arXiv subjects

R. Longland

Publications and source records attributed to R. Longland.

18 recordsLinked to original sources

arXiv2026

Core Breaking at Low Spin in $^{68}$Zn from Nuclear Resonance Fluorescence

Low-spin excited states in $^{68}$Zn have been studied at the High Intensity Gamma-Ray Source (HI$\gamma$S) from the ground state up to the particle emission threshold using the nuclear resonance fluorescence technique (NRF) and the newly developed Clover Array. Low-spin levels were excited by linearly-polarized, $2.90 - 9.79$ MeV photon beams. Spin-parity quantum numbers as well as associated $M1$ and $E1$ decay strengths were determined for a large fraction of the 158 states observed. In addition, long-duration coincidence measurements at 9.46 and 9.79 MeV enabled the investigation of the level scheme near the ground state. The results have been interpreted with shell-model calculations using two different model spaces and several effective interactions often used to describe nuclei in this mass region. While the structure near the ground state can be understood in terms of excitations involving solely valence nucleons, core breaking is required to account for the evolution of the total $M1$ strength at excitation energies above $\sim5$ MeV.

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arXiv2026

Implications for Type Ia Supernova Nucleosynthesis from an Experimentally Constrained $^{16}$O$(p,\alpha)^{13}$N Reaction Rate

The $^{16}$O$(p,\alpha)^{13}$N reaction plays a key role in shaping the $\alpha$-particle abundance during explosive oxygen burning in Type Ia supernovae. By enhancing $\alpha$-production, this reaction directly affects the calcium-to-sulphur (Ca/S) and argon-to-sulphur (Ar/S) ratios, which serves as a tracer of progenitor metallicity. However, recent work suggests that the rate must be enhanced by a factor of up to seven over the standard value to explain observed Ca/S ratios across a range of progenitor metallicities. To explore this impact, available experimental cross-section data for the $^{16}$O$(p,\alpha)^{13}$N reaction have been compiled and critically evaluated. Significant discrepancies are identified in the low-energy region ($E_{\mathrm{cm}}$ = 5.7--7.0 MeV), primarily due to limitations of the activation method. To resolve this, the first direct measurement at astrophysical energies has been performed using the MUSIC active-target detector. The new $^{16}$O$(p,\alpha)^{13}$N thermonuclear reaction rate is found to be approximately 1.5 times higher than the REACLIB rate in the temperature range T = 3--4 GK, with more constrained uncertainties that resolve the previously large spread among existing data. The suggested factor of seven enhancement is excluded and these results indicate that this reaction alone cannot fully explain the variation in the Ca/S and Ar/S ratios observed across different progenitor metallicities. Therefore, future work should focus on reducing the uncertainties in other key oxygen-burning reactions, particularly $^{16}$O+$^{16}$O and $^{12}$C+$^{16}$O. Further reducing the constraints on the $^{16}$O$(p,\alpha)^{13}$N rate is also needed to fully determine to whether a nuclear physics solution to this discrepancy is possible.

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arXiv2023

Spin-parities of sub-threshold resonances in the $^{18}$F(p, $α$)$^{15}$O reaction

The $^{18}$F(p, $α$)$^{15}$O reaction is key to determining the $^{18}$F abundance in classical novae. However, the cross section for this reaction has large uncertainties at low energies largely caused by interference effects. Here, we resolve a longstanding issue with unknown spin-parities of sub-threshold states in $^{19}$Ne that reduces these uncertainties. The $^{20}$Ne($^3$He, $^4$He)$^{19}$Ne neutron pick-up reaction was used to populate $^{19}$Ne excited states, focusing on the energy region of astrophysical interest ($\approx$ 6 - 7 MeV). The experiment was performed at the Triangle Universities Nuclear Laboratory using the high resolution Enge split-pole magnetic spectrograph. Spins and parities were found for states in the astrophysical energy range. In particular, the state at 6.133 MeV (E$_{r}^{\text{c.m.}} = -278$ keV) was found to have spin and parity of $3/2^+$ and we confirm the existence of an unresolved doublet close to 6.288 MeV (E$_{r}^{\text{c.m.}} = -120$ keV) with J$^π$ = $1/2^+$ and a high-spin state. Using these results, we demonstrate a significant factor of two decrease in the reaction rate uncertainties at nova temperatures.

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arXiv2023

New Constraints on Sodium Production in Globular Clusters From the $^{23}$Na$(^3$He$, \textbf{d})^{24}$Mg Reaction

The star to star anticorrelation of sodium and oxygen {is} a defining feature of globular clusters, but, to date, the astrophysical site responsible for this unique chemical signature remains unknown. Sodium enrichment within these clusters depends sensitively on reaction rate of the sodium destroying reactions $^{23}$Na$(p, γ)$ and $^{23}$Na$(p, α)$. In this paper, we report the results of a $^{23}$Na$(^3\text{He}, d)^{24}$Mg transfer reaction carried out at Triangle Universities Nuclear Laboratory using a $21$ MeV $^3$He beam. Astrophysically relevant states {in $^{24}$Mg} between $11 < E_x < 12$ MeV were studied using high resolution magnetic spectroscopy, thereby allowing the extraction of excitation energies and spectroscopic factors. Bayesian methods are combined with the distorted wave Born approximation to assign statistically meaningful uncertainties to the extracted spectroscopic factors. For the first time, these uncertainties are propagated through to the estimation of proton partial widths. Our experimental data are used to calculate the reaction rate. The impact of the new rates are investigated using asymptotic giant branch star models. It is found that while the astrophysical conditions still dominate the total uncertainty, intra-model variations on sodium production from the $^{23}$Na$(p, γ)$ and $^{23}$Na$(p, α)$ reaction channels are a lingering source of uncertainty.

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arXiv2022

Direct measurement of resonances in $^7$Be($α,γ$)$^{11}$C relevant to $νp$-process nucleosynthesis

We have performed the first direct measurement of two resonances of the $^7$Be($α,γ$)$^{11}$C reaction with unknown strengths using an intense radioactive $^7$Be beam and the DRAGON recoil separator. We report on the first measurement of the 1155 and 1110 keV resonance strengths of $1.73 \pm 0.25(stat.) \pm 0.40(syst.)$ eV and $125 ^{+27}_{-25}(stat.) \pm 15(syst.)$ meV, respectively. The present results have reduced the uncertainty in the $^7$Be($α,γ$)$^{11}$C reaction rate to $\sim$ 9.4-10.7% over T = 1.5-3 GK, which is relevant for nucleosynthesis in the neutrino-driven outflows of core-collapse supernovae ($νp$-process). We find no effect of the new, constrained reaction rate on $νp$-process nucleosynthesis.

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arXiv2022

First inverse kinematics measurement of resonances in $^7$Be($α,γ$)$^{11}$C relevant to neutrino-driven wind nucleosynthesis using DRAGON

A possible mechanism to explain the origin of the light $p$-nuclei in the Galaxy is the nucleosynthesis in the proton-rich neutrino-driven wind ejecta of core-collapse supernovae via the $νp$-process. However this production scenario is very sensitive to the underlying supernova dynamics and the nuclear physics input. As far as the nuclear uncertainties are concerned, the breakout from the $pp$-chains via the $^7$Be($α,γ$)$^{11}$C reaction has been identified as an important link which can influence the nuclear flow and therefore the efficiency of the $νp$-process. However its reaction rate is poorly known over the relevant temperature range, T = 1.5-3 GK. We report on the first direct measurement of two resonances of the $^7$Be($α,γ$)$^{11}$C reaction with previously unknown strengths using an intense radioactive $^7$Be beam from the ISAC facility and the DRAGON recoil separator in inverse kinematics. We have decreased the $^7$Be($α,γ$)$^{11}$C reaction rate uncertainty to $\sim$ 9.4-10.7% over the relevant temperature region.

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arXiv2022

The impact of $^{17}$O$+α$ reaction rate uncertainties on the s-process in rotating massive stars

Massive stars are crucial to galactic chemical evolution for elements heavier than iron. Their contribution at early times in the evolution of the Universe, however, is unclear due to poorly constrained nuclear reaction rates. The competing $^{17}$O($α,γ$)$^{21}$Ne and $^{17}$O($α,n$)$^{20}$Ne reactions strongly impact weak s-process yields from rotating massive stars at low metallicities. Abundant $^{16}$O absorbs neutrons, removing flux from the s-process, and producing $^{17}$O. The $^{17}$O($α,n$)$^{20}$Ne reaction releases neutrons, allowing continued s-process nucleosynthesis, if the $^{17}$O($α,γ$)$^{21}$Ne reaction is sufficiently weak. While published rates are available, they are based on limited indirect experimental data for the relevant temperatures and, more importantly, no uncertainties are provided. The available nuclear physics has been evaluated, and combined with data from a new study of astrophysically relevant $^{21}$Ne states using the $^{20}$Ne($d,p$)$^{21}$Ne reaction. Constraints are placed on the ratio of the ($α,n$)/($α,γ$) reaction rates with uncertainties on the rates provided for the first time. The new rates favour the ($α,n$) reaction and suggest that the weak s-process in rotating low-metallicity stars is likely to continue up to barium and, within the computed uncertainties, even to lead.

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arXiv2022

Experimental study of the $^{30}$Si($^{3}$He,$d$)$^{31}$P reaction and thermonuclear reaction rate of $^{30}$Si($p$,$γ$)$^{31}$P

[Background] Abundance anomalies in some globular clusters, such as the enhancement of potassium and the depletion of magnesium, can be explained in terms of an earlier generation of stars polluting the presently observed ones. It was shown that the potential range of temperatures and densities of the polluting sites depends on the strength of a few number of critical reaction rates. The reaction has been identified as one of these important reactions. [Purpose] The key ingredient for evaluating the thermonuclear reaction rate is the strength of the resonances which, at low energy, are proportional to their proton width. Therefore the goal of this work is to determine the proton widths of unbound 31P states. [Method] States in 31P were studied at the Maier-Leibnitz-Laboratorium using the one-proton transfer reaction. Deuterons were detected with the Q3D magnetic spectrometer. Angular distribution and spectroscopic factors were extracted for 27 states, and proton widths and resonance strengths were calculated for the unbound states. [Results] Several unbound states have been observed for the first time in a one-proton transfer reaction. Above 20 MK, the reaction rate is now entirely estimated from the observed properties of states. The reaction rate uncertainty from all resonances other than the resonance has been reduced down to less than a factor of two above that temperature. The unknown spin and parity of the resonance dominates the uncertainty in the rate in the relevant temperature range. [Conclusion] The remaining source of uncertainty on the reaction rate comes from the unknown spin and parity of the resonance which can change the reaction rate by a factor of ten in the temperature range of interest.

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arXiv2020

Bayesian Analysis of the $^{70}$Zn$(d, ^3\!\text{He}) ^{69}$Cu Transfer Reaction

Transfer reactions provide information about the single-particle nature of nuclear levels. In particular, the differential cross sections from these measurements are sensitive to the angular momentum of the transferred particle and the spectroscopic factor of the populated level. However, the process of extracting these properties is subject to uncertainties, both from experimental and theoretical sources. By integrating the distorted wave Born approximation into a Bayesian model, we propagate these uncertainties through to the spectroscopic factors and orbital angular momentum values. We use previously reported data of the proton pickup reaction $^{70}$Zn$(d, ^3\!\text{He}) ^{69}$Cu as an example. By accounting for uncertainties in the experimental data, optical model parameters, and reaction mechanism, we find that the extracted spectroscopic factors for low lying states of $^{69}$Cu are subject to large, asymmetric uncertainties ranging from $35 \%$ to $108 \%$. Additionally, Bayesian model comparison is employed to assign probabilities to each of the allowed angular momentum transfers. This method confirms the assignments for many states, but suggests that the data for a state lying at $3.70$ MeV is better characterized by an $\ell = 3$ transfer, rather than the previously reported $\ell = 2$.

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arXiv2020

Evaluation of the $^{13}$N($α$,p)$^{16}$O thermonuclear reaction rate and its impact on the isotopic composition of supernova grains

It has been suggested that hydrogen ingestion into the helium shell of massive stars could lead to high $^{13}$C and $^{15}$N excesses when the shock of a core-collapse supernova passes through its helium shell. This prediction questions the origin of extremely high $^{13}$C and $^{15}$N abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context $^{13}$N($α$,p)$^{16}$O the reaction plays an important role since it is in competition with $^{13}$N $β^+$-decay to $^{13}$C. The $^{13}$N($α$,p)$^{16}$O reaction rate used in stellar evolution calculations comes from the CF88 compilation with very scarce information on the origin of this rate. The goal of this work is to provide a recommended $^{13}$N($α$,p)$^{16}$O reaction rate, based on available experimental data. Unbound nuclear states in the $^{17}$F compound nucleus were studied using the spectroscopic information of the analog states in $^{17}$O nucleus that were measured at the Alto facility using the $^{13}$C($^7$Li,t)$^{17}$O alpha-transfer reaction, and spectroscopic factors were derived using a DWBA analysis. This spectroscopic information was used to calculate a recommended $^{13}$N($α$,p)$^{16}$O reaction rate with meaningful uncertainty using a Monte Carlo approach. The present $^{13}$N($α$,p)$^{16}$O reaction rate is found to be within a factor of two of the previous evaluation, with a typical uncertainty of a factor 2-3. The source of this uncertainty comes from the three resonances at $E_r^{c.m.} = 221$, 741 and 959 keV. This new error estimation translates to an overall uncertainty in the $^{13}$C production of a factor of 50. The main source of uncertainty on the re-evaluated $^{13}$N($α$,p)$^{16}$O reaction rate currently comes from the uncertain alpha-width of relevant $^{17}$F states.

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arXiv2020

Study of the $^{25}$Mg(d,p)$^{26}$Mg reaction to constrain the $^{25}$Al(p,$γ$)$^{26}$Si resonant reaction rates in nova burning conditions

The rate of the $^{25}$Al($p$,$γ$)$^{26}$Si reaction is one of the few key remaining nuclear uncertainties required for predicting the production of the cosmic $γ$-ray emitter $^{26}$Al in explosive burning in novae. This reaction rate is dominated by three key resonances ($J^π=0^{+}$, $1^{+}$ and $3^{+}$) in $^{26}$Si. Only the $3^{+}$ resonance strength has been directly constrained by experiment. A high resolution measurement of the $^{25}$Mg($d$,$p$) reaction was used to determine spectroscopic factors for analog states in the mirror nucleus, $^{26}$Mg. A first spectroscopic factor value is reported for the $0^{+}$ state at 6.256 MeV, and a strict upper limit is set on the value for the $1^{+}$ state at 5.691 MeV, that is incompatible with an earlier ($^{4}$He,$^{3}$He) study. These results are used to estimate proton partial widths, and resonance strengths of analog states in $^{26}$Si contributing to the $^{25}$Al($p$,$γ$)$^{26}$Si reaction rate in nova burning conditions.

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arXiv2014

Thermonuclear reaction rate of $^{18}$Ne($α$,$p$)$^{21}$Na from Monte-Carlo calculations

The $^{18}$Ne($α$,$p$)$^{21}$Na reaction impacts the break-out from the hot CNO-cycles to the $rp$-process in type I X-ray bursts. We present a revised thermonuclear reaction rate, which is based on the latest experimental data. The new rate is derived from Monte-Carlo calculations, taking into account the uncertainties of all nuclear physics input quantities. In addition, we present the reaction rate uncertainty and probability density versus temperature. Our results are also consistent with estimates obtained using different indirect approaches.

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arXiv2012

Is γ-ray emission from novae affected by interference effects in the 18F(p,α)15O reaction?

The 18F(p,α)15O reaction rate is crucial for constraining model predictions of the γ-ray observable radioisotope 18F produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, 19Ne, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between Jπ=3/2+ states near the proton threshold (Sp = 6.411 MeV) and a broad Jπ=3/2+ state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical S-factor at nova temperatures. Here we report a new measurement of states in this energy region using the 19F(3He,t)19Ne reaction. In stark contrast with previous assumptions we find at least 3 resonances between the proton threshold and Ecm=50 keV, all with different angular distributions. None of these are consistent with Jπ= 3/2+ angular distributions. We find that the main uncertainty now arises from the unknown proton-width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects 18F production by at least a factor of two in the model considered.

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arXiv2012

Nuclear structure of 30S and its implications for nucleosynthesis in classical novae

The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of silicon isotopes synthesized in novae, which are relevant for the identification of presolar grains of putative nova origin, were uncertain by a factor of 3. To investigate the level structure of 30S above the proton threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam gamma-ray spectroscopy experiments were performed. Differential cross sections of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born approximation calculations were performed to constrain the spin-parity assignments of the observed levels. An energy level scheme was deduced from gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction. Spin-parity assignments based on measurements of gamma-ray angular distributions and gamma-gamma directional correlation from oriented nuclei were made for most of the observed levels of 30S. As a result, the resonance energies corresponding to the excited states in 4.5 MeV - 6 MeV region, including the two astrophysically important states predicted previously, are measured with significantly better precision than before. The uncertainty in the rate of the 29P(p,gamma)30S reaction is substantially reduced over the temperature range of interest. Finally, the influence of this rate on the abundance ratios of silicon isotopes synthesized in novae are obtained via 1D hydrodynamic nova simulations.

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arXiv2011

White dwarf mergers and the origin of R Coronae Borealis stars

We present a nucleosynthesis study of the merger of a 0.4 solar masses helium white dwarf with a 0.8 solar masses carbon-oxygen white dwarf, coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.

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arXiv2011

Nucleosynthesis during the Merger of White Dwarfs and the Origin of R Coronae Borealis Stars

Many hydrogen deficient stars are characterised by surface abundance patterns that are hard to reconcile with conventional stellar evolution. Instead, it has been suggested that they may represent the result of a merger episode between a helium and a carbon-oxygen white dwarf. In this Letter, we present a nucleosynthesis study of the merger of a 0.4 M_sol helium white dwarf with a 0.8 M_sol carbon-oxygen white dwarf, by coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.

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arXiv2007

Experimental evidence of a natural parity state in $^{26}$Mg and its impact to the production of neutrons for the s process

We have studied natural parity states in $^{26}$Mg via the $^{22}$Ne($^{6}$Li,d)$^{26}$Mg reaction. Our method significantly improves the energy resolution of previous experiments and, as a result, we report the observation of a natural parity state in $^{26}$Mg. Possible spin-parity assignments are suggested on the basis of published $γ$-ray decay experiments. The stellar rate of the $^{22}$Ne($α$,$γ$)$^{26}$Mg reaction is reduced and may give rise to an increase in the production of s-process neutrons via the $^{22}$Ne($α$,n)$^{25}$Mg reaction.

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arXiv2004

Direct measurement of the 14N(p,g)15O S-factor

We have measured the 14N(p,g)15O excitation function for energies in the range E_p = 155--524 keV. Fits of these data using R-matrix theory yield a value for the S-factor at zero energy of 1.64(17) keV b, which is significantly smaller than the result of a previous direct measurement. The corresponding reduction in the stellar reaction rate for 14N(p,g)15O has a number of interesting consequences, including an impact on estimates for the age of the Galaxy derived from globular clusters.

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