J. F. Shriner

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.

Missing Level Corrections in Nuclear Resonances

Neutron and proton resonances provide detailed level density information. Due to experimental limitations some fraction of the resonances are not observed, and this missing fraction must be determined. The standard correction for missing levels uses the experimental widths and the Porter-Thomas distribution. Analysis of the spacing distribution should yield equivalent information. A general expression for an imperfect spacing distribution (with a fraction of levels randomly missing) was obtained with the maximum entropy principle. This formulation was tested extensively with numerical data and then applied to proton and neutron resonance data sets. Since in Random Matrix Theory the widths and spacings are not correlated, this method complements the conventional approach that considers only the widths. The two analysis methods are compared. PACS: 21.10.Ma, 24.60.Ky, 24.60.Dr, 25.40.Ny

Distribution of shell model reduced transition probabilities in 22Na

Abstract The distribution of reduced transition probabilities in 22Na has been studied using the shell model. B(M1) and B(E2) values have been calculated for transitions among the first 25 states with each spin from J = 0 to J = 8, positive parity, and isospins T = 0 or T = 1. No dependence on spin, isospin, electromagnetic character, or excitation energy was found. The results were consistent with a χ2(ν) distribution with ν = 1, suggesting that 22Na is chaotic in the energy range studied.

19Ne Levels Studied with the 18F(d,n)19Ne*(18F+p) Reaction

A good understanding of the level structure of 19Ne around the proton threshold is critical to estimating the destruction of long-lived 18F in novae. Here we report the properties of levels in 19Ne in the excitation energy range of 6.9 Ex 8.4 MeV studied via the proton-transfer 18F(d, n)Ne reaction at the Holifield Radioactive Ion Beam Facility. The populated 19Ne levels decay by breakup into p + 18F and + 15O particles. The results presented in this manuscript are those of levels that are simultaneously observed from the breakup into both channels. An s-wave state is observed at 1468 keV above the proton threshold, which is a potential candidate for a predicted broad J = 1/2+ state. The proton and partial widths are deduced to be p = 228 50 keV and = 130 30 keV for this state.

Small sample size effects in statistical analyses of eigenvalue distributions

The effects of small sample sizes on the statistical analysis of eigenvalue distributions were analyzed numerically. The behavior of the nearest-neighbor spacing distribution, the Δ3 statistic, and the linear correlation coefficient between adjacent spacings was studied, and the effects of missing or spurious levels and of unfolding an energy-dependent level density were explored. For small sample sizes the nearest-neighbor spacing distribution appears to be the most reliable of these three statistics.

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.

Detailed balance test of time reversal invariance with interfering resonances

Abstract Bunakov and Weidenmuller recently proposed that there may be large enhancement of time-reversal invariance violation in a test of detailed balance near two interfering resonances. In our (p, α) resonance data on 23 Na, 27 Al, 31 P, 35 Cl, and 39 K, there are 33 pairs of adjacent resonances with the same spin and parity. The difference in the differential cross sections for the (p, α) and (α, p) reactions was calculated for these resonance pairs using experimental values for the partial widths. The collision matrix elements were obtained for a hamiltonian H = H 0 + iH TRIV by following the approach of Moldauer. The differences show striking dependence on energy, angle, and the particular pair of resonances; the relative sensitivity varies by many orders of magnitude. These preliminary results suggest that this class of experiments may be much more sensitive than previous detailed balance tests.

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.

Parity violation in charged particle resonances

Abstract Recent parity nonconservation (PNC) measurements in nuclei use the compound nucleus as a laboratory for the study of symmetry breaking, with the symmetry breaking matrix elements treated as random variables. This approach is made appealing by the observation of very large parity violation in neutron resonances. One key issue is the mass dependence of the effective nucleon-nucleus weak interaction. The neutron measurements do not appear feasible for light nuclei. We have calculated PNC observables for charged particle resonance reactions for over 100 states in five s-d shell nuclides using experimental values of the resonance parameters. Analyzing powers show strong dependence on energy and angle, and vary greatly from resonance to resonance. Proposed PNC experiments are discussed.