K. A. Chipps

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.

Classical-NOVA CONTRIBUTION to the Milky Way's ²⁶Al abundance: exit channel of the key ²⁵Al(p,γ) ²⁶Si resonance.

Classical novae are expected to contribute to the 1809-keV Galactic γ-ray emission by producing its precursor 26Al, but the yield depends on the thermonuclear rate of the unmeasured 25Al(p,γ)26Si reaction. Using the β decay of 26P to populate the key J(π)=3(+) resonance in this reaction, we report the first evidence for the observation of its exit channel via a 1741.6±0.6(stat)±0.3(syst)  keV primary γ ray, where the uncertainties are statistical and systematic, respectively. By combining the measured γ-ray energy and intensity with other experimental data on 26Si, we find the center-of-mass energy and strength of the resonance to be E(r)=414.9±0.6(stat)±0.3(syst)±0.6(lit.)  keV and ωγ=23±6(stat)(-10)(+11)(lit.)  meV, respectively, where the last uncertainties are from adopted literature data. We use hydrodynamic nova simulations to model 26Al production showing that these measurements effectively eliminate the dominant experimental nuclear-physics uncertainty and we estimate that novae may contribute up to 30% of the Galactic 26Al.

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.

Development of ORRUBA: A Silicon Array for the Measurement of Transfer Reactions in Inverse Kinematics

The development of high quality radioactive beams has made possible the measurement of transfer reactions in inverse kinematics on unstable nuclei. Measurement of (d,p) reactions on neutron-rich nuclei yield data on the evolution of nuclear structure away from stability, and are of astrophysical interest. Experimentally, (d,p) reactions on heavy (Z=50) fission fragments are complicated by the strongly inverse kinematics, and relatively low beam intensities. Consequently, ejectile detection with high resolution in position and energy, a high dynamic range and a high solid angular coverage is required. The Oak Ridge Rutgers University Barrel Array (ORRUBA) is a new silicon detector array optimized for the measurement of (d,p) reactions in inverse kinematics.

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.

Isospin mixing reveals 30P(p, γ)31S resonance influencing nova nucleosynthesis

The thermonuclear ^{30}P(p,γ)^{31}S reaction rate is critical for modeling the final elemental and isotopic abundances of ONe nova nucleosynthesis, which affect the calibration of proposed nova thermometers and the identification of presolar nova grains, respectively. Unfortunately, the rate of this reaction is essentially unconstrained experimentally, because the strengths of key ^{31}S proton capture resonance states are not known, largely due to uncertainties in their spins and parities. Using the β decay of ^{31}Cl, we have observed the β-delayed γ decay of a ^{31}S state at E_{x}=6390.2(7)  keV, with a ^{30}P(p,γ)^{31}S resonance energy of E_{r}=259.3(8)  keV, in the middle of the ^{30}P(p,γ)^{31}S Gamow window for peak nova temperatures. This state exhibits isospin mixing with the nearby isobaric analog state at E_{x}=6279.0(6)  keV, giving it an unambiguous spin and parity of 3/2^{+} and making it an important l=0 resonance for proton capture on ^{30}P.

Design and Construction of a Gas Jet Target for RIB Experiements

PNNL is now part of the JENSA collaboration to produce a gas jet system for the Facility for Rare Isotope Beams (FRIB). This document is a status report for the gas jet working group to be delivered to the FRIB scientific advisory council (SAC). It briefly describes PNNL’s capability at constructing cost efficient and high detection efficiency HPGe arrays.

Spin assignments of 22Mg states through a 24Mg(p,t)22Mg measurement

The {sup 18}Ne({alpha},p){sup 21}Na reaction plays a crucial role in the ({alpha},p) process, which leads to the rapid proton capture process in X-ray bursts. The reaction rate depends upon properties of {sup 22}Mg levels above the {alpha} 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 {sup 24}Mg(p,t){sup 22}Mg 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 {sup 18}Ne({alpha},p){sup 21}Na resonances.

Neutron single particle structure in 131Sn and the r-process

Recent calculations suggest that, at late times in the r-process, the rate of neutron capture by {sup 130}Sn has a significant impact on nucleosynthesis. Direct capture into low-lying bound states is likely the dominant reaction in the r-process near the N=82 closed shell, so reaction rates are strongly impacted by the properties of neutron single particle states in this region. In order to investigate these properties, we have acquired (d,p) reaction data in the A{approx}132 region in inverse kinematics using {approx}630 MeV beams (4.85 MeV/u for {sup 130}Sn) and CD{sub 2} targets. An array of Si strip detectors, including SIDAR and an early implementation of the new Oak Ridge Rutgers University Barrel Array (ORRUBA), was used to detect reaction products. Preliminary results for the {sup 130}Sn(d,p){sup 131}Sn experiment are reported.