Jeffery Curtis Blackmon

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.

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.

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.

New Astrophysical Reaction Rates for 18F(p, α)15O and 18F(p, γ)19Ne

The rates of the thermonuclear 18F(p, α)15O and 18F(p,γ)19Ne reactions in hot astrophysical environments are needed to understand gamma-ray emission from nova explosions. The rates for these reactions have been uncertain due to discrepancies in recent measurements, as well as to a lack of a comprehensive examination of the available structure information in the compound nucleus 19Ne. We have examined the latest experimental measurements with radioactive and stable beams, and made estimates of the unmeasured 19Ne nuclear level parameters, to generate new rates with uncertainties for these reactions. The rates are expressed as numerical values over the temperature range relevant for stellar explosions, as well as analytical expressions as functions of temperature in a format suitable for use in astrophysical simulations. Comparisons with the previous rate calculations are carried out, and the astrophysical implications are briefly discussed.

Digital Electronics For The Versatile Array Of Neutron Detectors At Low Energies

A χ2 minimization algorithm has been developed to extract sub‐sampling‐time information from digitized waveforms, to be used to instrument the future Versatile Array of Neutron Detectors at Low energies. The algorithm performance has been characterized with a fast Arbitrary Function Generator, obtaining time resolution better than 1 ns for signals of amplitudes between 50 mV and 1V, with negligible walk in the whole range. The proof‐of‐principle measurement of the beta‐delayed neutron emission from 89Br indicates a resolution of 1 ns can be achieved in realistic experimental conditions.

Studies of Nuclei Close to 132Sn Using Single-Neutron Transfer Reactions

Neutron transfer reactions were performed in inverse kinematics using radioactive ion beams of 132Sn, 130Sn, and 134Te and deuterated polyethylene targets. Preliminary results are presented. The Q‐value spectra for 133Sn, 131Sn and 135Te reveal a number of previously unobserved peaks. The angular distributions are compatible with the expected lf7/2 nature of the ground state of 133Sn, and 2p3/2 for the 3.4 MeV state in 131Sn.

Single‐Particle Structure of Neutron‐Rich Nuclei

The d(82Ge,p) reaction has been measured in inverse kinematics at the Holifield Radioactive Ion Beam facility, enabling a study of the evolution of single‐particle structure above the N=50 shell gap for neutron‐rich nuclei.

Investigation of the 23Na(p, γ)24Mg and 23Na(p, α)20ne reactions via (3He, d) spectroscopy

States near the {sup 23}Na+p threshold in {sup 24}Mg were investigated using the {sup 23}Na({sup 3}He,d){sup 24}Mg reaction over the angular range of 5 deg. {<=}{theta}{sub lab}{<=}35 deg. at E({sup 3}He)=20 MeV. Spectroscopic factors were extracted for states corresponding to resonances in the {sup 23}Na(p,{gamma}){sup 24}Mg and {sup 23}Na(p,{alpha}){sup 20}Ne reactions. We find that one state, corresponding to a previously unobserved resonance at E{sub c.m.}=138 keV, may make a significant contribution to the rates of both reactions at low temperatures. Another state, corresponding to a possible resonance at E{sub c.m.}=37 keV may make a small contribution to the {sup 23}Na(p,{alpha}){sup 20}Ne reaction. New rates for the {sup 23}Na(p,{gamma}){sup 24}Mg and {sup 23}Na(p,{alpha}){sup 20}Ne reactions are presented and the astrophysical implications are discussed.

Neutron Transfer Reactions: Surrogates for Neutron Capture for Basic and Applied Nuclear Science

Neutron capture reactions on unstable nuclei are important for both basic and applied nuclear science. A program has been developed at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory to study single-neutron transfer (d,p) reactions with rare isotope beams to provide information on neutron-induced reactions on unstable nuclei. Results from (d,p) studies on {sup 130,132}Sn, {sup 134}Te and {sup 75}As are discussed.