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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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At least 523 records · Page 29
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DOE OSTI2020

γ -ray spectroscopy of astrophysically important states in Ca 39

Background: Nova explosions synthesize elements up to A ≃ 40 , and discrepancies exist between calculated and observed abundances of Ar and Ca created in the explosion. The K 38 ( p , γ ) Ca 39 reaction rate has been illustrated to be influential on these isotopic abundances at the endpoint of nova nucleosynthesis. The energies of the three most important resonances, corresponding to J π = 5 / 2 + excited states in the Ca 39 nucleus above the proton separation threshold, are uncertain and one has been measured with conflicting values [ E r = 679 ( 2 ) versus E r = 701 ( 2 ) keV] in previous experiments. Purpose: Reducing the uncertainties on the resonance energies would allow for a better understanding of the reaction rate. To improve these uncertainties, we searched for γ rays from the depopulation of the corresponding excited states in Ca 39 . Methods: We report a new measurement of these resonance energies via the observation of previously unobserved γ -ray transitions. These transitions were observed by studying the Ca 40 ( 3 He , α γ ) Ca 39 reaction with Gammasphere ORRUBA Dual Detectors for Experimental Structure Studies (GODDESS). The updated resonance energies were then used to calculate the K 38 ( p , γ ) Ca 39 reaction rate and assess its uncertainties. Results: In total, 23 new transitions were found, including three γ -ray transitions corresponding to the three J π = 5 / 2 + states of astrophysical interest at energies of 6156.2(16), 6268.8(22), and 6470.8(19) keV. These correspond to resonance energies in the K 38 ( p , γ ) Ca 39 reaction of 386(2), 498(2), and 701(2) keV. Conclusions: Updated K 38 ( p , γ ) Ca 39 reaction rate calculations reflect a reduced upper limit at nova temperatures. However, the lower-than-previously-measured energy of the 498-keV resonance and uncertainty in its resonance strength increases the upper limit of the rate close to previous estimates at 0.4 GK.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Neutron elastic scattering on calcium isotopes from chiral nuclear optical potentials

We formulate microscopic neutron-nucleus optical potentials from many-body perturbation theory based on chiral two- and three-body forces. The neutron self-energy is first calculated in homogeneous matter to second order in perturbation theory, which gives the central real and imaginary terms of the optical potential. Here, the real spin-orbit term is calculated separately from the density matrix expansion using the same chiral interaction as in the self-energy. Finally, the full neutron-nucleus optical potential is derived within the improved local density approximation utilizing mean-field models consistent with the chiral nuclear force employed. We compare the results of the microscopic calculations to phenomenological models and experimental data up to projectile energies of E = 200 MeV. Experimental elastic differential scattering cross sections and vector analyzing powers are generally well reproduced by the chiral optical potential, but we find that total cross sections are overestimated at high energies.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Neutron spin dynamics in polarized targets

We present neutron elastic scattering amplitude for arbitrary polarized target in irreducible spherical tensor representation. Here, the general approach for the description of neutron spin dynamics for the propagation trough the medium with an arbitrary polarization is discussed in a relation to the search for time reversal invariance violation in neutron scattering.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Dirac oscillator: An alternative basis for nuclear structure calculations

The isotropic harmonic oscillator supplemented by a strong spin-orbit interaction has been the cornerstone of nuclear structure since its inception more than seven decades ago. In this paper we introduce—or rather re-introduce—the “Dirac oscillator,” a fully relativistic basis that has all the desired attributes of the ordinary harmonic oscillator while naturally incorporating a strong spin-orbit coupling. To assess the power and flexibility of the Dirac oscillator basis in the solution of nuclear structure problems within the framework of covariant density-functional theory. Here, self-consistent calculations of binding energies and ground-state densities for a selected set of doubly magic nuclei are performed using the Dirac oscillator basis and are then compared against results obtained with the often-used Runge-Kutta method. Results obtained using the Dirac oscillator basis reproduce with high accuracy those derived using the Runge-Kutta method and suggest a clear path for a generalization to systems with axial symmetry. Although the harmonic oscillator with spin-orbit corrections has been the staple of the nuclear shell model since the beginning, the Dirac oscillator is practically unknown among the nuclear physics community. In this paper we illustrate the power and flexibility of the Dirac oscillator and suggest extensions to the study of systems without spherical symmetry, as required in constrained calculations of nuclear excitations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Preparation of excited states for nuclear dynamics on a quantum computer

We study two different methods to prepare excited states on a quantum computer, a key initial step to study dynamics within linear response theory. The first method uses unitary evolution for a short time T = O(√1 - F) to approximate the action of an excitation operator Ô with fidelity F and success probability P ≈ 1 – F. The second method probabilistically applies the excitation operator using the Linear Combination of Unitaries (LCU) algorithm. We benchmark these techniques on emulated and real quantum devices, using a toy model for thermal neutron-proton capture. Despite its larger memory footprint, the LCU-based method is efficient even on current generation noisy devices and can be implemented at a lower gate cost than a naive analysis would suggest. Here, these findings show that quantum techniques designed to achieve good asymptotic scaling on fault tolerant quantum devices might also provide practical benefits on devices with limited connectivity and gate fidelity.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Comparing different density-matrix expansions for long-range pion exchange

Empirical energy density functionals (EDFs) are generally successful in describing nuclear properties across the table of nuclides. But their limitations motivate using the density-matrix expansion (DME) to embed long-range pion interactions into a Skyrme functional. Recent results on the impact of the pion were both encouraging and puzzling, necessitating a careful re-examination of the DME implementation. In this study, we take the first steps, focusing on two-body scalar terms in the DME. Exchange energies with long-range one-pion contributions are well approximated by all DME implementations considered, with preference for variants that do not truncate at two derivatives in every EDF term. The use of the DME for chiral pion contributions is therefore supported by this investigation. For scalar-isovector energies it is important to treat neutrons and protons separately. The results are found to apply under broad conditions, although self-consistency is not yet tested.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Coriolis coupling effects in proton-pickup spectroscopic factors from 12 B

Spectroscopic factors to low-lying negative-parity states in Be 11 extracted from the B 12 ( d , He 3 ) Be 11 proton-removal reaction are interpreted within the rotational model. Earlier predictions of the p -wave proton-removal strengths in the strong-coupling limit of the Nilsson model underestimated the spectroscopic factors to the 3 / 2 1 - and 5 / 2 1 - states and suggested that deviations in the 1 + ground state of the odd-odd B 12 due to Coriolis coupling should be further explored. Furthermore, in this work we use the particle rotor model to take into account these effects and obtain a good description of the level scheme in B 11 , with a moderate K mixing of the proton Nilsson levels [110]1/2 and [101]3/2. This mixing, present in the 1 + bandhead of B 12 , is key to explaining the proton-pickup data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Hyperheavy spherical and toroidal nuclei: The role of shell structure

The properties of toroidal hyperheavy even-even nuclei and the role of toroidal shell structure are extensively studied within covariant density functional theory. The general trends in the evolution of toroidal shapes in the Z ≈ 130–180 region of the nuclear chart are established for the first time. These nuclei are stable with respect to breathing deformations. The most compact fat toroidal nuclei are located in the Z ≈ 136, N ≈ 206 region of the nuclear chart, but thin toroidal nuclei become dominant with increasing proton number and toward proton and neutron drip lines. The roles of toroidal shell structure, its regularity, supershell structure, and shell gaps as well as the role of different groups of the pairs of the orbitals in its formation are investigated in detail. The lowest in energy solutions at axial symmetry are characterized either by large shell gaps or low density of the single-particle states in the vicinity of the Fermi level in at least one of the subsystems (proton or neutron). Related quantum shell effects are expected to act against the instabilities in breathing and sausage deformations for these subsystems. The investigation with a large set of covariant energy density functionals reveals that substantial proton Z = 154 and 186 and neutron N = 228, 308, and 406 spherical shell gaps exist in all functionals. The nuclei in the vicinity of the combination of these particle numbers form the islands of stability of spherical hyperheavy nuclei. Furthermore, the study suggests that the N = 210 toroidal shell gap plays a substantial role in the stabilization of fat toroidal nuclei.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Centrality determination with a forward detector in the RHIC Beam Energy Scan

Recently, Chatterjee et al [1] used a hadronic transport model to estimate the resolution with which various experimental quantities select the impact parameter of relativistic heavy ion collisions at collision energies relevant to the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). Measures based on particle multiplicity at forward rapidity were found to be significantly worse than those based on midrapidity multiplicity. Using the same model, we show that a slightly more sophisticated measure greatly improves the resolution based on forward rapidity particles; this improvement persists even when the model is filtered through a realistic simulation of a recent upgrade detector to the STAR experiment. Furthermore, these results highlight the importance of optimizing centrality measures based on particles detected at forward rapidity, especially for experimental studies that search for a critical point in the QCD phase diagram. Such measurements usually focus on proton multiplicity fluctuations at midrapidity, hence selecting events based on multiplicity at midrapidity raises the possibility of nontrivial autocorrelations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Auxiliary function approach for determining symmetry energy at suprasaturation densities

Nuclear symmetry energy E sym (ρ) at density ρ is normally expanded or simply parameterized as a function of χ = (ρ – ρ 0 )/3ρ 0 in the form of Esym(ρ) ≈ S + Lχ + 2 –1 K sym χ 2 + 6 –1 J sym χ 3 + … using its magnitude S, slope L, curvature K sym and skewness J sym at the saturation density ρ0 of nuclear matter. Much progress has been made in recent years in constraining especially the S and L parameters using various terrestrial experiments and astrophysical observations. Furthermore, such kind of expansions/parameterizations do not converge at supra-saturation densities where χ is not small enough, hindering an accurate determination of high-density E sym (ρ) even if its characteristic parameters at ρ0 are all well determined by experiments/observations. By expanding the E sym (ρ) in terms of a properly chosen auxiliary function Π sym (χ,Θ sym ) with a parameter Θsym fixed accurately by an experimental E sym (ρ r ) value at a reference density ρr, we show that the shortcomings of the χ-expansion can be completely removed or significantly reduced in determining the high-density behavior of E sym (ρ). In particular, using two significantly different auxiliary functions, we show that the new approach effectively incorporates higher χ-order contributions and converges to the same E sym (ρ) much faster than the conventional χ-expansion at densities ≲3ρ 0 . Several quantitative demonstrations using Monte Carlo simulations are given.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Proton capture on 34 S in the astrophysical energy regime of O-Ne novae

Nuclear reaction sensitivity studies have shown that the final isotopic abundance of O-Ne nova nucleosynthesis is dependent on the 34 S(p,γ) 35 Cl reaction at astrophysical energies corresponding to peak nova burning temperatures of 0.1–0.4 GK. Isotopic ratios of the S, Cl, and Ar products are all used in various methods of cosmochemical analysis of presolar meteoritic grains. Due to the lack of direct experimental data, the 34 S+p reaction rate has been estimated using statistical modeling or information from indirect nucleon transfer experiments. In order to provide direct reaction information, here the resonance strengths of several low energy resonances, E c.m. = 272–495 keV, in the 34 S(p,γ) 35 Cl reaction were measured for the first time in inverse kinematics using the DRAGON recoil separator located at TRIUMF, Canada’s Particle Accelerator Centre in Vancouver.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Role of quadrupole deformation and continuum effects in the “island of inversion” nuclei 28,29,31 F

The peculiar properties of nuclei in the so-called “island of inversion” around Z = 10 and N = 20 are the focus of current nuclear physics research. Recent studies showed that 28 F has a negative-parity ground state and thus lies within the southern shore of the island of inversion, and 29 F presents a halo structure in its ground state, but it is unclear which effects, such as deformation, shell evolution due to tensor forces, or couplings to the continuum, lead to this situation. Here, we investigate the role of quadrupole deformation and continuum effects on the single-particle structure of 28,29,31 F from a relativistic mean-field approach and show how both phenomena can lead to a negative-parity ground state in 28 F and halo structures in 29,31 F. We solve the Dirac equation in the complex-momentum (Berggren) representation for a potential with quadrupole deformation at the first order obtained from relativistic mean-field calculations using the NL3 interaction and calculate the continuum level densities using the Green’s function method. We extract single-particle energies and widths from the continuum level densities to construct the Nilsson diagrams of 28,29,31 F in the continuum and analyze the evolution of both the widths and occupation probabilities of relevant Nilsson orbitals in 28 F and find that some amount of prolate deformation must be present. In addition, we calculate the density distributions for bound Nilsson orbitals near the Fermi surface in 29,31 F and reveal that, for a quadrupole deformation 0.3 ≤ β 2 ≤ 0.45 (prolate), characteristic halo tails appear at large distances. Using the relativistic mean-field approach in the complex-momentum representation with the Green’s function method, we demonstrate that in neutron-rich fluorine isotopes, while in the spherical case the pf shells are already inverted and close to the neutron emission threshold, a small amount of quadrupole deformation can dramatically reduce the gap between positive- and negative-parity states and increase the role of continuum states, ultimately leading to the negative parity in the ground state of 28 F and the halo structures in 29,31 F.

20 ≤ A ≤ 38↗
DOE OSTI2021

First measurement of proton decay from a transfer reaction to 21 Na

Decay protons from excited states in Na 21 populated through a previously reported Mg 24 ( p , α ) Na 21 transfer reaction [Cha et al. , Phys. Rev. C 96 , 025810 (2017) ] were analyzed to extract the proton branching ratios of the energy levels. Additionally, by utilizing 31-MeV proton beams from the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory and isotopically enriched Mg 24 solid targets, the decay protons were detected in coincidence with α particles from the ( p , α ) reaction using a silicon strip detector array. Proton decay branching ratios of several Na 21 levels were deduced for the p 0 and p 1 decay channels to the ground and first excited states in Ne 20 , respectively.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Influence of nonstatistical properties in nuclear structure on emission of prompt fission neutrons

We report the Hauser-Feshbach fission-fragment decay (HF 3 D) model is extended to calculate the prompt fission neutron spectrum (PFNS) for the thermal-neutron-induced fission on 235 U, where the evaporated neutrons from all possible fission-fragment pairs are aggregated. By studying model parameter sensitivities on the calculated PFNS as well as nonstatistical behavior of low-lying discrete level spin distribution, we conclude that discrepancies between the aggregation calculation and the experimental PFNS seen at higher neutron emission energies can be attributed to both the primary fission-fragment yield distribution and the possible high spin states that are not predicted by the statistical theory of nuclear structure.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗