Session ED: Nuclear Astrophysics I

Near proton threshold interference effects in the 18F destruction reaction 18F(p,α)15O

Predictions of $^{18}$F in classical nova explosions depend on the cross section of the $^{18}$F(p,$\alpha$)$^{15}$O reaction. Uncertainties in this cross section currently place a poor constraint on $^{18}$F production, and hence on the 511-keV gamma rays expected to be observed from its decay in the nova ejecta. An important source of these uncertainties comes from the incomplete knowledge of the interference effects between broad resonances (e.g. at E$_{CM}$ = 665 keV) and those near the proton-threshold. In order to understand these interference effects it is very important to accurately know the energies, spin and parities (J$^{\pi}$), and the widths of sub-threshold and unbound states. In this talk we will present the results of a $^{20}$Ne($^{3}$He,$\alpha$)$^{19}$Ne neutron pickup reaction performed at the Triangle Universities Nuclear Laboratory. By using its Enge Split-pole Spectrograph facility we obtained the energy and J$^{\pi}$ of states in the compound nucleus ($^{19}$Ne) most relevant for nova nucleosynthesis. In particular, we found that the sub-threshold states at 6.132 MeV and 6.290 MeV have J$^{\pi}$=3/2$^+$ and 7/2$^+$, respectively. We will present our experimental results and highlight their effect on the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate.   

Proton Emission from $^{31}$S and the $^{30}$P$(p,\gamma)^{31}$S Reaction Rate in Classical Novae

The $^{30}$P$(p,\gamma)^{31}$S reaction rate is the largest remaining source of uncertainty in the final abundances of nuclei produced in ONe novae. To further constrain the reaction rate, uncertainties in the properties of the key resonant states in $^{31}$S must be reduced. The $^{32}$S$(p,d)^{31}$S reaction was used to populate proton-unbound states in $^{31}$S, and their subsequent decay was observed. The measurement was carried out at the Texas A$\&$M Cyclotron Institute utilizing the LLNL Hyperion particle-gamma coincidence spectrometer. Reaction deuterons were detected in a silicon telescope located at forward angles, and decay protons were detected by a single silicon detector positioned at backward angles. Proton decay branching ratios and spin-parity assignments for several states in $^{31}$S will be presented.   

Constraining $^{30}$P($p$,$\gamma$)$^{31}$S for nova nucleosynthesis by measuring low-energy $^{31}$Cl $\beta$-delayed proton decays

Reducing uncertainties in the $^{30}$P($p$,$\gamma$)$^{31}$S reaction rate is important for understanding nucleosynthesis in ONe novae. This thermonuclear rate influences the isotopic ratios and chemical abundances of nova ejecta, constrains peak explosive temperatures, and provides insights into mixing between the underlying white dwarf and hydrogen-rich material from the donor star. The reaction proceeds primarily through proton capture into narrow resonance states lying just above the proton-emission threshold in $^{31}$S. We determined the intensity of a very weak proton branch for a single, potentially dominant resonance by measuring $^{31}$Cl $\beta$-delayed proton decays using the GADGET system. We report one of the smallest proton branching ratios ever measured for such low-energy resonances. Combined with a theoretical lifetime, we a report a lower limit on the total reaction rate.   

Astromers: Nuclear Isomers with Astrophysical Consequences

In astrophysical environments, large differences in the destruction rates of a nuclear isomer and its corresponding ground state can cause the nuclear levels fall out of or fail to reach thermal equilibrium. Without thermal equilibrium, there may not be a safe assumption about the distribution of occupation probability among the nuclear levels when computing nuclear reaction rates. We demonstrate a method to compute thermally-mediated transition rates between the ground state and long-lived isomers in nuclei; this allows the two states to be treated as separate species in a nucleosynthesis network calculation. We also establish criteria delimiting a thermalization temperature above which a nucleus may be considered a single species and below which it must be treated as two separate species: a ground state species, and an astrophysically relevant nuclear isomer (“astromer”) species. We show example applications to the well-known astromer Al-26 (tracer of star formation), as well as Kr-85 (s-process branch point), Cl-34, and Cd-113.   

Isomeric Contributions of $^{24}$Al to the $^{24}$Al(p,$\gamma$) Reaction

The $^{24}$Al(p,$\gamma$)$^{25}$Si reaction is relevant in the rp-process. Variations in the rate of this reaction can affect the abundances of several stable isotopes. Experimental information of this reaction is scarce, and the isomeric contributions have not yet been experimentally studied. Due to the large difference between the spins of the ground state (t$_{1/2}$ = 2.053s, J$^{\pi}$ = 4$^{+}$) and the isomeric first excited state (E$_{ex}$ = 0.426 MeV, t$_{1/2}$ = 130ms, J$^{\pi}$ = 1$^{+}$), each component could contribute separately to the overall reaction rate. A radioactive beam of $^{24}$Na mostly in its isomeric state has been developed and used to study the $^{24}$Na(d,p) reaction in inverse kinematics at the John D Fox Accelerator Laboratory at Florida State University. The $^{24}$Na(d,p) reaction was then used to study the astrophysically relevant $^{24}$Al(p,$\gamma$)$^{25}$Si reaction via the mirror symmetry of $^{25}$Na and $^{25}$Si. Spectroscopic information from the contributions of the isomeric state of $^{24}$Al to the overall rate of the $^{24}$Al(p,$\gamma$) reaction has been constrained and will be presented.   

Studying branching ratios of interest to the 26Al(p,gamma) reaction with the Enge Split-Pole Spectrograph at Florida State University

First science runs with the Enge Split-Pole Spectrograph (SE-SPS) at Florida State University utilizing the Silicon Array for Branching Ratio Experiments (SABRE) have recently been conducted. The 27Al(3He,t)27Si*(p)26Alg,m reaction was used to measure the branching ratios of states in 27Si of interest to the 26Alg,m(p,gamma)27Si reactions. These reactions are the primary destruction mechanism of 26Al in classical novae, affecting the amount of 26Al in the Galaxy, an important indicator of ongoing nucleosynthesis. SABRE was able to detect decay branching ratios at resonance energies 200-400 keV lower than previously measured. I will discuss this measurement and the calculation of the 26Alg,m(p,gamma)27Si reaction rates based on the results.   

Spectroscopic study of the 38Kg,m(d,p) reaction at ReA3 for nova nucleosynthesis

In ONe nova, the 38K(p,$\gamma$) bottleneck reaction impacts the nucleosynthetic flow to the Ca region, thereby affecting the final abundances around the endpoint of nova nucleosynthesis. Observations suggest that, in contrast to calculations, isotopic abundances of Ar and Ca appear to be enhanced with respect to solar abundances. Though many levels have been identified within the Gamow window, there is limited constraint on most of the resonance strengths, with only a single resonance with a well-constrained resonance strength from direct measurement. Thus far, experiments have focused on $\ell$=0 resonances, but as low-lying resonances lie at ~6 MeV excitation in 39Ca, $\ell$=1 states from the fp shell could also potentially contribute to the reaction rate. The situation is further compounded by a long-lived 38K isomer; potentially both this and the ground state play a role in the nucleosynthetic network. We utilized a beam containing both ground-state and isomeric components, to measure the 38K(d,p) reaction at the ReA3 facility. By constraining spin assignments and spectroscopic strengths of single neutron states in 39K, proton resonance strengths in 39Ca can be informed via mirror symmetry. Details of the experiment, the beam, and online results will be presented.   

Measurement of (p,n) reactions at astrophysical energies using recoil separators

We have developed an experimental technique for the measurement of (p,n) reactions with short-lived isotopes, using a stable hydrogen target, a neutron detector, and a recoil separator. In this presentation, we will discuss the details of the developed technique and the results from the first proof-of-principle experiment performed at the ReA3 facility at the National Superconducting Cyclotron Laboratory. In this experiment, partial cross-sections for the 40Ar(p,n)40K reaction were determined within an uncertainty of about 25{\%}, by operating a section of the ReA3 beamline as a recoil separator. The new technique is expected to be used in experiments with the SECAR recoil separator at the Facility for Rare Isotope Beams and provide experimental data relevant to nuclear astrophysics.