constructed. Most of the excited bound $^{32}\textbf{S}$ states that would correspond to allowed and first-forbidden $β$-decay transitions have been observed, demonstrating the potential of $β$-decay experiments to approach complete spectroscopy measurements at the next generation of radioactive beam facilities. The observed positive-parity levels are well matched by $sd$ shell-model calculations.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Updated β -decay measurement of neutron-rich Cu 74
Here, the β decay of neutron-rich 74 Cu has been studied at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. By using a high-resolution mass separator a purified 74 Cu beam was obtained, thus allowing decay through its isobar chain to stable 74 Ge without any decay chain member dominating. A total of 170γ rays were associated with 74 Cuβ decay with 111 placed in the 74 Zn level scheme. Updated β feeding intensities and estimated log(ft) values are presented, and new J π assignments are proposed using shell model calculations. The progression of simulated total absorption γ-ray spectroscopy (TAGS) based on proposed levels and β-feeding values from previous measurements to this evaluation are presented and demonstrate the need for a TAGS measurements for this and similar decays.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Nucleon- α scattering and resonances in 5 He and 5 Li with JISP16 and Daejeon16 NN interactions
The approach called the single-state harmonic oscillator representation of scattering equations (SS-HORSE) to analyze resonant states is generalized to the case of charged particle scattering by using the analytical properties of partial scattering amplitudes and is applied to the study of resonant states in the 5 Li nucleus and nonresonant s-wave proton-α scattering within the no-core shell model using the JISP16 and Daejeon16 NN interactions.We present also the results of calculations of neutron-α scattering and resonances in the 5 He nucleus with Daejeon16 and compare with results published previously using JISP16.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Nuclear kinetic density from ab initio theory
The nuclear kinetic density is one of many fundamental, nonobservable quantities in density functional theory (DFT) dependent on the nonlocal nuclear density. Often, approximations may be made when computing the density that may result in spurious contributions in other DFT quantities. With the ability to compute the nonlocal nuclear density from ab initio wave functions, it is now possible to estimate effects of such spurious contributions. Herein, we derive the kinetic density using ab initio nonlocal scalar one-body nuclear densities computed within the no-core shell model (NCSM) approach, utilizing two- and three-nucleon chiral interactions as the sole input. The ability to compute translationally invariant nonlocal densities allows us to gauge the impact of the spurious center-of-mass (c.m.) contributions in DFT quantities, such as the kinetic density, and provide ab initio insight into refining energy density functionals. The nonlocal nuclear densities are derived from the NCSM one-body densities calculated in second quantization. We present a review of c.m. contaminated and translationally invariant nuclear densities. We then derive an analytic expression for the kinetic density using these nonlocal densities, producing an ab initio kinetic density. The ground-state nonlocal densities of He 4 , 6 , 8 , C 12 , and O 16 are used to compute the kinetic densities of the aforementioned nuclei. The impact of c.m. removal techniques in the density are discussed and compared to a procedure applied in DFT. The results of this work can be extended to other fundamental quantities in DFT. The use of a general nonlocal density allows for the calculation of fundamental quantities taken as input in theories such as DFT. This allows benchmarking c.m. removal procedures and provides a bridge for comparison between ab initio and DFT many-body techniques.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Nuclear level densities and γ -ray strength functions in samarium isotopes
The γ-strength functions and level densities in the quasicontinuum of 147,149 Sm isotopes have been extracted from particle-γ coincidences using the Oslo method. The nuclei of interest were populated via (p,d) reactions on pure 148,150 Sm targets and the reaction products were recorded by the Hyperion array. An upbend in the low-energy region of the γSF has been observed. The systematic analysis of the γSF for a range of Sm isotopes highlights the interplay between scissors mode and the upbend. In conclusion, shell-model calculations show reasonable agreement with the experimental γSFs and confirm the correspondence between the upbend and scissors mode.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Uncertainties in tellurium-based dark matter searches stemming from nuclear structure uncertainties
Using tellurium dioxide as a target, we calculate uncertainties on 90% upper confidence limits of Galilean effective field theory (Galilean EFT) couplings to a weakly interacting massive particle (WIMP) dark matter candidate due to uncertainties in nuclear shell models. We find that these uncertainties in naturally occurring tellurium isotopes are comparable across the different Galilean EFT couplings to uncertainties in xenon, with some reaching over 100%. We also consider the effect these nuclear uncertainties have on estimates of the annual modulation of dark matter from these searches, finding that the uncertainties in the modulation amplitude are proportional to the nonmodulating upper confidence limit uncertainties. We also show that the determination of the modulation phase is insensitive to changes in the nuclear model for a given isotope.
79 ASTRONOMY AND ASTROPHYSICS↗
Structure of Ni 78 from First-Principles Computations
Doubly magic nuclei have a simple structure and are the cornerstones for entire regions of the nuclear chart. Theoretical insights into the supposedly doubly magic 78 Ni and its neighbors are challenging because of the extreme neutron-to-proton ratio and the proximity of the continuum. In this study, we predict the J π = 2$+\atop{1}$ state in 78 Ni from a correlation with the J π = 2$+\atop{1}$ state in 48 Ca using chiral nucleon-nucleon and three-nucleon interactions. Our results confirm that 78 Ni is doubly magic, and the predicted low-lying states of 79,80 Ni open the way for shell-model studies of many more rare isotopes.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Prediction for a Four-Neutron Resonance
Here, we utilize various ab initio approaches to search for a low-lying resonance in the four-neutron (4n) system using the JISP16 realistic NN interaction. Our most accurate prediction is obtained using a J-matrix extension of the no-core shell model and suggests a 4n resonant state at an energy near E r = 0.8 MeV with a width of approximately Γ = 1.4 MeV.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Electromagnetic Moments of Radioactive 136 Te and the Emergence of Collectivity 2p ⊕ 2n outside of Double-Magic 132 Sn
Radioactive 136 Te has two valence protons and two valence neutrons outside of the 132 Sn double shell closure, providing a simple laboratory for exploring the emergence of collectivity and nucleon- nucleon interactions. Coulomb excitation of 136 Te on a titanium target was utilized to determine an extensive set of electromagnetic moments for the three lowest-lying states, including B(E2;0$+\atop{1}$→ 2$+\atop{1}$ ), Q(2$+\atop{1}$ ), and g(2$+\atop{1}$ ). The results indicate that the first-excited state, 2$+\atop{1}$ , composed of the simple 2p ⊕ 2n system, is prolate deformed, and its wave function is dominated by neutron degrees of freedom, but not to the extent previously suggested. It is demonstrated that extreme sensitivity of g(2$+\atop{1}$) to the proton and neutron contributions to the wave function provides unique insight into the nature of emerging collectivity, and g(2$+\atop{1}$ ) was used to differentiate among several state-of-the-art theoretical calculations. Finally, our results are best described by the most recent shell model calculations.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Novel Δ J = 1 Sequence in Ge 78 : Possible Evidence for Triaxiality
Here, a sequence of low-energy levels in 78 32 Ge 46 has been identified with spins and parity of 2 + , 3 + , 4 + , 5 + , and 6 + . Decays within this band proceed strictly through ΔJ=1 transitions, unlike similar sequences in neighboring Ge and Se nuclei. Above the 2+ level, members of this sequence do not decay into the ground-state band. Moreover, the energy staggering of this sequence has the phase that would be expected for a γ-rigid structure. The energies and branching ratios of many of the levels are described well by shell-model calculations. However, the calculated reduced transition probabilities for the ΔJ=2 in-band transitions imply that they should have been observed, in contradiction with the experiment. Within the calculations of Davydov, Filippov, and Rostovsky for rigid-triaxial rotors with γ=30°, there are sequences of higher-spin levels connected by strong ΔJ=1 transitions which decay in the same manner as those observed experimentally, yet are calculated at too high an excitation energy.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Evidence for Coexisting Shapes through Lifetime Measurements in Zr98
The lifetimes of the first excited 2+, 4+, and 6+ states in Zr98 were measured with the recoil-distance Doppler shift method in an experiment performed at GANIL. Excited states in Zr98 were populated using the fission reaction between a 6.2 MeV/u U238 beam and a Be9 target. The γ rays were detected with the EXOGAM array in correlation with the fission fragments identified by mass and atomic number in the VAMOS++ spectrometer. Our result shows a very small B(E2;21+→01+) value in Zr98, thereby confirming the very sudden onset of collectivity at N=60. The experimental results are compared to large-scale Monte Carlo shell model and beyond-mean-field calculations. The present results indicate the coexistence of two additional deformed shapes in this nucleus along with the spherical ground state. © 2018 American Physical Society.
60 APPLIED LIFE SCIENCES↗
Enhanced Electric Dipole Strength for the Weakly Bound States in Ne 27
In this paper, an enhanced low-energy electric dipole (E1) strength is identified for the weakly bound excited states of the neutron-rich isotope 27 Ne. The Doppler-shift lifetime measurements employing a combination of the γ-ray tracking array GRETINA, the plunger device, and the S800 spectrograph determine the lower limit of 0.030 e 2 fm 2 or 0.052 W.u. for the 1/2 + → 3/2 – E1 transition in 27 Ne, representing one of the strongest E1 strengths observed among the bound discrete states in this mass region. This value is at least 30 times larger than that measured for the 3/2 – decay to the 3/2 + gs ground state. A comparison of the present results to large-scale shell-model calculations points to an important role of core excitations and deformation in the observed E1 enhancement, suggesting a novel example of the electric dipole modes manifested in weakly bound deformed systems.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Nuclear Charge Radii of B 10 , 11
The first laser spectroscopic determination of the change in the nuclear charge radius for a five-electron system is reported. This is achieved by combining high-accuracy ab initio mass-shift calculations and a high-accuracy measurement of the isotope shift in the 2s 2 2p 2 P 1/2 → 2s 2 3s 2 S 1/2 ground state transition in boron atoms. Accuracy is increased by orders of magnitude for the stable isotopes 10,11 B and the results are used to extract their difference in the mean-square charge radius < r$^{2}_{c}$ > 11– < r$^{2}_{c}$ >10= –0.49(12) fm 2 . The result is qualitatively explained by a possible cluster structure of the boron nuclei and quantitatively used to benchmark new ab initio nuclear structure calculations using the no-core shell model and Green’s function Monte Carlo approaches. These results are the foundation for a laser spectroscopic determination of the charge radius of the proton-halo candidate 8 B.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗