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 (133)Sn that lie outside the double shell closure present at the short-lived nucleus (132)Sn. 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 (132)Sn.

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The (d,p) neutron transfer and (d,d) elastic scattering reactions were measured in inverse kinematics using a radioactive ion beam of 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 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 208 Pb.

Recent calculations suggest that the rate of neutron capture by (130)Sn has a significant impact on late-time nucleosynthesis in the r process. Direct capture into low-lying bound states is expected to be significant in neutron capture near the N=82 closed shell, so r-process reaction rates may be strongly impacted by the properties of neutron single particle states in this region. In order to investigate these properties, the (d,p) reaction has been studied in inverse kinematics using a 630 MeV beam of (130)Sn (4.8 MeV/u) and a (CD(2))(n) target. An array of Si strip detectors, including the Silicon Detector Array and an early implementation of the Oak Ridge Rutgers University Barrel Array, was used to detect reaction products. Results for the (130)Sn(d, p)(131)Sn reaction are found to be very similar to those from the previously reported (132)Sn(d, p)(133)Sn reaction. Direct-semidirect (n,γ) cross section calculations, based for the first time on experimental data, are presented. The uncertainties in these cross sections are thus reduced by orders of magnitude from previous estimates.

Executive SummaryRadiation portal monitors used for interdiction of illicit materials at borders include highly sensitive neutron detection systems. The main reason for having neutron detection capability is to detect fission neutrons from plutonium. Most currently deployed radiation portal monitors (RPMs) use neutron detectors based upon 3 He-filled gas proportional counters, which are the most common large area neutron detector. This type of neutron detector is used in the RPMs installed in international locations made by TSA and others, and in the Ludlum and Science Applications International Corporation RPMs deployed primarily for domestic applications. There is a declining supply of 3 He in the world and, thus, methods to reduce the use of this gas in RPMs with minimal changes to the current system designs and sensitivity to cargo-borne neutrons are being investigated.Four technologies have been identified as being currently commercially available and potential alternative neutron detectors to replace the use of 3 He in RPMs. These technologies are: 1) Boron trifluoride-filled proportional counters, 2) Boron-lined proportional counters, 3) Lithium-loaded glass fibers, and 4) Coated wavelength-shifting plastic fibers.Reported here is a summary of the testing carried out at Pacific Northwest National Laboratory on these technologies to date, as well as measurements on 3 He tubes at various pressures. Details on these measurements are available in the referenced reports. Sponsors of these tests include the Department of Energy (DOE), Department of Homeland Security (DHS), the Department of Defense (DoD), and internal Pacific Northwest National Laboratory funds.The purpose of this testing was to measure the efficiency and gamma sensitivity of the various neutron detection systems and configurations to determine which of these technologies could meet the neutron detection requirements while not exceeding the current footprint of the 3 He-based neutron module in the RPMs. The measurements made as part of this testing included the response of each system to moderated neutrons and to a high gamma-ray exposure rate. As part of this testing, various configurations of 3 Hebased detectors were also measured. The results reported here are from a limited set of tests to measure the capability of each technology listed above to meet the basic requirements. Additional requirements, such as robustness to different environmental conditions, would need to be tested prior to implementation.The requirements used in this testing are from the specification for RPMs developed for the domestic deployments under the Radiation Portal Monitor Project (RPMP). These requirements allow for testing of individual modules with 252 Cf, a common industrial neutron source. Simulations were performed that indicate the TSA system has comparable efficiency per unit surface area, and therefore if the technology meets the RPMP specification, it will likely meet the requirements of the Second Line of Defense (SLD) program for the TSA RPM in the sam...