Search NASA⌕ Search

Engineering topics

Charity, R. J.

Publications and source records attributed to Charity, R. J..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Evolution of shell gaps in the neutron-poor calcium region from invariant-mass spectroscopy of 37,38 Sc, 35 Ca, 34 K

A fast secondary beam of 37 Ca impinged on a 9 Be target resulting in a set of reactions populating proton-rich nuclei including 35 Ca and the first observations of 37,38 Sc and 34 K. Invariant-mass spectroscopy, used to reconstruct proton decays for these nuclei, yielded three new ground-state masses and information on their low-lying structures. The newly measured mass excesses are: ΔM( 37 Sc) = 3500(410) keV, ΔM( 38 Sc) = –4656(14) keV, and ΔM( 34 K) = –1487(17) keV. These nuclei straddle the well-known Z = 20 shell closure as well as the N = 16 subshell closure. Furthermore, trends in separation energies help elucidate how nuclear structure evolves showing a fading of the Z = 20 shell gap for N ≥ 18 and indications of a N = 16 subshell gap.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

In-beam γ -ray spectroscopy of negative-parity states of 37 K populated in dissipative reactions

In-beam γ-ray spectroscopy was used to study excited states of the neutron-deficient nucleus 37 K populated in fast-beam inelastic-scattering and proton-removal reactions at high-momentum loss. New γ-ray transitions and γγ coincidence relationships were established using the γ-ray tracking array GRETINA. The extension of the level scheme up to the first (13/2 – ) state highlights the potential of this recently demonstrated population pathway for studies of isospin symmetry involving mirror-energy differences. Furthermore, the nature of the newly identified states is discussed in comparison to shell-model calculations with the FSU cross-shell effective interaction. The calculated occupation numbers of individual orbitals are shown to offer a consistent explanation of the measured mirror-energy differences between 37 K and 37 Ar.

20 ≤ A ≤ 38↗
DOE OSTI2023

Probing proton cross-shell excitations through the two-neutron removal from 38 Ca

Bound states of the neutron-deficient, near-dripline nucleus 36 Ca were populated in two-neutron removal from the ground state of 38 Ca, a direct reaction sensitive to the single-particle configurations and couplings of the removed neutrons in the projectile wave function. Final-state exclusive cross sections for the formation of 36 Ca and the corresponding longitudinal momentum distributions, both determined through the combination of particle and γ-ray spectroscopy, are compared to predictions combining eikonal reaction theory and shell-model two-nucleon amplitudes from the USDB, USDC, and ZBM2 effective interactions. The final-state cross-section ratio σ($2$$^{+}_{1}$) / σ (0 + ) shows particular sensitivity and is approximately reproduced only with the two-nucleon amplitudes from the ZBM2 effective interaction that includes proton cross-shell excitations into the pf shell. Characterizing the proton pf-shell occupancy locally and schematically, an increase of the sd – pf shell gap by 250 keV yields an improved description of this cross-section ratio and simultaneously enables a reproduction of the B(E2;$0$$^{+}_{1}$ → $2$$^{+}_{1}$) excitation strength of 36 Ca. Furthermore, this highlights an important aspect if a new shell-model effective interaction for the region was to be developed on the quest to model the neutron-deficient Ca isotopes and surrounding nuclei whose structure is impacted by proton cross-shell excitations.

20 ≤ A ≤ 38↗
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 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 OSTI2023

Invariant-mass spectroscopy of 10 B, 11 C, 14 F, 16 F, and 18 Na

The invariant-mass method is used to study the structure of a number of light proton-rich isotopes utilizing fast beams. Reactions where the projectile picks up a proton have been used to study d-wave resonances in 14 F, 16 F, and 18 Na. While the 16 F and 18 Na results are consistent with previous studies, the 14 F results are not consistent with the only previous work. Here, we have tentatively identified the 4 + member of a rotational band in 10 B, which is the analog of well-known states with strong α-cluster structure in 10 Be and 10 C. Finally, spin and parities of newly observed states in 11 C which decay sequentially into three-body exit channels have been determined or restricted.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Measurement of the $\textit{B}(\textit{E}2 ↑)$ strengths of 36 Ca and 38 Ca

In this work, the $B(E2, 0^+_1 → 2^+_1)$ strengths of 36 Ca and 38 Ca are measured to be 131(20) $e^2\text{fm}^4$ and 101(11) $e^2\text{fm}^4$, respectively. The $\textit{B}(\textit{E}2)$ value for 36 Ca required a measurement of the p/γ branching ratio because the 2 + state is proton unbound. This branching ratio is $B_p$=0.087(8). These $\textit{B}(\textit{E}2)$ and branching-ratio values can be reproduced in the shell-model with the ZMB2 interaction, an interaction that predicts the $\textit{Z} = 20 \textit{sd}$-shell closure is incomplete with large proton pf-shell occupancies in the ground state. These occupancies are at odds with other shell-model and energy-density-functional calculations of 36 Ca. New data are used to provide an update on constraints of the density dependence of the symmetry energy through mirror charge-radii differences as well as to help reduce uncertainties of the astrophysical important 35 K(p,γ) reaction.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Exploiting dissipative reactions to perform in-beam γ-ray spectroscopy of the neutron-deficient isotopes 38,39 Ca

The neutron-deficient Ca isotopes continue to attract attention due to their importance for testing isospin symmetry and their relevance in capture reactions of interest for nova nucleosynthesis and the shape of light curves in Type I x-ray bursts. To date, spectroscopic information on 38,39 Ca is largely limited to data on lower-spin excited states. Here, we report in-beam γ-ray spectroscopy of complementary higher-spin, complex-structure states in 39 Ca populated in fast-beam-induced, momentum-dissipative processes leading to neutron pickup onto excited configurations of the projectile, 9 Be( 38 Ca*, 39 Ca + γ)X. Such a dissipative reaction was recently characterized for the case of inelastic scattering of 38 Ca off 9 Be, 9 Be( 38 Ca, 38 Ca + γ)X. Additional data and discussion on the nuclear structure of 38 Ca is also presented. Furthermore, an explanation for the more-complex-structure states, populated with small cross sections in one-nucleon knockout reactions, and observed in the tails of their longitudinal momentum distributions, is also offered.

20 ≤ A ≤ 38↗
DOE OSTI2022

Dissipative Reactions with Intermediate-Energy Beams: A Novel Approach to Populate Complex-Structure States in Rare Isotopes

A novel pathway for the formation of multiparticle-multihole excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of 38 Ca on a Be target. The negative-parity, complex-structure final states in 38 Ca are observed following the in-beam γ-ray spectroscopy of events in the 9 Be( 38 Ca, 38 Ca + γ)X reaction in which the scattered projectile loses longitudinal momentum of order Δp || = 700 MeV/c. Here, the characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly energetic projectile-target collision. Unlike the far-less-dissipative, surface-grazing reactions usually exploited for the in-beam γ-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multiparticle configurations in rare isotopes—final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Nucleon–nucleon correlations in the extreme oxygen isotopes

There has been an upsurge of interest in two-nucleon decays thanks to the studies of nucleon–nucleon correlations. In our previous work (2021 Phys. Rev. Lett. 126 142501), based on a novel time-dependent three-body approach, we demonstrated that the energy and angular correlations of the emitted nucleons can shed light on the structure of nucleonic pairs formed inside the nucleus. In this work, we apply the new framework to study the decay dynamics and properties of some extreme proton-rich and neutron-rich oxygen isotopes, including two-proton (2p) decays of 11,12 O and two-neutron (2n) decay of 26 O. Here we show that the low- ℓ components of 11,12 O wave functions, which are affected by continuum and configuration-interaction effects, strongly impact decay dynamics and asymptotic correlations. In the calculated wave functions of 11,12 O, diproton and cigarlike structures merge together during the tunneling process and the resulting energy and angular correlations are very consistent with the experimental data. The asymptotic correlations of the 2n decay of 26 O dramatically change as the 2n decay energy approaches the zero-energy threshold. The small reported value of Q 2n suggests that the 2n decay of this nucleus can be understood in terms of the universal phase-space limit.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Proton decay spectroscopy of S 28 and Cl 30

States in 28 S and 30 Cl have been studied using one- and two-proton decay spectroscopy. In the first spectrometer setting, states in 28 S were populated following one-neutron knockout from a fast 29 S beam. Three new states are observed in 28 S from one- and two-proton decay. For the two-proton case the nature of the decay was investigated and found to proceed via sequential two-proton emission. For the second setting, states in 30 Cl were populated via one-proton knockout from a fast 31 Ar beam. The decay energy of the ground and first excited state were measured, with the ground-state decay energy found to be in disagreement with a previous measurement. Here, the spin and parity of these two 30 Cl states were inferred from shell-model calculations.

20 ≤ A ≤ 38↗
DOE OSTI2022

Two-proton decay from α-cluster states in 10 C and 11 N

Two-proton decay from excited states in 10 C and 11 N has been characterized from an analysis of the 2p + 2α and 3p + 2α exit channels. Data from four previously published invariant-mass studies associated with inelastic excitation, multinucleon knockout, and neutron pickup reactions have been considered. A rotational band in 10 C was identified built on the second 0 + state which has strong α-cluster molecular structure. The members of this band were tentatively identified up to J π = 4 + and all identified states have a prompt 2p decay branch. The spins of the 0 + and 2 + members were deduced from comparisons of the momentum correlations in their 2p branches to those from known 2p emitters. The 2 + and the tentative 4 + members are both above the α threshold and exhibit large reduced decay widths for α emission. This band exhibits a similar moment of inertia to its analog in the mirror nucleus 10 Be. Evidence is also presented for a molecular band in 11 N built on the second 3/2 – state which 2p decays. Furthermore, tentative members of this band have been assigned up to J π = 9/2 – and their excitation energies also match their likely analogs in the mirror nucleus 11 Be.

6 ≤ A ≤ 19↗
DOE OSTI2021

First Observation of the Four-Proton Unbound Nucleus 18 Mg

18 Mg was observed, for the first time, by the invariant-mass reconstruction of 14 O + 4p events. The ground-state decay energy and width are E T = 4.865(34) MeV and Γ = 115(100) keV, respectively. The observed momentum correlations between the five particles are consistent with two sequential steps of prompt 2p decay passing through the ground state of 16 Ne. The invariant-mass spectrum also provides evidence for an excited state at an excitation energy of 1.84(14) MeV, which is likely the first excited 2 + state. As this energy exceeds that for the 2 + state in 20 Mg, this observation provides an argument for the demise of the N = 8 shell closure in nuclei far from stability. Furthermore, in open systems this classical argument for shell strength is compromised by Thomas-Ehrman shifts.

6 ≤ A ≤ 19↗
DOE OSTI2021

Spectroscopy of 10 N with the invariant-mass method

Proton decays of $^{10}$N states has been investigated with the invariant-mass technique using data from two reactions. In the first experiment, $^{10}$N states were created via multi-nucleon knockout from a fast $^{13}$O beam. The second experiment involved proton pickup from a $^9$Be target to a fast $^9$C beam. Both data sets produce similar distributions with a peak centered at a decay energy of 2.8 MeV and a width of $\approx 2.5$ MeV. This result is consistent with a previous study using multi-nucleon transfer reaction which was originally fit with an $\ell$ = 0 resonance but later interpreted as an $\ell$=1 resonance. This later interpretation is affirmed as the proton pickup reaction should favor $\ell$ = 1. This strength is located near the predicted energies of two $\ell$ = 1 resonances in calculations using complex scaling and the Gamow shell model. Furthermore, the multi-nucleon knockout data also show excess strength below the main peak which is interpreted as contributions from one or more $\ell$ = 0 resonances.

6 ≤ A ≤ 19↗
DOE OSTI2021

Using spin alignment of inelastically excited nuclei in fast beams to assign spins: The spectroscopy of 13 O as a test case

Excited states in O 13 were investigated using inelastic scattering of an E / A = 69.5 MeV O 13 beam off of a Be 9 target. The excited states were identified in the invariant-mass spectra of the decay products. Both single-proton and sequential two-proton decays of the excited states were examined. Furthermore, for a number of the excited states, the protons were emitted with strong anisotropy where emissions transverse to the beam axis are favored. The measured proton-decay angular distributions were compared to predictions from distorted-wave Born-approximation calculations of the spin alignment which was shown to be largely independent of the excitation mechanism. The deduced O 13 level scheme is compared to ab initio no-core shell model with continuum predictions. The lowest-energy excited states decay isotropically consistent with predictions of strong proton 1 s 1 / 2 structure. Above these states in the level scheme, we observed a number of higher-spin states not predicted within the model. Possibly these are associated with rotational bands built on deformed cluster configurations predicted by antisymmetrized molecular dynamics calculations. The spin alignment mechanism is shown to be useful for making spin assignments and may have widespread use.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Observation of the Exotic Isotope 13 F Located Four Neutrons beyond the Proton Drip Line

A 13 F resonance was observed following a charge-exchange reaction between a fast 13 O beam and a 9 Be target. The resonance was found in the invariant-mass distribution of 3p+ 10 C events and probably corresponds to a 5/2 + excited state. The ground state was also expected to be populated, but was not resolved from the background. The observed level decays via initial proton emissions to both the ground and first 2 + state of 12 O, which subsequently undergo 2p decay. In addition there may also be a significant proton decay branch to the second 2+ level in 12 O. The wavefunction associated with the observed level may be collectivized due to coupling to the continuum as is it located just above the threshold for proton decay to the second $2^{+}_{2}$ state of 12 O.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Single-nucleon knockout cross sections for reactions producing resonance states at or beyond the drip line

Single-nucleon knockout cross sections from fast secondary beams of the proton-drip-line nuclei 9 C, 13 O, and 17 Ne on a 9 Be target have been studied with emphasis on the production of resonance states. These states were identified by their invariant mass, and resonances with two-, three-, and five-body exit channels were examined. The measured cross sections for these states were compared with eikonal-model predictions using shell-model or variational Monte Carlo spectroscopic factors. The experimental yields were found to be suppressed relative to the model predictions, especially when a well-bound neutron or proton is removed. This suppression exceeds that found systematically in measured inclusive cross sections to particle-bound final states. In neutron knockout from 9 C and 13 O projectiles, this suppression of the unbound ground-state residuals yield is a factor of two to three times larger than that found in the bound final-state studies. Modifications to the structure of these systems due to coupling of the shell-model configurations to the continuum is expected to contribute to this extra suppression, especially when the final state is a near-threshold resonance. Furthermore, other considerations including the role of nuclear dynamics may be required to explain all the observed trends.

6 ≤ A ≤ 19↗