K. W. Kemper

Florida State University, Tallahassee, Florida, United States

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Publications (530)1331.91 Total impact

  • [Show abstract] [Hide abstract]
    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.
    Physical Review C 04/2015; 91(4). DOI:10.1103/PhysRevC.91.041302 · 3.88 Impact Factor
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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.
    Physical Review C 04/2015; 91(4). DOI:10.1103/PhysRevC.91.044612 · 3.88 Impact Factor
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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.
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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.
    Nuclear Physics A 02/2015; 939. DOI:10.1016/j.nuclphysa.2015.02.006 · 2.50 Impact Factor
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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.
    Physical Review Letters 02/2015; 114(7):071101. · 7.73 Impact Factor
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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.
    Nanoscience and Nanotechnology Letters 02/2015; 7(2). DOI:10.1166/nnl.2015.1898 · 1.44 Impact Factor
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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.
    Journal of Physics Conference Series 12/2014; 569(1). DOI:10.1088/1742-6596/569/1/012004
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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}
    Physical Review Letters 10/2014; 114(7). DOI:10.1103/PhysRevLett.114.071101 · 7.73 Impact Factor
  • Physical Review C 10/2014; 90(4). DOI:10.1103/PhysRevC.90.047601 · 3.88 Impact Factor
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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".
    European Physical Journal A 09/2014; 50(9). DOI:10.1140/epja/i2014-14145-3 · 2.42 Impact Factor
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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.
    Physical Review C 09/2014; 90(3). DOI:10.1103/PhysRevC.90.034301 · 3.88 Impact Factor
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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.
    Physical Review C 07/2014; 90(1). DOI:10.1103/PhysRevC.90.011305 · 3.88 Impact Factor
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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.
    Physical Review C 07/2014; 91(4). DOI:10.1103/PhysRevC.91.044317 · 3.88 Impact Factor
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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.
    Nuclear Physics A 07/2014; 927. DOI:10.1016/j.nuclphysa.2014.04.018 · 2.50 Impact Factor
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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.
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    ABSTRACT: Background. Many important $\alpha$-particle induced reactions for nuclear astrophysics may only be measured using indirect techniques due to small cross sections at the energy of interest. One of such indirect technique, is to determine the Asymptotic Normalization Coefficients (ANC) for near threshold resonances extracted from sub-Coulomb $\alpha$-transfer reactions. This approach provides a very valuable tool for studies of astrophysically important reaction rates since the results are practically model independent. However, the validity of the method has not been directly verified. Purpose. The aim of this letter is to verify the technique using the $^{16}$O($^6$Li,$d$)$^{20}$Ne reaction as a benchmark. The $^{20}$Ne nucleus has a well known $1^-$ state at excitation energy of 5.79 MeV with a width of 28 eV. Reproducing the known value with this technique is an ideal opportunity to verify the method. Method. The 1$^-$ state at 5.79 MeV is studied using the $\alpha$-transfer reaction $^{16}$O($^6$Li,$d$)$^{20}$Ne at sub-Coulomb energies. Results. The partial $\alpha$ width for the $1^-$ state at excitation energy of 5.79 MeV is extracted and compared with the known value, allowing the accuracy of the method to be evaluated. Conclusions. This study demonstrates that extracting the Asymptotic Normalization Coefficients using sub-Coulomb $\alpha$-transfer reactions is a powerful tool that can be used to determine the partial $\alpha$ width of near threshold states that may dominate astrophysically important nuclear reaction rates. \end{description}
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    ABSTRACT: Complete angular distributions of the 14C + 11B elastic and inelastic scattering at the energy \( E_{lab}({}^{11}B)\) = 45 MeV were measured for the ground and excited states of 11B. The data were analyzed within the optical model and the coupled-reaction-channels method. The parameters of the 14C + 11B optical potential and 11B deformation were deduced. The contributions of one- and two-step transfers of nucleons and clusters into the 14C + 11B elastic and inelastic scattering were calculated. The isotopic differences between the 14C + 11B and 12,13C + 11B scatterings are presented and clearly show the importance of the large angle scattering data for understanding the interaction between light heavy ions.
    European Physical Journal A 01/2014; 50(1). DOI:10.1140/epja/i2014-14004-3 · 2.42 Impact Factor
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    ABSTRACT: The separation between single particle levels in nuclei plays the dominant role in determining the location of the neutron drip line. The separation also provides a test of current crossed shell model interactions if the experimental data is such that multiple shells are involved. The present work uses the $^{14}$N(d, p)$^{15}$N reaction to extract the 2s$_{1/2}$, and 1d$_{5/2}$ total neutron single particle strengths and then compares these results with a shell model calculation using a p-sd crossed shell interaction to identify the J$^\pi$ of all levels in $^{15}$N up to 12.8 MeV in excitation.
    Acta Physica Polonica Series B 10/2013; 45(2). DOI:10.5506/APhysPolB.45.159 · 1.00 Impact Factor
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    N. Keeley, K. W. Kemper, K. Rusek
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    ABSTRACT: Elastic scattering data for 11Li, 6Li, and 6He + 208Pb at incident energies of 29.8 MeV, 29 MeV, and 18 MeV, respectively, were analyzed by means of coupled discretized continuum channels (CDCC) calculations. Dynamic polarization potentials (DPPs) of the trivially equivalent local potential (TELP) type were derived from these calculations and compared. The dipole polarizability factor α for 11Li obtained by fitting a Coulomb polarization potential to the long-range part of the real DPP is consistent with the theoretical value of 5.7 fm3. These results point to the dineutron model providing a very good description of 11Li breakup coupling effects. The much larger Coulomb dipole polarizability of 11Li suggests the persistence of large deviations from Rutherford scattering at sub-barrier energies for medium mass targets, a prediction confirmed by CDCC calculations.
    Physical Review C 07/2013; 88(1). DOI:10.1103/PhysRevC.88.017602 · 3.88 Impact Factor

Publication Stats

4k Citations
1,331.91 Total Impact Points

Institutions

  • 1969–2015
    • Florida State University
      • Department of Physics
      Tallahassee, Florida, United States
  • 2007
    • GSI Helmholtzzentrum für Schwerionenforschung
      • ExtreMe Matter Institute EMMI and Research Division
      Darmstadt, Hesse, Germany
  • 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
  • 1992
    • Universität Basel
      Bâle, Basel-City, Switzerland
  • 1991
    • Stanford University
      • Department of Physics
      Palo Alto, California, United States
  • 1989
    • King Fahd University of Petroleum and Minerals
      Az̧ Z̧ahrān, Eastern Province, Saudi Arabia