The rise of space-based astronomy has revealed that X-ray bursts (XRBs) represent the most frequent stellar explosions to occur in our galaxy. These bursts are driven by thermonuclear flashes on accreting neutron stars, and lead to the formation of some of the most neutron-deficient nuclei known to exist. However, it has been found that so-called “waiting points” along the rapid proton (rp) capture process path can halt the progress of nucleosynthesis. Moreover, it has been postulated that a competition between the astrophysical 59 Cu(p,γ) 60 Zn and 59 Cu(p,α) 56 Ni reactions may even result in the formation of a nickel-copper (NiCu) cycle that traps the flux of material between 56 Ni and 60 Zn. Here, we report on the spectroscopic measurement of proton-unbound resonant states in 60 Zn, which govern the rate of the 59 Cu(p,γ) 60 Zn reaction in XRBs. Incorporating these results into stellar-model calculations, we provide the first experimental evidence in support of a NiCu cycle and limit its contribution within rp-process flow to a maximum of ∼38%. In assuming maximum NiCu cycling, our models highlight an increase in the production of low-mass isotopes which may influence Urca cooling processes in neutron star crusts.

In our systematic research on reactions with weakly bound nuclei at suband nearbarrier energies, we have studied the system 8B+natZr at the sub-barrier energy of 26.5 MeV. Our measurements, performed at the TriSol radioactive beam facility of the University of Notre Dame, include angular distributions of both elastic scattering and breakup, for the determination of the total reaction and breakup cross sections as well as the direct-to-total reaction cross section ratio. Preliminary results of the breakup analysis will be presented, supported by Continuum Discretized Coupling Channel calculations.

Elastic scattering measurements for the reaction 8B+90Zr at the sub-barrier energy of 26.5 MeV (~ 0.9 VC.B.) were carried out in a recent experiment, realized at the TriSol radioactive beam facility of the University of Notre Dame. This experiment was performed in continuation of our previous work studying 8B on the heavier target 208Pb, alongside with breakup measurements for the same reaction. Final goal is the determination of total reaction and breakup cross sections and the deduction of the direct to total cross section ratio. Preliminary experimental data for elastic scattering will be presented and compared with OMP and CDCC calculations.

The ^24 Mg( α ,p) ^27 Al reaction was measured using the solenoid spectrometer for nuclear astrophysics at the University of the Notre Dame to study the astrophysical ^24 Mg( α ,p) ^27 Al reaction rate. Alpha beams from the 10-MV FN tandem accelerator impinged on ^24 Mg solid targets which were made using the vacuum evaporation method on ^12 C foils. A total of 43 beam energies were used. Recoiling protons from the ^24 Mg( α ,p) ^27 Al reaction were detected using a double-sided position sensitive silicon detector array mounted on the solenoid. Energies, times of flight, and flight distances of particles were measured for particle identification. Protons associated with a wide range of excitation energies E_x = 12.42–14.44 MeV in ^28 Si were identified.

The cross section of the C13(α,n)O16 reaction is needed for nuclear astrophysics and applications to a precision of 10% or better, yet inconsistencies among 50 years of experimental studies currently lead to an uncertainty of ≈15%. Using a state-of-the-art neutron detection array, we have performed a high resolution differential cross section study covering a broad energy range. These measurements result in a dramatic improvement in the extrapolation of the cross section to stellar energies potentially reducing the uncertainty to ≈5% and resolving long standing discrepancies in higher energy data.Received 18 March 2023Revised 5 September 2023Accepted 17 January 2024DOI:https://doi.org/10.1103/PhysRevLett.132.062702© 2024 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasH & He burningH & He induced nuclear reactionsHydrostatic stellar nucleosynthesisNeutron physicsResonance reactionsS-factors processProperties6 ≤ A ≤ 19TechniquesNuclear data analysis & compilationNuclear Physics

The reaction dynamics for the proton halo nucleus $^{8}\mathrm{B}$ $+$ $^{\mathrm{nat}}\mathrm{Zr}$ is explored through an elastic scattering measurement at the sub-Coulomb barrier energy of 26.5 MeV. The differential angular distribution has been measured and the total reaction cross section as well as the interaction distance are derived via an optical model analysis. The present result is combined with relevant values for $^{8}\mathrm{B}$ in comparison with $^{6}\mathrm{He}$, $^{7}\mathrm{Be}$, $^{6,7}\mathrm{Li}$, and $^{16}\mathrm{O}$ on various targets through a consistent optical model analysis at sub- and near-barrier energies. The results demonstrate the proton halo nature of this exotic nucleus, which exhibits larger values for both the total reaction and interaction radii observables, than those determined for the proton-rich radioactive nucleus $^{7}\mathrm{Be}$ as well as for other stable weakly bound projectiles. Similar results are found for the neutron halo nucleus $^{6}\mathrm{He}$. The present elastic scattering results are also described well with continuum-discretized coupled-channels calculations, exhibiting a weak coupling to continuum.

The Cabibbo-Kobayashi-Maskawa quark mixing matrix currently does not satisfy unitarity at the 2σ-level. This could be the result of an inaccurate value of one or both of its largest matrix elements Vus and Vud. In the case of Vud, the most precise measurement is obtained from the f t-value measurements of superallowed beta-transitions between 0+ states. The accuracy of this determination can, in turn, be tested by extracting Vud in other transitions including superallowed transitions between mirror nuclei. The Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is currently under construction at the Nuclear Science Laboratory of the University of Notre Dame to perform such a determination, with the goal of shedding more light on this tension with unitarity. St. Benedict will take a radioactive ion beam produced by TwinSol, thermalize it in a large volume gas catcher, then transport it in two separate differentially-pumped volumes using a radio-frequency (RF) carpet and a radio-frequency quadrupole (RFQ) ion guide before injecting it in an RFQ trap to create cool ion bunches for injection in the measurement Paul trap. In this paper, we detail the installation of the beam preparation components of St. Benedict, and present the results of the first RIBs successfully stopped and extracted from its gas catcher.

We present a method for measurement analyses based on probabilistic deep neural networks that provide several advantages over conventional analyses with phenomenological models. These include predicting physical quantities directly from data, the rapid generation of statistically robust uncertainties, and the ability to bypass some parameters that may induce ambiguities and complications in data analysis. As deep learning methods make predictions through "black boxes," the uncertainty quantification is typically challenging. We use a probabilistic framework that provides thorough uncertainty quantification and is straightforward to follow in practice. With the network architecture based on the Transformer, we demonstrate the current method for predicting nuclear resonance parameters from scattering data using the phenomenological R-matrix model.

The cross section of the ^{13}C(α,n)^{16}O reaction is needed for nuclear astrophysics and applications to a precision of 10% or better, yet inconsistencies among 50 years of experimental studies currently lead to an uncertainty of ≈15%. Using a state-of-the-art neutron detection array, we have performed a high resolution differential cross section study covering a broad energy range. These measurements result in a dramatic improvement in the extrapolation of the cross section to stellar energies potentially reducing the uncertainty to ≈5% and resolving long standing discrepancies in higher energy data.

The reaction dynamics for the proton halo nucleus 8 B + nat Zr is explored through an elastic scattering measurement at the sub-Coulomb barrier energy of 26.5 MeV. The differential angular distribution has been measured and the total reaction cross section as well as the interaction distance are derived via an optical model analysis. The present result is combined with relevant values for 8 B in comparison with 6 He, 7 Be, 6 , 7 Li, and 16 O on various targets through a consistent optical model analysis at sub- and near-barrier energies. The results demonstrate the proton halo nature of this exotic nucleus, which exhibits larger values for both the total reaction and interaction radii observables, than those determined for the proton-rich radioactive nucleus 7 Be as well as for other stable weakly bound projectiles. Similar results are found for the neutron halo nucleus 6 He. The present elastic scattering results are also described well with continuum-discretized coupled-channels calculations, exhibiting a weak coupling to continuum.

Physics models typically contain adjustable parameters to reproduce measured data. While some parameters correspond directly to measured features in the data, others are unobservable. These unobservables can, in some cases, cause ambiguities in the extraction of observables from measured data, or lead to questions on the physical interpretation of fits that require these extra parameters. We propose a method based on deep learning to extract values of observables directly from the data without the need for unobservables. The key to our approach is to label the training data for the deep learning model with only the observables. After training, the deep learning model can determine the values of observables from measured data with no ambiguities arising from unobservables. We demonstrate this method on the phenomenological R-matrix that is widely utilized in nuclear physics to extract resonance parameters from cross section data. Our deep learning model based on Transformers successfully predicts nuclear properties from measurements with no need for the channel radius and background pole parameters required in traditional R-matrix analyses. Details and limitations of this method, which may be useful for studies of a wide range of phenomena, are discussed.

In this work, we report the measurement of elastic and Coulomb break-up channels in 6He+208Pb collisions at Elab = 19.3 MeV, close to the Coulomb barrier of this system ∼ 19 MeV. In the context of the astrophysical r-process, the reaction 4He(2n,γ)6He has been proposed to be a key reaction in the path of synthesizing seed nuclei for the r-process, as 12C, in an environment composed mainly of alpha particles and neutrons. Based on a theoretical approach for treating three body reactions by means of which its reaction rate can be inferred, our experimental approach aims to obtain an indirect measurement of the reaction rate of 4He(2n,γ)6He by measuring the Coulomb breakup of 6He under the intense electric field produced by a 208Pb target nucleus. The experiment was carried out at the TriSol facility operated in the Nuclear Science Laboratory of the University of Notre Dame, USA, which delivered a 6He beam together with other contaminants. Particular care must be taken for the alpha particles produced in the production reaction.

The cross section of the 13C(alpha, n)16O reaction is needed for nuclear astrophysics and applications to a precision of 10% or better, yet inconsistencies among 50 years of experimental studies currently lead to an uncertainty of approximate to 15%. Using a state-of-the-art neutron detection array, we have performed a high resolution differential cross section study covering a broad energy range. These measurements result in a dramatic improvement in the extrapolation of the cross section to stellar energies potentially reducing the uncertainty to approximate to 5% and resolving long standing discrepancies in higher energy data.

The Mg-25,Mg-26(alpha, n)Si-28,Si-29 reactions have been shown to be influential to Al-26 production in massive stars. The previously measured data sets for these reactions have discrepant results, and further study is warranted. The first measurements are reported of the total reaction cross sections of these reactions using direct recoil detection. The results are in good agreement with previous data sets based upon differential cross section studies. Astrophysical reaction rates based on the experimental data are reported and are within a factor of 1.5 of previous statistical model estimates.

Recent measurement of the fusion excitation function for $^{17}\mathrm{O}+^{12}\mathrm{C}$ reported the significant suppression of the fusion cross section at ${E}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\approx}14$ MeV. This suppression was hypothesized to signal the existence of a $^{16}\mathrm{O}+n+^{12}\mathrm{C}$ molecular configuration. Using the active-target detector MuSIC@Indiana provided an effective means of reexamining the fusion excitation function for $^{17}\mathrm{O}+^{12}\mathrm{C}$. The accuracy of this thick-target measurement is strengthened through comparison with the thin-target measurement of the excitation function for $^{17}\mathrm{F}+^{12}\mathrm{C}$. The result provides important information about the dependence of the average fusion cross section for the oxygen isotopic chain on neutron excess.

The rate of the final step in the astrophysical αp process, the ^{34}Ar(α,p)^{37}K reaction, suffers from large uncertainties due to a lack of experimental data, despite having a considerable impact on the observable light curves of x-ray bursts and the composition of the ashes of hydrogen and helium burning on accreting neutron stars. We present the first direct measurement constraining the ^{34}Ar(α,p)^{37}K reaction cross section, using the Jet Experiments in Nuclear Structure and Astrophysics gas jet target. The combined cross section for the ^{34}Ar,Cl(α,p)^{37}K,Ar reaction is found to agree well with Hauser-Feshbach predictions. The ^{34}Ar(α,2p)^{36}Ar cross section, which can be exclusively attributed to the ^{34}Ar beam component, also agrees to within the typical uncertainties quoted for statistical models. This indicates the applicability of the statistical model for predicting astrophysical (α,p) reaction rates in this part of the αp process, in contrast to earlier findings from indirect reaction studies indicating orders-of-magnitude discrepancies. This removes a significant uncertainty in models of hydrogen and helium burning on accreting neutron stars.

The Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is currently under construction at the University of Notre Dame Nuclear Science Laboratory. It is designed to measure the beta-neutrino angular correlation parameter in superallowed mixed beta-decay transitions between mirror nuclei in order to extract the Fermi-to-Gamow Teller mixing ratio and test theoretical corrections entering in the determination of the Vud element of the Cabibbo–Kobayashi–Maskawa quark mixing matrix. St. Benedict includes a large volume gas catcher to thermalize the radioactive ion beam from TwinSol, a differentially-pumped extraction system, a radiofrequency quadrupole ion cooler and buncher, and a Paul trap for the observation and detection of the β decay products. The status of each of these components will be presented.

The $^{25}\mathrm{Al}(p,\ensuremath{\gamma})^{26}\mathrm{Si}$ reaction bypasses production of potentially observable $^{26}\mathrm{Al}$ in novae and strongly influences the nova contribution to galactic $^{26}\mathrm{Al}$. The nuclear structure of $^{26}\mathrm{Si}$ directly impacts estimates of the astrophysical $^{25}\mathrm{Al}(p,\ensuremath{\gamma})^{26}\mathrm{Si}$ reaction rate, and reactions such as $^{28}\mathrm{Si}(p,t)^{26}\mathrm{Si}$ have been used to understand this structure. Since the original publication of $^{28}\mathrm{Si}(p,t)^{26}\mathrm{Si}$ data [D. W. Bardayan et al., Phys. Rev. C 65, 032801(R) (2002)], a number of subsequent publications have greatly clarified the level scheme of $^{26}\mathrm{Si}$, and this article reports on a reconsideration of these original data in light of the new results.

We report here on the recent upgrade of the TwinSol radioactive nuclear beam (RNB) facility at the University of Notre Dame. The new TriSol system includes a magnetic dipole to provide a second beamline and a third solenoid which acts to reduce the size of the radioactive beam on target.

Precise measurements of nuclear beta decays provide a unique insight into the Standard Model due to their connection to the electroweak interaction. These decays help constrain the unitarity or non-unitarity of the Cabibbo–Kobayashi–Maskawa (CKM) quark mixing matrix, and can uniquely probe the existence of exotic scalar or tensor currents. Of these decays, superallowed mixed mirror transitions have been the least well-studied, in part due to the absence of data on their Fermi to Gamow-Teller mixing ratios (ρ). At the Nuclear Science Laboratory (NSL) at the University of Notre Dame, the Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is being constructed to determine the ρ for various mirror decays via a measurement of the beta–neutrino angular correlation parameter (aβν) to a relative precision of 0.5%. In this work, we present an overview of the St. Benedict facility and the impact it will have on various Beyond the Standard Model studies, including an expanded sensitivity study of ρ for various mirror nuclei accessible to the facility. A feasibility evaluation is also presented that indicates the measurement goals for many mirror nuclei, which are currently attainable in a week of radioactive beam delivery at the NSL.