Search

Background: The uncertainty in the 29P(p, ɣ )30S reaction rate over 0.1 ≤ T ≤ 1.3GK was previously determined to span approximately four orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the Ex = 4.7-4.8 MeV 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. Purpose: (a) To investigate the level structure of 30S above the proton threshold [4394.9(7) keV] via chargedparticle spectroscopy using the 32S(p, t)30S reaction and in-beam y-ray spectroscopy using the 28Si(³He, ny)30S reaction to calculate the 29P(p, ɣ)30S reaction rate, (b) To explore the impact of this rate on the abundances of silicon isotopes synthesized in novae. Methods: Differential cross sections of the 32S(p, t)30S reaction were measured at 34.5 MeV. Distorted-wave Bom approximation calculations were performed to constrain the spin-parity assignments of the observed levels, including the two astrophysically important levels. An energy-level scheme was deduced from ɣ-ɣ coincidence measurements using the 28Si(³He, nɣ)30S reaction. Spin-parity assignments based on measurements of ɣ-ray angular distributions and ɣ-ɣ directional correlation from oriented nuclei were made for most of the observed levels of 30S. Results: The resonance energies corresponding to the states with 4.5 MeV ≤ Ex ≤ 6 MeV, ncluding the two astrophysically important states predicted previously, are measured with significantly better precision than before. The spin-parity assignments of both astrophysically important resonances are confirmed. The uncertainty in the rate of the 29P(p, ɣ)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 ID hydrodynamic nova simulations. Conclusions: The uncertainty in the 29P(p, ɣ)30S reaction rate is reduced to the point that it no longer affects the silicon isotopic abundance ratios significantly, and, thus, the results of our nova hydrodynamic simulation for the nucleosynthesis in the Si-Ca mass region are more reliable than before. [ABSTRACT FROM AUTHOR]

The structure of proton-unbound 30S states strongly determines the thermonuclear 29P(p,γ)30S reaction rate at temperatures characteristic of explosive hydrogen burning in classical novae and type I x-ray bursts. Specifically, the rate had been previously predicted to be dominated by two low-lying, unobserved, levels in the Ex= 4.7-4.8 MeV region, with spin and parity assignments of 3+ and 2+. In recent experimental work, two candidate levels were observed with energies of 4.699 MeV and 4.814 MeV, but no experimental information on their spins and parities was obtained. We have performed an in-beam γ-ray spectroscopy study of 30S with the 28Si(3He, nγ)30S reaction. By constructing the decay schemes of proton-unbound states with a γ-γ coincidence analysis of their decay γ rays, their Jπ values were inferred from a comparison to the known decay schemes of the corresponding mirror states in 30Si. For the two aforementioned states, our results strongly corroborate the spin-parity assignments assumed in recent evaluations of the 29P(p,γ)30S reaction rate. [ABSTRACT FROM AUTHOR]

Background: The presolar grains originating in oxygen-neon novae may be identified more easily than those of other stellar sources if their sulfur isotopic ratios (33S/32S and 34S/32S) are compared with the theoretical ones. The accuracy of such a comparison depends on reliable 33S(p,γ)34Cl and 34S(p,γ)35Cl reaction rates at the nova temperature regime. The latter rate has recently been computed based on experimental input, and many new excited states in 35Cl were discovered above the proton threshold. As a result, the experimental 34S(p,γ)35Cl rate was found to be less uncertain and 2-5 times smaller than the theoretical one. Consequently, the simulated 34S/32S isotopic ratio for nova presolar grains was predicted to be smaller than that of type II supernova grains by a factor of 1.5 to 3.7. Purpose: The present study was performed to confirm the existence of these new resonances, and to improve the remaining uncertainties in the 34S(p,γ)35Cl reaction rate. Methods: Energies and spin-parities of the 35Cl excited levels were investigated via high-resolution charged-particle spectroscopy with an Enge split-pole spectrograph using the 32S(α,p)35Cl reaction. Differential cross sections of the outgoing protons were measured at Eα=21 MeV. Distorted-wave Born approximation calculations were carried out to constrain the spin-parity assignments of observed levels, with special attention to those significant in determination of the 34S(p,γ)35Cl reaction rate over the nova temperature regime. Results: The existence of these newly discovered states are largely confirmed, although a few states were not observed in this study. The spins and parities of a few 35Cl states were assigned tentatively for the first time. Conclusions: The present 34S(p,γ)35Cl experimental thermonuclear reaction rate at 0.1-0.4 GK is consistent within 1σ with the previous evaluation. However, our rate uncertainty is larger than before due to a more realistic treatment of the uncertainties in the rate input. In comparison with the previous rate evaluation, where the high and low rates differed by less than a factor of 2 over the nova temperature regime, the ratio of the present limit rates is at most a factor of 3.5 at 0.12 GK. At temperatures above 0.2 GK, we recommend the future work to focus on determination of the unknown properties of four excited states of 35Cl: 6643, 6761, 6780, and 6800 keV. [ABSTRACT FROM AUTHOR]

Background: Discrepancies exist between the observed abundances of argon and calcium in oxygen-neon nova ejecta and those predicted by nova models. An improved characterization of the 38K(p,γ)39Ca reaction rate over the nova temperature regime (∼0.1-0.4 GK), and thus the nuclear structure of 39Ca above the proton threshold [5770.92(63) keV], is necessary to resolve these contradictions. Purpose: The present study was performed to search for low-spin proton resonances in the 38K+p system, and to improve the uncertainties in energies of the known astrophysically significant proton resonances in 39Ca. Methods: The level structure of 39Ca was investigated via high-resolution charged-particle spectroscopy with an Enge split-pole spectrograph using the 40Ca(³He,α)39Ca reaction. Differential cross sections were measured over six laboratory angles at 21 MeV. Distorted-wave Born approximation calculations were performed to constrain the spin-parity assignments of observed levels with special attention to those significant in determination of the 38K(p,γ)39Ca reaction rate over the nova temperature regime. Results: The resonance energies corresponding to two out of three astrophysically important states at 6154(5) and 6472.2(24) keV are measured with better precision than previous charged-particle spectroscopy measurements. A tentatively new state is discovered at 5908(3) keV. The spin-parity assignments of a few of the astrophysically important resonances are determined. Conclusions: The present 38K(p,γ)39Ca upper limit thermonuclear reaction rate at 0.1-0.4 GK is higher than that determined by Christian et al. [Phys. Rev. C 97, 025802 (2018)] by at most a factor of 1.4 at 0.1 GK. [ABSTRACT FROM AUTHOR]

Proton-unbound excited states of 31S have been populated with the 32S(d, t)31 S reaction at a beam energy of 24 MeV. Tritons corresponding to 31S states with EX(31,S) ≈ 6.3-7.1 MeV were momentum analyzed with a high resolution quadrupole-dipole-dipole-dipole (Q3D) magnetic spectrograph at six angles ranging from (θlab = 15°to 58.5°. We report a statistically significant detection of an astrophysically important state at EX(31 S) = 6402 (2) keV, whose existence as a third state in this region has been under debate. Using updated A = 31 nuclear structure information, we present a new set of proposed 31S-31P mirror assignments for 31S, in which this state is tentatively assigned a spin of 7/2. This level, corresponding to a 30P + p resonance at 271 keV, is likely to have a significant influence on the 30P(p, γ)31 reaction rate in explosive hydrogen burning in classical novae. [ABSTRACT FROM AUTHOR]

The 17O(p,γ)18F reaction plays an important role in hydrogen-burning nucleosynthesis. Conflicting values for the low-energy behavior of its cross section exist in the literature. We present direct measurements of the astrophysical S factor of the 17O(p,γ)18F reaction at center-of-mass energies between 250 and 500 keV. These measurements were conducted in inverse kinematics at the DRAGON recoil separator. [ABSTRACT FROM AUTHOR]

Model predictions of the amount of the radioisotope 26A1 produced in hydrogen-burning environments require reliable estimates of the thermonuclear rates for the 26gAl(p,γ) 27Si and 26mAl(p,γ)27Si reactions. These rates depend upon the spectroscopic properties of states in 27Si within about 1 MeV of the 26sAl + p threshold (Sp = 7463 keV). We have studied the 28Si(3He,α)27Si reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. For the first time with a transfer reaction, we have constrained Jπ values for states in 27Si over Ex = 7.0-8.1 MeV through angular distribution measurements. Aside from a few important cases, we generally confirm the energies and spin-parity assignments reported in a recent γ-ray spectroscopy study. The magnitudes of neutron spectroscopic factors determined from shell-model calculations are in reasonable agreement with our experimental values extracted using this reaction. [ABSTRACT FROM AUTHOR]

Background: Type I x-ray bursts are the most frequently observed thermonuclear explosions in the galaxy, resulting from thermonuclear runaway on the surface of an accreting neutron star. The 30S(α,p) reaction plays a critical role in burst models, yet insufficient experimental information is available to calculate a reliable, precise rate for this reaction. Purpose: Our measurement was conducted to search for states in 34Ar and determine their quantum properties. In particular, natural-parity states with large α-decay partial widths should dominate the stellar reaction rate. Method: We performed the first measurement of 30S+α resonant elastic scattering up to a center-of-mass energy of 5.5 MeV using a radioactive ion beam. The experiment utilized a thick gaseous active target system and silicon detector array in inverse kinematics. Results: We obtained an excitation function for 30S(α,α) near 150° in the center-of-mass frame. The experimental data were analyzed with R-matrix calculations, and we observed three new resonant patterns between 11.1 and 12.1 MeV, extracting their properties of resonance energy, widths, spin, and parity. Conclusions: We calculated the resonant thermonuclear reaction rate of 30S(α,p) based on all available experimental data of 34Ar and found an upper limit about one order of magnitude larger than a rate determined using a statistical model. The astrophysical impact of these two rates has been investigated through one-zone postprocessing type I x-ray burst calculations. We find that our new upper limit for the 30S(α,p)33Cl rate significantly affects the predicted nuclear energy generation rate during the burst. [ABSTRACT FROM AUTHOR]

The total cross section of the 12C+16O fusion reaction has been measured at low energies to investigate the role of this reaction during late stellar evolution burning phases. A high-intensity oxygen beam, produced by the 5 MV pelletron accelerator at the University of Notre Dame, impinged on a thick, ultrapure graphite target. Protons and γ rays were simultaneously measured in the center-of-mass energy range from 3.64 to 5.01 MeV for singles and from 3.73 to 4.84 MeV for coincidence events, using silicon and Ge detectors. Statistical model calculations were employed to interpret the experimental results. The emergence of a new resonance-like broad structure and a decreasing trend in the S-factor data towards lower energies (opposite to previous data) are found for the 12C+16O fusion reaction. Based on these results the uncertainty range of the reaction rate within the temperature range of late stellar burning environments is discussed. [ABSTRACT FROM AUTHOR]