Showing papers by "Goran Arbanas published in 2012"

Neutron single particle structure in 131Sn and direct neutron capture cross sections.

Abstract: Recent calculations suggest that the rate of neutron capture by 130Sn 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 130Sn (4.8 MeV/u) and a (CD2)n target. An array of Si strip detectors, including SIDAR and an early implementation of the ORRUBA, was used to detect reaction products. Results for the 130Sn(d, p)131Sn reaction are found to be very similar to those from the previously reported 132Sn(d, p)133Sn reaction. Direct-semidirect (n, ) 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.

Structures of exotic 131 , 133 Sn isotopes and effect on r -process nucleosynthesis

Abstract: Background: Four strong single-particle bound levels with strikingly similar level spacings have recently been measured in Sn-131 and Sn-133. This similarity has not yet been addressed by a theoretical nuclear structure model. Information on these single-particle bound levels, as well as on resonant levels above the neutron capture threshold, is also needed to determine neutron capture cross sections-and corresponding capture reaction rates-on Sn-130,Sn-132. The Sn-130(n,gamma) rate was shown in a recent sensitivity study to significantly impact the synthesis of heavy elements in the r-process in supernovae. Purpose: Understand the structure of bound and resonant levels in Sn-131,Sn-133, and determine if the densities of unbound resonant levels are sufficiently high to warrant statistical model treatments of neutron capture on Sn-130,Sn-132. Method: Single-particle bound and resonant levels for Sn-131,Sn-133 are self-consistently calculated by the analytical continuation of the coupling constant (ACCC) method based on a relativistic mean field (RMF) theory with BCS approximation. Results: We obtain four strong single-particle bound levels in both Sn-131,Sn-133 with an ordering that agrees with experiments and spacings that, while differing from experiment, are consistent between the Sn isotopes. We also find at most one single-particle level in the effective energy range for neutron captures in the r-process. Conclusions: Our RMF + ACCC + BCS model successfully reproduces observed single-particle bound levels in (131,13)3Sn and self-consistently predicts single-particle resonant levels with densities too low for widely used traditional statistical model treatments of neutron capture cross sections on Sn-130,Sn-132 employing Fermi gas level density formulations.

Extending the Kawai-Kerman-McVoy Statistical Theory of Nuclear Reactions to Intermediate Structure via Doorways

Abstract: Kawai, Kerman, and McVoy have shown that a statistical treatment of many open channels that are coupled by direct reactions leads to modifications of the Hauser- Feshbach expression for energy-averaged cross section (Ann. of Phys. 75, 156 (1973)). The energy averaging interval for this cross section is on the order of the width of sin- gle particle resonances, 1 MeV, revealing only a gross structure in the cross section. When the energy-averaging interval is decreased down to a width of a doorway state, 0:1 MeV, a so-called intermediate structure may be observed in cross sections. We extend the Kawai-Kerman-McVoy theory into the intermediate structure by leveraging a theory of doorway states developed by Feshbach, Kerman, and Lemmer (Ann. of Phys. 41, 230 (1967)). As a by-product of the extension, an alternative derivation of the central result of the Kawai-Kerman-McVoy theory is suggested. We quantify the e ect of the ap- proximations used in derivation by performing numerical computations for a large set of compound nuclear states.

Convergence of Legendre Expansion of Doppler-Broadened Double Differential Elastic Scattering Cross Section

Abstract: Convergence properties of Legendre expansion of a Doppler-broadened double-differential elastic neutron scattering cross section of {sup 238}U near the 6.67 eV resonance at temperature 10{sup 3} K are studied. A variance of Legendre expansion from a reference Monte Carlo computation is used as a measure of convergence and is computed for as many as 15 terms in the Legendre expansion. When the outgoing energy equals the incoming energy, it is found that the Legendre expansion converges very slowly. Therefore, a supplementary method of computing many higher-order terms is suggested and employed for this special case.

Covariance Matrix of a Double-Differential Doppler-Broadened Elastic Scattering Cross Section

Abstract: Legendre moments of a double-differential Doppler-broadened elastic neutron scattering cross section on {sup 238}U are computed near the 6.67 eV resonance at temperature T = 10{sup 3} K up to angular order 14. A covariance matrix of these Legendre moments is computed as a functional of the covariance matrix of the elastic scattering cross section. A variance of double-differential Doppler-broadened elastic scattering cross section is computed from the covariance of Legendre moments.