K. W. Kemper

Florida State University, Tallahassee, Florida, United States

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Publications (559)1316.76 Total impact

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    Full-text · Article · Jan 2016 · Nuclear Physics A
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    N. Keeley · K. W. Kemper · K. Rusek
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    ABSTRACT: The population of the 9.50 MeV 9/21+ resonance in C13 by single-neutron transfer reactions is expected to be dominated by the two-step route through the C12 21+ (4.44 MeV) state, with another possible contribution via the strongly excited 31- (9.64 MeV) resonance in C12. However, we find that a good description of the angular distribution for population of this state via the C12(d,p)C13 reaction is only possible when both direct 01+ - g9/2 and two-step (via the 4.44 MeV C12 21+ state) 21+ - d5/2 paths are included in a coupled reaction channel calculation. While the calculated angular distribution is almost insensitive to the presence of the two-step path via the 9.64 MeV C12 31- resonance, despite a much greater contribution to the wave function from the 31- - f7/2 configuration, its inclusion is required to fit the details of the experimental angular distribution. The very large interference between the various components of the calculations, even when these are small, arises through the "kinematic" effect associated with the different transfer routes.
    Full-text · Article · Nov 2015 · Physical Review C
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    N. Keeley · K. W. Kemper · K. Rusek
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    ABSTRACT: The population of the 9.50 MeV 9/2+ resonance in 13C by single neutron transfer reactions is expected to be dominated by the two-step route through the 12C 2+ (4.44 MeV) state, with another possible contribution via the strongly excited 3- (9.64 MeV) resonance in 12C. However, we find that a good description of the angular distribution for population of this state via the 12C(d,p)13C reaction is only possible when both direct 0+ x g_9/2 and two-step (via the 4.44 MeV 12C 2+ state) 2+ x d_5/2 paths are included in a coupled reaction channel calculation. While the calculated angular distribution is almost insensitive to the presence of the two-step path via the 9.64 MeV 12C 3- resonance, despite a much greater contribution to the wave function from the 3- x f_7/2 configuration, its inclusion is required to fit the details of the experimental angular distribution. The very large interference between the various components of the calculations, even when these are small, arises through the ``kinematic'' effect associated with the different transfer routes.
    Full-text · Article · Nov 2015
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    ABSTRACT: Full angular distributions for 11B + 14N elastic and inelastic scattering were measured at ( ). The data were analyzed within the optical model and coupled-reaction-channels method which included the reorientations of 11B and 14N as well as the more important one- and two-step transfer reactions. The 11B + 14N optical potential parameters as well as deformation parameters of these nuclei were deduced. The 11B + 14N potential and data were compared with those of previously reported 11B + 15N data to explore isotopic differences in the scattering process. As has been shown in other cases of 11B scattering, the enhanced large-angle elastic and inelastic scattering data mostly arise from the ground-state reorientation of 11B.
    No preview · Article · Sep 2015 · Nuclear Physics A
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    ABSTRACT: Shell evolution is studied in the neutron-rich silicon isotopes 36,38,40 Si using neutron single-particle strengths deduced from one-neutron knockout reactions. Configurations involving neutron excita- tions across the N = 20 and N = 28 shell gaps are quantified experimentally in these rare isotopes. Comparisons with shell model calculations show that the tensor force, understood to drive the col- lective behavior in 42 Si with N = 28, is already important in determining the structure of 40 Si with N = 26. New data relating to cross-shell excitations provide the first quantitative support for repulsive contributions to the cross-shell T = 1 interaction arising from three-nucleon forces.
    No preview · Article · Apr 2015 · Physical Review C
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    K. Rusek · N. Keeley · K. W. Kemper · A. T. Rudchik
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    ABSTRACT: The most pronounced reaction channels in the $^{6}\mathrm{Li}+^{18}\mathrm{O}$ system were studied by means of the continuum-discretized coupled-channel and coupled-reaction-channel methods to investigate their effects on the elastic scattering. It is shown that, whereas breakup coupling provides no contribution to the observed rise in the backward-angle elastic scattering angular distribution, coupling to the single-neutron pickup alone enhances the elastic scattering cross section in this region by up to two orders of magnitude.
    Full-text · Article · Apr 2015 · Physical Review C
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    ABSTRACT: Background: Accurate knowledge of the $^{13}$C($\alpha$,$n$)$^{16}$O reaction cross section is important for the understanding of the s-process in AGB stars, since it is considered to be the main source of neutrons. The sub-threshold 1/2$^+$ state at excitation energy of 6.356 MeV in $^{17}$O has a strong influence on the reaction cross section at energies relevant for astrophysics. Several experiments have been performed to determine the contribution of this state to the $^{13}$C($\alpha,n)^{16}$O reaction rate. Nevertheless, significant discrepancies between different measurements remain. Purpose: The aim of this work is to investigate these discrepancies. Method: An 8 MeV $^{13}$C beam (below the Coulomb barrier) was used to study the $\alpha$-transfer reaction $^6$Li($^{13}$C,$d$)$^{17}$O. Results: The squared Coulomb modified ANC of the 1/2$^+$ state in $^{17}$O measured in this work is $(\tilde C^{^{17}\text{O}(1/2+)}_{\alpha-^{13}\text{C}})^2=3.6\pm0.7\hspace{0.2cm}\text{fm}^{-1}$. Conclusions: Discrepancy between the results of $\alpha$-transfer experiments have been resolved. However, some discrepancy with the most recent measurement using the Trojan Horse method remains.
    Preview · Article · Mar 2015
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    ABSTRACT: Angular distributions of the 11B + 15N elastic and inelastic scattering were measured at (15N) = 84 MeV ( ). The data were analyzed within the optical model and coupled-reaction-channels method. The elastic and inelastic scattering, reorientations of 11B in ground and excited states and 15N in excited states as well as the more important one- and two-step transfer reactions were included in the channels-coupling scheme. The 11B + 15N optical potential parameters as well as deformation parameters of these nuclei were deduced. The contributions of one- and two-step transfers in the 11B + 15N elastic and inelastic scattering channels were estimated. The large angle cross section is found to be structureless and of the same order of magnitude ∼0.1–0.2 mb/sr as that for 11B + 16O and 11B + 14C.
    Full-text · Article · Feb 2015 · Nuclear Physics A
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    ABSTRACT: The ^{12}C(α,γ)^{16}O reaction plays a fundamental role in astrophysics and needs to be known with accuracy better than 10%. Cascade γ transitions through the excited states of ^{16}O are contributing to the uncertainty. We constrained the contribution of the 0^{+} (6.05 MeV) and 3^{-} (6.13 MeV) cascade transitions by measuring the asymptotic normalization coefficients for these states using the α-transfer reaction ^{6}Li(^{12}C,d)^{16}O at sub-Coulomb energy. The contribution of the 0^{+} and 3^{-} cascade transitions at 300 keV is found to be 1.96±0.3 and 0.12±0.04 keV b for destructive interference of the direct and resonance capture and 4.36±0.45 and 1.44±0.12 keV b for constructive interference, respectively. The combined contribution of the 0^{+} and 3^{-} cascade transitions to the ^{12}C(α,γ)^{16}O reaction cross section at 300 keV does not exceed 4%. Significant uncertainties have been dramatically reduced.
    Full-text · Article · Feb 2015 · Physical Review Letters
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    ABSTRACT: In order to ensure the safety of humans and equipment in the presence of proton radiation, it is essential that accurate dosimetry and effective shielding be utilized. Polymers have emerged as promising candidates for proton radiation shielding, and the addition of carbon nanotubes (CNTs) could provide essential improvements to their mechanical and electrical properties. This study evaluates the dosimetry capabilities and proton radiation shielding effectiveness of CNT-epoxy composites. Single wall (SWCNT) and multiwall carbon nanotubes (MWCNT) were dispersed in an epoxy matrix and subjected to proton irradiation of energies ranging from 8–12 MeV. The nanocomposites' shielding capabilities against different energetic proton beams were measured by tracking the beam's energy before and after penetrating the samples. The electrical volume resistivity of the samples was measured pre and post irradiation. It was found that while the addition of CNTs did not affect the shielding effectiveness, it decreased the electrical resistivity of the epoxy samples by five orders of magnitude; greatly increasing their resistance to damage from electrostatic discharge while maintaining excellent shielding properties. Proton radiation was shown to further reduce the electrical resistivity, indicating potential for use as a real-time dosimeter.
    Full-text · Article · Feb 2015 · Nanoscience and Nanotechnology Letters
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    ABSTRACT: Clustering phenomena in 10Be and 18O were studied by means of resonance elastic scattering of α-particles on 6He and 14C. Excitation functions for α+6He and α+14C were measured and detailed R-matrix analyses of the excitation functions was performed. We compare the experimental results with the predictions of modern theoretical approaches and discuss properties of cluster rotational bands.
    No preview · Article · Dec 2014 · Journal of Physics Conference Series
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    ABSTRACT: New angular distribution data for 7Li + 14N elastic and inelastic scattering at the energy Elab(14N) = 80MeV (c.m. 26.7MeV) are presented. The data were analyzed within the optical model and the coupled-reaction-channels method using a channels-coupling scheme that included the 7Li and 14N inelastic scattering channels, spin reorientations of 7Li and 14N as well as most important transfer reactions. The low-energy excited states of 7Li and 14N were assumed to be collective in nature. The 7Li + 14N potential parameters as well as deformation parameters of 7Li and 14N were deduced. The 7Li + 14N potential and the data were compared with those of 6Li + 14N to observe isotopic differences. The enhanced large-angle elastic and inelastic scattering data are shown to have a large contribution from the ground-state reorientation of 7Li.
    No preview · Article · Oct 2014 · European Physical Journal A
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    ABSTRACT: Background. The $^{12}$C($\alpha,\gamma$)$^{16}$O reaction plays a fundamental role in astrophysics because its cross section near 300 keV in c.m. determines the $^{12}$C/$^{16}$O ratio at the end of the helium burning stage of stellar evolution. The astrophysically desired accuracy of better than 10\% has not been achieved. Cascade $\gamma$ transitions through the excited states of $^{16}$O are contributing to the uncertainty. Purpose. To measure the Asymptotic Normalization Coefficients (ANCs) for the 0$^+$ (6.05 MeV) and 3$^-$ (6.13 MeV) excited states in $^{16}$O and provide constraints on the cross section for the corresponding cascade transitions. Method. The ANCs were measured using the $\alpha$-transfer reaction $^{12}$C($^6$Li,$d$)$^{16}$O performed at sub-Coulomb energies for both the entrance and exit channels. Results. The ANCs for the 0$^+$(6.05 MeV), 3$^-$(6.13 MeV), 2$^+$(6.92 MeV) and 1$^-$(7.12 MeV) states in $^{16}$O have been measured. The contribution of the 0$^+$ and 3$^-$ cascade transitions to the $^{12}$C($\alpha,\gamma$)$^{16}$O reaction S-factor was found to be 1.9$\pm$0.3 keV b and 0.5$\pm$0.09 keV b respectively. Conclusions. Significant uncertainties related to the 6.05 MeV 0$^+$ and 6.13 MeV 3$^-$ cascade transitions have been eliminated. The combined contribution of the 0$^+$ and 3$^-$ cascade transitions to the $^{12}$C($\alpha,\gamma$)$^{16}$O reaction cross section at 300 keV does not exceed 2\%. \end{description}
    Full-text · Article · Oct 2014 · Physical Review Letters
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    ABSTRACT: The transverse analyzing powers T20T were obtained from a polarized beam study of the reaction C12(Li7,α)N15 at Elab(7Li)=34 MeV for several well known, strongly populated and isolated states in N15 to determine if they contained a sufficiently distinctive signature to be used to provide a test against which selectively populated and strong, but not well understood states, such as the one at 13.17 MeV could be used to determine their spins and parities. Unfortunately, current finite range distorted wave Born approximation and coupled channels Born approximation calculations were not able to describe the data to the well-known states. The current work presents these experimental analyzing powers with the expectation that they can be used as a test of future detailed reaction model calculations.
    No preview · Article · Oct 2014 · Physical Review C
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    N. Keeley · K. W. Kemper · K. Rusek
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    ABSTRACT: Accurate elastic scattering angular distribution data measured at bombarding energies just above the Coulomb barrier have shapes that can markedly differ from or be the same as the expected classical Fresnel scattering pattern depending on the structure of the projectile, the target or both. Examples are given such as 18O + 184W and 16O + 148,152Sm where the expected rise above Rutherford scattering due to Coulomb-nuclear interference is damped by coupling to the target excited states, and the extreme case of 11Li scattering, where coupling to the 9Li + n + n continuum leads to an elastic scattering shape that cannot be reproduced by any standard optical model parameter set. The recent availability of high quality 6He, 11Li and 11Be data provides further examples of the influence that coupling effects can have on elastic scattering. Conditions for strong projectile-target coupling effects are presented with special emphasis on the importance of the beam-target charge combination being large enough to bring about the strong coupling effects. Several measurements are proposed that can lead to further understanding of strong coupling effects by both inelastic excitation and nucleon transfer on near-barrier elastic scattering. A final note on the anomalous nature of 8B elastic scattering is presented as it possesses a more or less normal Fresnel scattering shape whereas one would a priori not expect this due to the very low breakup threshold of 8B. The special nature of 11Li is presented as it is predicted that no matter how far above the Coulomb barrier the elastic scattering is measured, its shape will not appear as Fresnel like whereas the elastic scattering of all other loosely bound nuclei studied to date should eventually do so as the incident energy is increased, making both 8B and 11Li truly "exotic".
    Full-text · Article · Sep 2014 · European Physical Journal A
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    ABSTRACT: The structure of the neutron-rich silicon isotopes $^{36,38,40}\mathrm{Si}$ was studied by one-neutron and one-proton knockout reactions at intermediate beam energies. We construct level schemes for the knockout residues $^{35,37,39}\mathrm{Si}$ and $^{35,37,39}\mathrm{Al}$ and compare knockout cross sections to the predictions of an eikonal model in conjunction with large-scale shell-model calculations. The agreement of these calculations with the present experiment lends support to the microscopic explanation of the enhanced collectivity in the region of $^{42}\mathrm{Si}$. We also present an empirical method for reproducing the observed low-momentum tails in the parallel momentum distributions of knockout residues.
    No preview · Article · Sep 2014 · Physical Review C
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    ABSTRACT: We have performed measurements of the $0_\mathrm{g.s.}^+ \rightarrow 2_1^+$ excitations in the neutron-rich isotopes $^{48,50}$Ca via inelastic proton scattering on a liquid hydrogen target, using the GRETINA $\gamma$-ray tracking array. A comparison of the present results with those from previous measurements of the lifetimes of the $2_1^+$ states provides us the ratio of the neutron and proton matrix elements for the $0_\mathrm{g.s.}^+ \rightarrow 2_1^+$ transitions. These results allow the determination of the ratio of the proton and neutron effective charges to be used in shell model calculations of neutron-rich isotopes in the vicinity of $^{48}$Ca.
    Full-text · Article · Jul 2014 · Physical Review C
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    ABSTRACT: New $^{14}$N(d,p) angular distribution data were taken at a deuteron bombarding energy of 16 MeV to locate all narrow single particle neutron states up to 15 MeV in excitation. A new shell model calculation is able to reproduce all levels in $^{15}$N up to 11.5 MeV and is used to characterize a narrow single particle level at 11.236 MeV and to provide a map of the single particle strengths. The known levels in $^{15}$N are then used to determine their mirrors in the lesser known nucleus $^{15}$O. The 2s$_{1/2}$ and 1d$_{5/2}$ single particle centroid energies are determined for the $^{15}$N$-^{15}$O mirror pair as: $^{15}$N $(\text{2s}_{1/2}) = 8.08$ MeV, $^{15}$O $(\text{2s}_{1/2}) = 7.43$ MeV, $^{15}$N $(\text{1d}_{5/2}) = 7.97$ MeV, and $^{15}$O $(\text{1d}_{5/2}) = 7.47$ MeV. These results confirm the degeneracy of these orbits and that the $^{15}$N$-^{15}$O nuclei are where the transition between the $\text{2s}_{1/2}$ lying below the $\text{1d}_{5/2}$ to lying above it, takes place. The $\text{1d}_{3/2}$ single particle strength is estimated to be centered around 13 MeV in these nuclei.
    Preview · Article · Jul 2014 · Physical Review C
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    ABSTRACT: Angular distributions of the 6Li(18O, 17O)7Li reaction were measured at Elab(O18)=114 MeV for ground and excited states of the exit channel nuclei for the first time. The data were analyzed within the coupled-reaction-channels method (CRC). The 6Li + 18O elastic and inelastic scattering channels as well as simplest one- and two-step reactions were included in the coupled-reaction-channels scheme. The 7Li + 17O potential was deduced by fitting CRC calculations to the reaction data. The spectroscopic amplitudes for single nucleon and nuclear cluster configurations were calculated within the translationally invariant shell model. Isotopic differences of the 7Li + 16, 17, 18O and 7, 8Li + 17O potentials and the reaction mechanisms were studied.
    No preview · Article · Jul 2014 · Nuclear Physics A
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    ABSTRACT: Background: Clustering phenomena in $N \neq Z$ nuclei provide an opportunity to understand the interplay between cluster and nucleon degrees of freedom. Purpose:To study resonances in the $^{18}$O spectrum, populated in $^{14}$C+$\alpha$ elastic scattering. Method: The Thick Target Inverse Kinematics (TTIK) technique was used to measure the excitation function for the $^{14}$C+$\alpha$ elastic scattering. A 42 MeV $^{14}$C beam was used to populate states of excitation energy up to 14.9 MeV in $^{18}$O. The analysis was performed using a multi-level, multi-channel R-Matrix approach. Results: Detailed spectroscopic information, including spin-parities, partial $\alpha$- and neutron- decay widths and dimensionless reduced widths, was obtained for excited states in $^{18}$O between 8 and 14.9 MeV in excitation energy. Cluster-Nucleon Configuration Interaction Model calculations of the same quantities are performed and compared to the experimental results. Conclusions: Strong fragmentation of large $\alpha$-cluster strengths is observed in the spectrum of $^{18}$O making the $\alpha$-cluster structure of $^{18}$O quite different from the pattern of known quasi-rotational bands of alternating parity that are characteristic of $N=Z$, even-even nuclei like $^{16}$O and $^{20}$Ne.
    Preview · Article · Jun 2014 · Physical Review C

Publication Stats

5k Citations
1,316.76 Total Impact Points

Institutions

  • 1969-2015
    • Florida State University
      • Department of Physics
      Tallahassee, Florida, United States
  • 2014
    • Louisiana State University
      • Department of Physics & Astronomy
      Baton Rouge, Louisiana, United States
  • 2001
    • Staffordshire University
      Stoke-upon-Trent, England, United Kingdom
  • 2000
    • Earlham College
      Richmond, California, United States
  • 1998
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1978-1998
    • Australian National University
      • Department of Nuclear Physics
      Canberra, Australian Capital Territory, Australia
  • 1993
    • State University of New York College at Geneseo
      Geneseo, New York, United States
  • 1992
    • Stanford University
      • Department of Physics
      Stanford, California, United States