arXiv subjects

B. M. Sherrill

Publications and source records attributed to B. M. Sherrill.

25 records · Page 2Linked to original sources

arXiv2006

Measurement of the Gamow-Teller Strength Distribution in 58Co via the 58Ni(t,3He) reaction at 115 MeV/nucleon

Electron capture and beta decay play important roles in the evolution of pre-supernovae stars and their eventual core collapse. These rates are normally predicted through shell-model calculations. Experimentally determined strength distributions from charge-exchange reactions are needed to test modern shell-model calculations. We report on the measurement of the Gamow-Teller strength distribution in 58Co from the 58Ni(t,3He) reaction with a secondary triton beam of an intensity of ~10^6 pps at 115 MeV/nucleon and a resolution of \~250 keV. Previous measurements with the 58Ni(n,p) and the 58Ni(d,2He) reactions were inconsistent with each other. Our results support the latter. We also compare the results to predictions of large-scale shell model calculations using the KB3G and GXPF1 interactions and investigate the impact of differences between the various experiments and theories in terms of the weak rates in the stellar environment. Finally, the systematic uncertainties in the normalization of the strength distribution extracted from 58Ni(3He,t) are described and turn out to be non-negligible due to large interferences between the dL=0, dS=1 Gamow-Teller amplitude and the dL=2, dS=1 amplitude.

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arXiv2005

New $^{32}$Cl(p,$γ$)$^{33}$Ar reaction rate for astrophysical rp-process calculations

The $^{32}$Cl(p,$γ$)$^{33}$Ar reaction rate is of potential importance in the rp-process powering type I X-ray bursts. Recently Clement et al. \cite{CBB04} presented new data on excitation energies for low lying proton unbound states in $^{33}$Ar obtained with a new method developed at the National Superconducting Cyclotron Laboratory. We use their data, together with a direct capture model and a USD shell model calculation to derive a new reaction rate for use in astrophysical model calculations. In particular, we take into account capture on the first excited state in $^{32}$Cl, and also present a realistic estimate of the remaining uncertainties. We find that the $^{32}$Cl(p,$γ$)$^{33}$Ar reaction rate is dominated entirely by capture on the first excited state in $^{32}$Cl over the whole temperature range relevant in X-ray bursts. In the temperature range from 0.2 to 1 GK the rate is up to a factor of 70 larger than the previously recommended rate based on shell model calculations only. The uncertainty is now reduced from up to a factor of 1000 to a factor of 3 at 0.3-0.7 GK and a factor of 6 at 1.5 GK.

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arXiv2003

Proton-tetraneutron elastic scattering

We analyze the elastic scattering of protons on a 4n system. This was used as part of the detection technique of a recent experiment [1] to search for the 4n (tetraneutron) as a bound particle. We show that it is unlikely that this process alone could yield the events reported in ref. [1], unless the 4n has an anomalously large backward elastic scattering amplitude.

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arXiv2001

Electromagnetic Dissociation of 8B and the Rate of the 7Be(p,gamma)8B Reaction in the Sun

In an effort to better determine the 7Be(p,gamma)8B reaction rate, we have performed inclusive and exclusive measurements of the Coulomb dissociation of 8B. The former was a study of longitudinal momentum distributions of 7Be fragments emitted in the Coulomb breakup of intermediate energy 8B beams on Pb and Ag targets. Analysis of these data yielded the E2 contribution to the breakup cross section. In the exclusive measurement, we determined the cross section for the Coulomb breakup of 8B on Pb at low relative energies in order to infer the astrophysical S factor for the 7Be(p,gamma)8B reaction. Interpreting the measurements with 1st-order perturbation theory, we obtained SE2/SE1 = 4.7 (+ 2.0,- 1.3) times 10^-4 at Erel = 0.6 MeV, and S17(0) = 17.8 (+ 1.4,- 1.2) eV b. Semiclassical 1st-order perturbation theory and fully quantum mechanical continuum-discretized coupled channels analyses yield nearly identical results for the E1 strength relevant to solar neutrino flux calculations, suggesting that theoretical reaction mechanism uncertainties need not limit the precision of Coulomb breakup determinations of the 7Be(p,gamma)8B S factor. A recommended value of S17(0) based on a weighted average of this and other measurements is presented.

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arXiv2001

S17(0) Determined from the Coulomb Breakup of 83 MeV/nucleon 8B

A kinematically complete measurement was made of the Coulomb dissociation of 8B nuclei on a Pb target at 83 MeV/nucleon. The cross section was measured at low relative energies in order to infer the astrophysical S factor for the 7Be(p,gamma)8B reaction. A first-order perturbation theory analysis of the reaction dynamics including E1, E2, and M1 transitions was employed to extract the E1 strength relevant to neutrino-producing reactions in the solar interior. By fitting the measured cross section from Erel = 130 keV to 400 keV, we find S17(0) = 17.8 (+1.4, -1.2) eV b.

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arXiv2000

Spectroscopy of 13,14B via the one-neutron knockout reaction

The single-nucleon knockout reactions 9Be(14B, 13B + gamma)X and 197Au(14B, 13B + gamma)X, at an incident energy of 60 MeV per nucleon, have been used to probe the structure of 14B and of the core fragment 13B. A dominant 2s configuration is deduced for the neutron in the ground state of 14B. The longitudinal momentum distribution for this state is consistent with "neutron halo" structure. Spin assignments for 13B excited states at 3.48 and 3.68 MeV are proposed based on the observed spectroscopic factors for one-neutron removal.

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arXiv1998

Measurement of E2 Transitions in the Coulomb Dissociation of 8B

In an effort to understand the implications of Coulomb dissociation experiments for the determination of the 7Be(p,gamma)8B reaction rate, longitudinal momentum distributions of 7Be fragments produced in the Coulomb dissociation of 44 and 81 MeV/nucleon 8B beams on a Pb target were measured. These distributions are characterized by asymmetries interpreted as the result of interference between E1 and E2 transition amplitudes in the Coulomb breakup. At the lower beam energy, both the asymmetries and the measured cross sections are well reproduced by perturbation theory calculations, allowing a determination of the E2 strength.

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