M. S. Smith

The magic nature of 132 Sn explored through the single-particle states of 133 Sn

Atomic nuclei have a shell structure in which nuclei with ‘magic numbers’ of neutrons and protons are analogous to the noble gases in atomic physics. Only ten nuclei with the standard magic numbers of both neutrons and protons have so far been observed. The nuclear shell model is founded on the precept that neutrons and protons can move as independent particles in orbitals with discrete quantum numbers, subject to a mean field generated by all the other nucleons. Knowledge of the properties of single-particle states outside nuclear shell closures in exotic nuclei is important for a fundamental understanding of nuclear structure and nucleosynthesis (for example the r-process, which is responsible for the production of about half of the heavy elements). However, as a result of their short lifetimes, there is a paucity of knowledge about the nature of single-particle states outside exotic doubly magic nuclei. Here we measure the single-particle character of the levels in 133Sn that lie outside the double shell closure present at the short-lived nucleus 132Sn. We use an inverse kinematics technique that involves the transfer of a single nucleon to the nucleus. The purity of the measured single-particle states clearly illustrates the magic nature of 132Sn.

Halo Nucleus Be11: A Spectroscopic Study via Neutron Transfer

The best examples of halo nuclei, exotic systems with a diffuse nuclear cloud surrounding a tightly bound core, are found in the light, neutron-rich region, where the halo neutrons experience only weak binding and a weak, or no, potential barrier. Modern direct-reaction measurement techniques provide powerful probes of the structure of exotic nuclei. Despite more than four decades of these studies on the benchmark one-neutron halo nucleus 11Be, the spectroscopic factors for the two bound states remain poorly constrained. In the present work, the 10Be d;p reaction has been used in inverse kinematics at four beam energies to study the structure of 11Be. The spectroscopic factors extracted using the adiabatic model were found to be consistent across the four measurements and were largely insensitive to the optical potential used. The extracted spectroscopic factor for a neutron in an n j 2s1=2 state coupled to the ground state of 10Be is 0.71(5). For the first excited state at 0.32 MeV, a spectroscopic factor of 0.62(4) is found for the halo neutron in a 1p1=2 state.

Neutron capture on 130Sn during r-process freeze-out

We examine the role of neutron capture on 130Sn during r-process freeze-out in the neutrino-driven wind environment of the core-collapse supernova. We find that the global r-process abundance pattern is sensitive to the magnitude of the neutron capture cross section of 130Sn. The changes to the abundance pattern include not only a relative decrease in the abundance of 130Sn and an increase in the abundance of 131Sn, but also a shift in the distribution of material in the rare earth and third peak regions.

New constraints on the 18F(p,α) 15O rate in novae from the (d, p) reaction

The degree to which the (p,gamma) and (p,alpha) reactions destroy 18F at temperatures 1-4x10^8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma-ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold (E_x=6.411 MeV in 19Ne). To gain further information about these resonances, we have used a radioactive 18F beam from the Holifield Radioactive Ion Beam Facility to selectively populate corresponding mirror states in 19F via the inverse d(18F,p)19F neutron transfer reaction. Neutron spectroscopic factors were measured for states in 19F in the excitation energy range 0-9 MeV. Widths for corresponding proton resonances in 19Ne were calculated using a Woods-Saxon potential. The results imply significantly lower 18F(p,gamma)19Ne and 18F(p,alpha)15O reaction rates than reported previously, thereby increasing the prospect of observing the 511-keV annihilation radiation associated with the decay of 18F in the ashes ejected from novae.

A New 17F(p, γ)18Ne Reaction Rate and Its Implications for Nova Nucleosynthesis

Proton capture by 17F plays an important role in the synthesis of nuclei in nova explosions. A revised rate for this reaction, based on a measurement of the 1H(17F, p)17F excitation function using a radioactive 17F beam at Oak Ridge National Laboratorys Holifield Radioactive Ion Beam Facility, is used to calculate the nucleosynthesis in nova outbursts on the surfaces of 1.25 and 1.35 M☉ ONeMg white dwarfs and a 1.00 M☉ CO white dwarf. We find that the new 17F (p, γ)18Ne reaction rate changes the abundances of some nuclides (e.g., 17O) synthesized in the hottest zones of an explosion on a 1.35 M☉ white dwarf by more than a factor of 104 compared to calculations using some previous estimates for this reaction rate, and by more than a factor of 3 when the entire exploding envelope is considered. In a 1.25 M☉ white dwarf nova explosion, this new rate changes the abundances of some nuclides synthesized in the hottest zones by more than a factor of 600, and by more than a factor of 2 when the entire exploding envelope is considered. Calculations for the 1.00 M☉ white dwarf nova show that this new rate changes the abundance of 18Ne by 21% but has negligible effect on all other nuclides. Comparison of model predictions with observations is also discussed.

Impact of nuclear reaction rate uncertainties on Nova models

Abstract We have, for the first time, determined the effect of nuclear reactions rate uncertainties on nova model predictions by simultaneously considering uncertainties in all relevant reaction rates. Our unique Monte Carlo approach enables a robust determination of uncertainties of nuclear origin in predictions of synthetized abundances, including radioisotopes which may be observable. This technique also enables us to identify which reactions most influence the production of each isotope, thereby guiding future measurements.

Direct reaction measurements with a 132Sn radioactive ion beam

The (d,p) neutron transfer and (d,d) elastic scattering reactions were measured in inverse kinematics using a radioactive ion beam of {sup 132}Sn at 630 MeV. The elastic scattering data were taken in a region where Rutherford scattering dominated the reaction, and nuclear effects account for less than 8% of the elastic scattering cross section. The magnitude of the nuclear effects, in the angular range studied, was found to be independent of the optical potential used, allowing the transfer data to be normalized in a reliable manner. The neutron-transfer reaction populated a previously unmeasured state at 1363 keV, which is most likely the single-particle 3p{sub 1/2} state expected above the N=82 shell closure. The data were analyzed using finite-range adiabatic-wave calculations and the results compared with the previous analysis using the distorted-wave Born approximation. Angular distributions for the ground and first-excited states are consistent with the previous tentative spin and parity assignments. Spectroscopic factors extracted from the differential cross sections are similar to those found for the one-neutron states beyond the benchmark doubly magic nucleus {sup 208}Pb.

Search for astrophysically important Ne 19 levels with a thick-target F 18 ( p , p ) F 18 measurement

The rates of the {sup 18}F(p,{alpha}){sup 15}O and {sup 18}F(p,{gamma}){sup 19}Ne reactions in astrophysical environments depend on the properties of {sup 19}Ne levels above the {sup 18}F+p threshold. There are at least eight levels in the mirror nucleus {sup 19}F for which analogs have not been observed in {sup 19}Ne in the excitation energy range E{sub x}=6.4-7.6 MeV. These levels may significantly enhance the {sup 18}F+p reaction rates, and thus we have made a search for these levels by measuring the {sup 1}H({sup 18}F,p){sup 18}F excitation function over the energy range E{sub c.m.}=0.3-1.3 MeV. We have identified and measured the properties of a newly observed level at E{sub x}=7.420{+-}0.014 MeV, which is most likely the mirror to the J{sup {pi}}=(7/2){sup +} {sup 19}F level at 7.56 MeV. We have additionally found a significant discrepancy with a recent compilation for the properties of a {sup 19}Ne state at E{sub x}=7.5 MeV and set upper limits on the proton widths of missing levels.

Design of a New Recoil Separator for Measurements of Radiative Capture Reactions in Astrophysics

The rates of proton‐ and alpha‐capture reactions on unstable proton‐rich nuclei are needed to understand the energy generation and element synthesis occurring in novae, X‐ray bursts, and other explosions. Direct measurements of the cross sections of some of these reactions are now possible with radioactive beams and a recoil separator. A new device for such measurements, the Separator for CApture Reactions [SECAR], is being designed for use at the Facility for Rare Isotope Beams (FRIB). The specifications and preliminary conceptual design will be discussed along with plans for the first set of measurements.

Reactions of a 10 Be beam on proton and deuteron targets

The extraction of detailed nuclear structure information from transfer reactions requires reliable, well-normalized data, as well as optical potentials and a theoretical framework demonstrated to work well in the relevant mass and beam energy ranges. It is rare that the theoretical ingredients can be tested well for exotic nuclei owing to the paucity of data. The halo nucleus