K. W. Kemper

Florida State University, Tallahassee, Florida, United States

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Publications (512)1183.47 Total impact

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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}
    10/2014;
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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".
    09/2014;
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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). · 3.72 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.
    07/2014;
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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; · 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.
    06/2014;
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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}
    06/2014;
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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.
    10/2013;
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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). · 3.72 Impact Factor
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    ABSTRACT: Accurate differential cross sections have been measured at specific beam energies and angles to be used in a method proposed previously for the simultaneous quantification of light elements (Z<11Z<11) present in evaporated liquid biological samples. Targets containing 1H, 7Li, 12C, 16O, 19F, 28Si and 197Au have been bombarded with 13 MeV 6Li3+ and 20 MeV 16O5+ beams. The 16O + 1H, 16O + 12C, 16O + 16O, 16O + 19F, 16O + 28Si and 16O + 197Au cross sections, shown to be consistent with the Rutherford formula predictions at 15° and 20°, have been used to determine cross sections for the 6Li + 1H, 6Li + 12C, 6Li + 16O, 6Li + 19F, 6Li + 28Si and 6Li + 197Au scatterings respectively at 17.5°, 24°, 25°, 26°, 28° and 30°. Although 6Li + 7Li cross sections have not been obtained from 16O + 7 Li cross sections, they have been determined from measured 6Li + 19F cross sections and, in addition, used to obtain 16O + 7Li cross sections at 15° and 20°. The reliability of the new cross sections determined in this investigation for the 6Li + 1H, 6Li + 7Li and 6Li + 19F scatterings is based on the Rutherford behavior of the measured 6Li + 197Au scattering data as expected and the consistency observed between the 6Li + 12C, 6Li + 16O and 6Li + 28Si cross sections obtained in this work and previously reported values. This research has important implications in applied physics.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 06/2013; 305:16–21. · 1.19 Impact Factor
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    ABSTRACT: Resonance scattering with rare isotope beams provides direct access to continuum properties of exotic nuclei and can serve as a stringent test for modern theoretical approaches. Properties of neutron deficient isotope 8B, that were studied using resonance scattering of protons of 7Be, are discussed and compared to the predictions of the ab initio theories. New experimental data on clustering in 10Be studied using 6He+α resonance elastic scattering is presented.
    04/2013;
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    ABSTRACT: Motivation: Detailed experimental knowledge of the level structure of light weakly bound nuclei is necessary to guide the development of new theoretical approaches that combine nuclear structure with reaction dynamics. Purpose: The resonant structure of 8B is studied in this work. Method: Excitation functions for elastic and inelastic 7Be+p scattering were measured using a 7Be rare isotope beam. Excitation energies ranging between 1.6 and 3.4 MeV were investigated. An R-matrix analysis of the excitation functions was performed. Results: New low-lying resonances at 1.9, 2.5, and 3.3 MeV in 8B are reported with spin-parity assignment 0+, 2+, and 1+, respectively. Comparison to the Time Dependent Continuum Shell (TDCSM) model and ab initio no-core shell model/resonating-group method (NCSM/RGM) calculations is performed. This work is a more detailed analysis of the data first published as a Rapid Communication. [J.P. Mitchell, et al, Phys. Rev. C 82, 011601(R) (2010)] Conclusions: Identification of the 0+, 2+, 1+ states that were predicted by some models at relatively low energy but never observed experimentally is an important step toward understanding the structure of 8B. Their identification was aided by having both elastic and inelastic scattering data. Direct comparison of the cross sections and phase shifts predicted by the TDCSM and ab initio No Core Shell Model coupled with the resonating group method is of particular interest and provides a good test for these theoretical approaches.
    Physical Review C 03/2013; 87(5). · 3.72 Impact Factor
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    ABSTRACT: Existing data for the 6Li + 16O elastic scattering at $ E_{c.m.}=3.27$ -36.8MeV were analyzed within the optical model and coupled-reaction-channels method. The 6Li + 16O elastic and inelastic scattering as well as the reorientation of 6Li and the simplest transfer reactions were included in the coupled-channels scheme. The 6Li + 16O potential parameters at different energies as well as their energy dependence were deduced by the use of the dispersion relation between the real and imaginary parts of the potential. The contributions of the 6Li reorientation and transfer reactions to the 6Li + 16O elastic scattering channel were estimated at the different energies. It is found that the potential scattering dominates the interaction at smaller angles for all energies, with the transfers contributing significantly at larger angles at the highest energies studied. The real part of the 6Li + 16O deduced potential is in reasonable agreement with that of a corresponding folding potential. The isotopic difference between the 6Li + 16O and 7Li + 16O optical potentials is larger for the imaginary potentials.
    European Physical Journal A 01/2013; 49(6). · 2.04 Impact Factor
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    ABSTRACT: Inverse kinematics scattering of 18O on 6Li at ElabElab(18O) = 114 MeV was measured to obtain elastic and inelastic scattering cross sections. In this way cross sections for excited states in 6Li and 18O were determined. The data were analyzed within the optical model and coupled reaction channel method. The 6Li + 18O optical potential as well as the 6Li and 18O deformation parameters were deduced. Contributions of different nuclear processes to the 6Li + 18O elastic and inelastic scattering were explored. The isotopic differences between the 6,76,7Li + 18O and 6Li + 16,1816,18O potential parameters were determined.
    Nuclear Physics A. 01/2013;
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    ABSTRACT: The role of proton shell effects in the structure of the N=28 isotones 45Cl and 44S has been studied via one-proton knockout from 45Cl. We compare measured γ-ray intensities, inclusive and partial knockout cross sections, and the inclusive momentum distribution of outgoing 44S particles with shell-model and reaction-theory predictions. The strong population in this reaction of the recently identified 41+ state in 44S, identified through its subsequent γ-ray decay energy, makes a compelling case for a Jπ=3/2+ ground state in 45Cl.
    Physical Review C 10/2012; 86(4). · 3.72 Impact Factor
  • N. Keeley, K. W. Kemper, K. Rusek
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    ABSTRACT: Existing 10Be(d,t)9Be neutron pickup data are analyzed using the same bound-state form factors for the 〈10Be|9Be+n〉 overlap as in a recent analysis of single-neutron knockout [ Grinyer et al. Phys. Rev. Lett. 106 162502 (2011)]. While the knockout data were well described by a bound-state form factor calculated using the variational Monte Carlo (VMC) technique, including an appropriate neutron binding potential, the 10Be(d,t)9Be pickup data are significantly overpredicted using this form factor. In addition, the no-core shell model (NCSM) and VMC form factors yield the same calculated pickup cross sections whereas the knockout results using these form factors differed by 20%. We explore possible sources of ambiguity in the pickup calculations that affect our ability to compare the absolute magnitudes in cross sections between these two very different reactions.
    Physical Review C 07/2012; 86(1). · 3.72 Impact Factor
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    ABSTRACT: The determination of the electric quadrupole transition strength between the ground state and first excited state with spin-party of J^π=2^+ (the B(E2; 0^+-> 2^+) value) in an even-even nucleus provides a measurement of the low-lying quadrupole collectivity. The B(E2) values for ^34,36,38,40,42Si were measured via intermediate-energy Coulomb excitation at NSCL. The secondary beams were produced by fragmentation of ^48Ca primary beam and guided onto a high-Z target. De-excitation gamma rays indicating the inelastic process were detected around the target position with the high efficiency scintillator array CAESAR in coincidence with scattered projectiles tracked on an event-by-event basis in the S800 spectrograph. The results comprise the first measurements of the quadrupole collectivity of ^40Si and ^42Si and probe the persistence of the N=28 magic number. The measured B(E2) values are compared to large-scale shell model calculations and provide insight into the evolution of shell structure and deformation in this region.
    2011 Fall Meeting of the APS Division of Nuclear Physics; 10/2011
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    ABSTRACT: The neutron-rich N=28 nucleus 44S was studied using the two-proton knockout reaction from 46Ar at intermediate beam energy. We report the observation of four new excited states, one of which is a strongly prolate deformed 4+ state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the “intruder” configuration responsible for the ground-state deformation. Consequently, we do not have three coexisting shapes in 44S, but three coexisting configurations, corresponding to zero-, one-, and two-neutron particle-hole excitations.
    Phys. Rev. C. 06/2011; 83(6).
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    ABSTRACT: Excited states in the very neutron-rich nuclei 35Mg and 33Na were populated in the fragmentation of a 38Si projectile beam on a Be target at 83 MeV/u beam energy. We report on the first observation of γ-ray transitions in 35Mg, the odd-N neighbor of 34Mg and 36Mg, which are known to be part of the ‘island of inversion” around N=20. The results are discussed in the framework of large-scale shell-model calculations. For the A=3Z nucleus 33Na, a new γ-ray transition was observed that is suggested to complete the γ-ray cascade 7/2+→5/2+→3/2gs+ connecting three neutron two-particle–two-hole intruder states that are predicted to form a close-to-ideal K=3/2 rotational band in the strong-coupling limit.
    Physical Review C 04/2011; 83(4). · 3.72 Impact Factor
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    ABSTRACT: New angular distributions for the 7Li(18O,17N)8Be reaction at an energy of Elab(18O)=114 MeV for the ground states of 8Be and 17N and the excited states of 17N were measured. These data and 18O(d,3He)17N reaction data taken at Ed=52 MeV were analyzed within the coupled-reaction-channels method using 7Li+18O and 18O + d optical potentials deduced from previous elastic and inelastic scattering results. Shell-model spectroscopic amplitudes were used in the analysis. Both reactions are dominated by single proton transfer. Calculations show that heavy-ion reactions of the type studied in this work can be used to identify final-state spins when measurements are carried to small angles.
    Physical Review C 02/2011; 83(2). · 3.72 Impact Factor

Publication Stats

4k Citations
1,183.47 Total Impact Points

Institutions

  • 1969–2014
    • Florida State University
      • Department of Physics
      Tallahassee, Florida, United States
  • 2009
    • Simon Bolívar University
      • Departamento de Física
      Baruta, Estado Miranda, Venezuela
  • 2007
    • Rzeszów University
      Zheshev, Subcarpathian Voivodeship, Poland
  • 1997–2007
    • Michigan State University
      • Department of Physics and Astronomy
      East Lansing, MI, United States
  • 2001
    • GANIL
      Caen, Lower Normandy, France
  • 1998
    • Australian National University
      • Department of Nuclear Physics
      Canberra, Australian Capital Territory, Australia
  • 1994
    • University of Birmingham
      Birmingham, England, United Kingdom