Neutron Single Particle Structure in $^{131}\mathrm{Sn}$ and Direct Neutron Capture Cross Sections

Neutron Single Particle Structure in ${}^{131}{Sn}$ and Direct Neutron Capture Cross Sections

R. L. Kozub, G. Arbanas, A. S. Adekola, D. W. Bardayan, J. C. Blackmon, K. Y. Chae, K. A. Chipps, J. A. Cizewski, L. Erikson, R. Hatarik, W. R. Hix, K. L. Jones, W. Krolas, J. F. Liang, Z. Ma, C. Matei, B. H. Moazen, C. D. Nesaraja, S. D. Pain, D. Shapira, J. F. Shriner, Jr., M. S. Smith, and T. P. Swan

Phys. Rev. Lett. 109, 172501 – Published 24 October 2012

Abstract

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.

Received 29 May 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.172501

Authors & Affiliations

R. L. Kozub¹, G. Arbanas², A. S. Adekola^3,4, D. W. Bardayan⁵, J. C. Blackmon^5,*, K. Y. Chae^6,7, K. A. Chipps⁸, J. A. Cizewski⁴, L. Erikson^8,†, R. Hatarik^4,‡, W. R. Hix^5,6, K. L. Jones⁶, W. Krolas⁹, J. F. Liang⁵, Z. Ma⁶, C. Matei^10,§, B. H. Moazen⁶, C. D. Nesaraja⁵, S. D. Pain^4,5, D. Shapira⁵, J. F. Shriner, Jr.¹, M. S. Smith⁵, and T. P. Swan^4,11

¹Department of Physics, Tennessee Technological University, Cookeville, Tennessee 38505, USA
²Reactor and Nuclear Systems Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6171, USA
³Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA
⁴Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854-8019, USA
⁵Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁶Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
⁷Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea
⁸Department of Physics, Colorado School of Mines, Golden, Colorado 80401, USA
⁹Institute of Nuclear Physics, PAN, 31-342 Krakow, Poland
¹⁰Oak Ridge Associated Universities, Building 6008, P.O. Box 2008, Oak Ridge, Tennessee 37831-6374, USA
¹¹Department of Physics, University of Surrey, Guilford, Surrey, GU2 7XH, United Kingdom

^*Present address: Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA.
^†Present address: Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA.
^‡Present address: Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
^§Present address: European Commission-Joint Research Centre, Institute for Reference Materials and Measurements, Retieseweg 111 2440 GEEL, Belgium.

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Vol. 109, Iss. 17 — 26 October 2012

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Figure 1
$Q$ -value spectrum of protons from a forward-angle strip detector of the ORRUBA, covering laboratory angles between about 69° and 90°. Approximate ${}^{131}{Sn}$ excitation energies, assumed spins and parities, and nominal position of the unobserved ground state are also shown. The strong peak region is fitted with four Gaussians of equal width, for which the $χ^{2}$ per degree of freedom is 1.46. The background in the low $Q$ -value region (low proton energies) is owing mostly to setting analysis thresholds close to detector noise levels.Reuse & Permissions
Figure 2
The dashed arrows connect energy levels observed in the ( $d$ , $p$ ) reaction on ${}^{130}{Sn}$ (left) and ${}^{132}{Sn}$ (right) [8]. The energy corresponding to the ${}^{133}{Sn}$ ground state has been shifted to align with the lowest state observed in ${}^{131}{Sn}$ . (For a $Q$ -value comparison, the energy levels of ${}^{133}{Sn}$ should be shifted upward by 197 keV, so neutron thresholds are aligned.) The states flagged with an asterisk (*) were the strongest in the respective experiments. Lower known states in ${}^{131}{Sn}$ [6] are also shown, but they were not observed in the present work.Reuse & Permissions
Figure 3
Angular distributions of protons from ORRUBA detectors, extracted by using bins of 5° angular width in the laboratory. The 3986- and 4655-keV distributions have fewer points, owing to low proton energies being below detector thresholds at some angles. Dashed curves are distorted wave Born approximation calculations using the optical parameters of Strömich et al. [15], and the solid curves are the same calculations except for using the BG parameters of Sen, Riley, and Udagawa [27] in the proton channel [obtained by fitting ${}^{136}{Xe} (p, p) {}^{136}{Xe}$ elastic scattering data]. The “standard” bound state parameters of $r_{0} = 1.25 fm$ and $a = 0.65 fm$ were used for all cases. For each level, the dashed and solid curves are shown for the same spectroscopic factor. Dotted curves are $2 f_{7 / 2}$ and $2 f_{5 / 2}$ calculations for the 3404- and 3986-keV levels, respectively (both with $S = 1.00$ ), to illustrate the strong preference at small angles for $ℓ = 1$ assignments for these states. The exact finite range code DWUCK5 [28] was used for the calculations. See the text for discussion of tentative $J^{π}$ assignments.Reuse & Permissions
Figure 4
Direct-semidirect capture cross sections for the ${}^{130}{Sn} (n, γ) {}^{131}{Sn}$ reaction computed by using the code CUPIDO [21] for real parts of a phenomenological Koning-Delaroche [22] optical model potential are plotted for levels in Table . An upper limit for the combined DSD capture into the $1 h_{11 / 2}$ , $2 d_{3 / 2}$ , $3 s_{1 / 2}$ , $2 d_{5 / 2}$ , and $1 g_{7 / 2}$ orbitals (labeled other) is not included in the total DSD. Shown for comparison (single points) are the calculations of Rauscher et al. [4] for 30 keV neutrons using the finite range droplet (FRDM), the Hartree-Fock-Bogoliubov (HFB), and relativistic mean field theory (RMFT) models. Of these, only the FRDM predicted both the $3 p_{3 / 2}$ and $3 p_{1 / 2}$ single-neutron states to be bound.Reuse & Permissions

Physical Review Letters

Neutron Single Particle Structure in Sn131 and Direct Neutron Capture Cross Sections

Phys. Rev. Lett. 109, 172501 – Published 24 October 2012

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Neutron Single Particle Structure in ${}^{131}{Sn}$ and Direct Neutron Capture Cross Sections