[en] To make measurements with the intense (but often contaminated) radioactive beams available today, one often needs to identify the reaction products to determine the events of interest. The low energies required for many astrophysics measurements make impossible the use of traditional energy loss techniques, and additional constraints are required. We demonstrate a simple technique to measure the risetimes of silicon strip-detector signals and show partial discrimination can be obtained even at energies below 1 MeV/u

[en] The 19F nucleus has been studied extensively. However, there have been no comprehensive experimental studies of 18F+n single-particle components in 19F, and no measure of neutron vacancies in the 18F ground state, as such experiments require a (radioactive) 18F target or beam. We have used the 2H(18F,p)19F reaction to selectively populate states in 19F that are of 18F+n character. The 108.5-MeV radioactive 18F+9 beam was provided by the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. Proton-recoil coincidence data were taken for both -decaying and particle-stable final states. Angular distributions and spectroscopic factors were measured for nine proton groups, corresponding to 13 states in 19F. The results are compared to shell model calculations

[en] While recent measurements have substantially improved our understanding of the ¹⁸F(p, α)¹⁵O reaction that is important in novae, the production of ¹⁸F is still uncertain by more than 2 orders of magnitude, due in large part to the contribution of a resonance located at E_cm = 330 keV. We developed a new technique to study resonant (p, α) reactions and employed it to measure properties of the E_cm = 183 keV resonance in ¹⁷O(p, α)¹⁴N which had been previously reported to decrease ¹⁸F production in ONeMg novae by as much as a factor of 10. The previous results were confirmed using the new technique and we now propose to use this technique to study the ¹⁸F(p, α)¹⁵O reaction

[en] Studying (α,p) reactions of astrophysical interest with radioactive beams presents serious challenges because of the difficult nature of the target. We have developed a new approach for the measurement of (α,p) reactions using heavy ion beams in inverse kinematics and have measured the ⁴He(¹⁹F,¹H)²²Ne reaction as a demonstration. ¹⁹F beams were used at the Holifield Radioactive Ion Beam Facility (HRIBF) tandem accelerator in Oak Ridge National Laboratory (ORNL) to bombard a large scattering chamber filled with ⁴He gas. Recoiling protons from the reactions were detected by the large area annular silicon strip detector array (SIDAR). The ¹⁹F(α,p) and ¹⁹F(α,p') excitation functions were measured over the energy range of E_c.m. ∼ 1-2.1 MeV, demonstrating the viability of the technique

[en] The ¹⁸Ne(α,p)²¹Na reaction plays a crucial role in the (α,p) process, which leads to the rapid proton capture process in x-ray bursts. The reaction rate depends upon properties of ²²Mg levels above the α threshold at 8.14 MeV. Despite recent studies of these levels, only the excitation energies are known for most with no constraints on the spins. We have studied the ²⁴Mg(p,t)²²Mg reaction at the Oak Ridge National Laboratory (ORNL) Holifield Radioactive Ion Beam Facility (HRIBF), and by measuring the angular distributions of outgoing tritons, we provide some of the first experimental constraints on the spins of astrophysically important ¹⁸Ne(α,p)²¹Na resonances

[en] The ¹⁸Ne(α,p)²¹Na reaction plays a crucial role in the (α,p) process, which leads to the rapid proton capture process in X-ray bursts. The reaction rate depends upon properties of ²²Mg levels above the α threshold at 8.14 MeV. Despite recent studies of these levels, only the excitation energies are known for most with no constraints on the spins. We have studied the ²⁴Mg(p,t)²²Mg reaction at the Oak Ridge National Laboratory (ORNL) Holifield Radioactive Ion Beam Facility (HRIBF), and by measuring the angular distributions of outgoing tritons, we provide the first experimental constraints on the spins of astrophysically-important ¹⁸Ne(α,p)²¹Na resonances

[en] We directly measure the ¹⁷F(p,γ)¹⁸Ne resonant reaction using a mixed beam of ¹⁷F and ¹⁷O at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory (ORNL). The astrophysically important 3⁺ resonance at ∼600 keV above the proton threshold in ¹⁸Ne is found to have a partial width Γ_γ = 56 ± 24(stat) ± 30(sys) meV, in reasonable agreement with the theoretically predicted width. A 2σ upper limit on the direct capture of S(E) = 65 keV b is determined at an energy of 800 keV. Experimental techniques and astrophysical implications are discussed.

[en] The ⁷⁵As(d,pγ) reaction in inverse kinematics as a surrogate for neutron capture was performed at Oak Ridge National Laboratory using a deuterated plastic target. The intensity of the 165 keV γ-ray from ⁷⁶As in coincidence with ejected protons, from exciting ⁷⁶As above the neutron separation energy populating a compound state, was measured. A tight geometry of four segmented germanium clover γ-ray detectors together with eight ORRUBA-type silicon-strip charged-particle detectors were used to optimize geometric acceptance. The preliminary analysis of the ⁷⁵As experiment and the future plans of the (d,pγ) surrogate campaign in inverse kinematics are discussed.

[en] The rate of the 25Al(p,g)26Si reaction in novae is important to the production of galactic 26Al, an important astronomical observable for its 1.809 MeV gamma ray. 26Al has been catalogued near massive stars and found in presolar grains. The 25Al proton capture reaction serves to bypass 26Alg:s: production since the subsequent 26Si beta decay preferentially populates instead the isomeric state of 26Al. Several recent studies have aimed to locate and characterize the first l = 0 resonances in 26Si above the 25Al+p threshold, which would dominate the reaction rate at nova temperatures. We have studied several resonances in 25Al(p,g)26Si via the 28Si(p,t)26Si* reaction at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. Several higherenergy states in 26Si are given spin and parity assignments for the first time, and possible new states are observed. In addition to measuring the angular distributions of the tritons, a partial implementation of the Oak Ridge Rutgers University Barrel Array (ORRUBA) was used to measure the coincident protons emitted from the decay of states in 26Si above the proton threshold. We observe that the first l = 0 state above the proton threshold decays essentially 100% of the time via proton emission, resulting in a proton branching ratio consistent with one.

[en] Calculations of r-process nucleosynthesis rely significantly on nuclear structure models as input, which are not well tested in the neutron-rich regime, due to the paucity of experimental data on the majority of these nuclei. High quality radioactive beams have recently made possible the measurement of (d,p) reactions on unstable nuclei in inverse kinematics, which can yield information on the development of single-neutron structure away from stability in close proximity to suggested r-process paths. The Oak Ridge Rutgers University Barrel Array (ORRUBA) has been developed for the measurement of such reactions. An early partial implementation of ORRUBA has been utilized to measure the ¹³²Sn(d,p)¹³³Sn and ¹³⁴Te(d,p)¹³⁵Te reactions for the first time