Publications

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    ABSTRACT: The 9Be(28Mg,27Na) one-proton removal reaction with a large proton separation energy of Sp(28Mg)=16.79 MeV is studied at intermediate beam energy. Coincidences of the bound 27Na residues with protons and other light charged particles are measured. These data are analyzed to determine the percentage contributions to the proton removal cross section from the elastic and inelastic nucleon removal mechanisms. These deduced contributions are compared with the eikonal reaction model predictions and with the previously measured data for reactions involving the re- moval of more weakly-bound protons from lighter nuclei. The role of transitions of the proton between different bound single-particle configurations upon the elastic breakup cross section is also quantified in this well-bound case. The measured and calculated elastic breakup fractions are found to be in good agreement.
    12/2014;
  • J. A. Tostevin, A. Gade
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    ABSTRACT: There is now a large and increasing body of experimental data and theoretical analyses for reactions that remove a single nucleon from an intermediate-energy beam of neutron- or proton-rich nuclei. In each such measurement, one obtains the inclusive cross section for the population of all bound final states of the mass A-1 reaction residue. These data, from different regions of the nuclear chart, and that involve weakly- and strongly-bound nucleons, are compared with theoretical expectations. These calculations include an approximate treatment of the reaction dynamics and shell-model descriptions of the projectile initial state, the bound final states of the residues, and the single-particle strengths computed from their overlap functions. The results are discussed in the light of recent data, more exclusive tests of the eikonal dynamical description, and calculations that take input from more microscopic nuclear structure models.
    09/2014;
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    ABSTRACT: Cross sections of 1n-removal reactions from the neutron-rich nucleus ^{37}Mg on C and Pb targets and the parallel momentum distributions of the ^{37}Mg residues from the C target have been measured at 240 MeV/nucleon. A combined analysis of these distinct nuclear- and Coulomb-dominated reaction data shows that the ^{37}Mg ground state has a small 1n separation energy of 0.22_{-0.09}^{+0.12} MeV and an appreciable p-wave neutron single-particle strength. These results confirm that ^{37}Mg lies near the edge of the "island of inversion" and has a sizable p-wave neutron halo component, the heaviest such system identified to date.
    Physical Review Letters 06/2014; 112(24):242501. · 7.73 Impact Factor
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    ABSTRACT: Cross sections of 1n-removal reactions from the neutron-rich nucleus ^{37}Mg on C and Pb targets and the parallel momentum distributions of the ^{37}Mg residues from the C target have been measured at 240 MeV/nucleon. A combined analysis of these distinct nuclear- and Coulomb-dominated reaction data shows that the ^{37}Mg ground state has a small 1n separation energy of 0.22_{-0.09}^{+0.12} MeV and an appreciable p-wave neutron single-particle strength. These results confirm that ^{37}Mg lies near the edge of the "island of inversion" and has a sizable p-wave neutron halo component, the heaviest such system identified to date.
    Physical Review Letters 06/2014; 112(24):242501. · 7.73 Impact Factor
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    ABSTRACT: The halo structure of Ne31 is studied using 1n-removal reactions on C and Pb targets at 230 MeV/nucleon. A combined analysis of the cross sections of these nuclear and Coulomb dominated reactions that feed directly the Ne30 ground-state reveals Ne31 to have a small neutron separation energy, 0.15-0.10+0.16 MeV, and spin-parity 3/2-. Consistency of the data with reaction and large-scale shell-model calculations identifies Ne31 as deformed and having a significant p-wave halo component, suggesting that halos are more frequent occurrences at the neutron drip line.
    Physical Review Letters 04/2014; 112(14):142501. · 7.73 Impact Factor
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    ABSTRACT: Excited states in the neutron-rich N=38, 36 nuclei Ti60 and Ti58 were populated in nucleon-removal reactions from V61 projectiles at 90 MeV/nucleon. The γ-ray transitions from such states in these Ti isotopes were detected with the advanced γ-ray tracking array GRETINA and were corrected event by event for large Doppler shifts (v/c∼0.4) using the γ-ray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutron-rich N=36, 38 Fe and Cr toward the Ti and Ca isotones. In fact, Ti58,60 provide some of the most neutron-rich benchmarks accessible today for calculations attempting to determine the structure of the potentially doubly magic nucleus Ca60.
    Physical Review Letters 03/2014; 112(11):112503. · 7.73 Impact Factor
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    ABSTRACT: Excited states in the neutron-rich N=38,36 nuclei \nuc{60}{Ti} and \nuc{58}{Ti} were populated in nucleon-removal reactions from \nuc{61}{V} projectiles at 90~MeV/nucleon. The \gamma-ray transitions from such states in these Ti isotopes were detected with the advanced \gamma-ray tracking array GRETINA and were corrected event-by-event for large Doppler shifts (v/c \sim 0.4) using the \gamma-ray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutron-rich N=36,38 Fe and Cr toward the Ti and Ca isotones. In fact, \nuc{58,60}{Ti} provide some of the most neutron-rich benchmarks accessible today for calculations attempting to determine the structure of the potentially doubly-magic nucleus \nuc{60}{Ca}.
    02/2014;
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    Y. Kucuk, J. A. Tostevin
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    ABSTRACT: The heaviest particle-bound carbon isotope, $^{22}$C, is thought to have a Borromean three-body structure. We discuss and compare four-body, i.e. three-body projectile plus target, reaction model calculations of reaction cross sections for such systems that use the fast adiabatic approximation. These methods are efficient and well-suited for quantitative analyses of reactions of neutron-rich nuclei with light target nuclei at secondary beam energies of $\approx$300 MeV/nucleon, as are now becoming available. Using $^{22}$C projectile wave functions, modelled as a $^{20}$C core plus two-valence neutron three-body system, we compare the predictions of the adiabatic model of the reaction both without and with the additional eikonal model approximations that have been used previously.
    12/2013; 89(3).
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    ABSTRACT: The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process beta-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes.
    10/2013;
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    ABSTRACT: The first spectroscopy of excited states in ^{52}Ni (T_{z}=-2) and ^{51}Co (T_{z}=-3/2) has been obtained using the highly selective two-neutron knockout reaction. Mirror energy differences between isobaric analogue states in these nuclei and their mirror partners are interpreted in terms of isospin nonconserving effects. A comparison between large-scale shell-model calculations and data provides the most compelling evidence to date that both electromagnetic and an additional isospin nonconserving interactions for J=2 couplings, of unknown origin, are required to obtain good agreement.
    Physical Review Letters 08/2013; 111(7):072501. · 7.73 Impact Factor
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    ABSTRACT: The first spectroscopy of excited states in 52Ni (Tz=2) and 51Co (Tz=-3/2) has been obtained using the highly selective two-neutron knockout reaction. Mirror energy differences between isobaric analogue states in these nuclei and their mirror partners are interpreted in terms of isospin nonconserving effects. A comparison between large scale shell-model calculations and data provides the most compelling evidence to date that both electromagnetic and an additional isospin nonconserving interactions for J=2 couplings, of unknown origin, are required to obtain good agreement.
    Physical Review Letters 08/2013; 111(7):072501. · 7.73 Impact Factor
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    ABSTRACT: Theories of $(d,p)$ reactions frequently use a formalism based on a transition amplitude that is dominated by the components of the total three-body scattering wave function where the spatial separation between the incoming neutron and proton is confined by the range of the $n$-$p$ interaction, $V_{np}$. By comparison with calculations based on the CDCC method we show that the $(d,p)$ transition amplitude is dominated by the first term of the expansion of the three-body wave function in a complete set of Weinberg states. We use the \nuc{132}{Sn}(d,p)\nuc{133} {Sn} reaction at 30 and 100 MeV as examples of contemporary interest. The generality of this observed dominance and its implications for future theoretical developments are discussed.
    Physical Review C 06/2013; 87(6). · 3.72 Impact Factor
  • J. A. Tostevin, B. A. Brown, E. C. Simpson
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    ABSTRACT: Newly published 42Si γ-ray spectra and a final-state-inclusive 42Si production cross section value, obtained in a higher-statistics intermediate-energy two-proton removal experiment from 44S, are considered in terms of the final-state-exclusive cross sections computed using proposed shell-model effective interactions for nuclei near N=28. Specifically, we give cross section predictions when using the two nucleon amplitudes of the two-proton overlaps <42Si(Jπ)|44S> computed using the newly proposed sdpf-mu shell-model Hamiltonian. We show that these partial cross sections or their longitudinal momentum distributions should enable a less-tentative interpretation of the measured gamma-ray spectra and provide a more quantitative assessment of proposed shell-model Hamiltonians in this interesting and challenging region of the chart of nuclides.
    Physical Review C 02/2013; · 3.72 Impact Factor
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    ABSTRACT: Results are presented from a one-neutron knockout experiment at relativistic energies of ~ 420 A MeV on 51-55Sc using the GSI Fragment Separator as a two-stage magnetic spectrometer and the MINIBALL array for gamma-ray detection. Inclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the contributions corresponding to knockout from the nup_1/2, nup_3/2, (L = 1) and nuf_7/2, nuf_5/2 (L = 3) neutron orbitals. The observed L = 1 and L = 3 contributions are compared with theoretical cross-sections using eikonal knockout theory and spectroscopic factors from shell model calculations using the GXPF1A interaction. The measured inclusive knockout cross-sections generally follow the trends expected theoretically and given by the spectroscopic strength predicted from the shell model calculations. However, the deduced L = 1 cross-sections are generally 30-40% higher while the L = 3 contributions are about a factor of two smaller than predicted. This points to a promotion of neutrons from the nuf_7/2 to nup_3/2 the orbital indicating a weakening of the N = 28 shell gap in these nuclei. While this is not predicted for the phenomenological GXPF1A interaction such a weakening is predicted by recent calculations using realistic low-momentum interactions V_lowk obtained by evolving a chiral N3LO nucleon-nucleon potential.
    European Physical Journal A 12/2012; 48(12):191 1-10. · 2.42 Impact Factor
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    J. A. Tostevin, B. A. Brown, E. C. Simpson
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    ABSTRACT: Newly-published $^{42}$Si gamma-ray spectra and a final-state-inclusive $^{42}$Si production cross section value, obtained in a higher-statistics intermediate-energy two-proton removal experiment from $^{44}$S, are considered in terms of the final-state-exclusive cross sections computed using proposed shell-model effective interactions for nuclei near N=28. Specifically, we give cross section predictions when using the two nucleon amplitudes of the two-proton overlaps $<^{42}$Si$ (J^\pi)\,|\,^{44}$S$>$ computed using the newly-proposed {\sc sdpf-mu} shell-model Hamiltonian. We show that these partial cross sections or their longitudinal momentum distributions should enable a less-tentative interpretation of the measured gamma-ray spectra and provide a more quantitative assessment of proposed shell-model Hamiltonians in this interesting and challenging region of the chart of nuclides.
    11/2012;
  • E. C. Simpson, J. A. Tostevin
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    ABSTRACT: We discuss intermediate-energy single-nucleon removal reactions from deformed projectile nuclei. The removed nucleon is assumed to originate from a given Nilsson model single-particle state and the inclusive cross sections, to all rotational states of the residual nucleus, are calculated. We investigate the sensitivity of both the stripping cross sections and their momentum distributions to the assumed size of the model space in the Nilsson model calculations and to the shape of the projectile and residue. We show that the cross sections for small deformations follow the decomposition of the Nilsson state in a spherical basis. In the case of large and prolate projectile deformations the removal cross sections from prolate-like Nilsson states, having large values for the asymptotic quantum number nz, are reduced. For oblate-like Nilsson states, with small nz, the removal cross sections are increased. Whatever the deformation, the residue momentum distributions are found to remain robustly characteristic of the orbital angular momentum decomposition of the initial state of the nucleon in the projectile.
    Physical Review C 11/2012; 86(5). · 3.72 Impact Factor
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    ABSTRACT: We report final-state-exclusive measurements of the light charged fragments in coincidence with ^{26}Ne residual nuclei following the direct two-proton removal from a neutron-rich ^{28}Mg secondary beam. A Dalitz-plot analysis and comparisons with simulations show that a majority of the triple-coincidence events with two protons display phase-space correlations consistent with the (two-body) kinematics of a spatially correlated pair-removal mechanism. The fraction of such correlated events, 56(12)%, is consistent with the fraction of the calculated cross section, 64%, arising from spin S=0 two-proton configurations in the entrance-channel (shell-model) ^{28}Mg ground state wave function. This result promises access to an additional and more specific probe of the spin and spatial correlations of valence nucleon pairs in exotic nuclei produced as fast secondary beams.
    Physical Review Letters 11/2012; 109:202505. · 7.73 Impact Factor
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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
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    J. A. Tostevin
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    ABSTRACT: Many of the most exotic neutron-proton asymmetric nuclei are produced in relatively small numbers in high-energy fragmentation reactions. They are produced as fast secondary beams with energies of 100 MeV per nucleon or more. Developments made and recent results that both exploit and assess fast one- and two-nucleon removal reactions from such secondary beams are reviewed. This includes very recent work that interfaces the sudden, eikonal reaction models used with more ab-initio nuclear structure inputs. The potential use of neutron pick-up reactions to study particle-like states in exotic nuclei is also outlined.
    Journal of Physics Conference Series 09/2012; 381(1):2007-.
  • Y. Kucuk, J. A. Tostevin
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    ABSTRACT: There are now several well-studied instances where very neutron-rich light nuclei at or near the neutron drip-line, such as 6He, 11Li and 14Be, have been found to have a Borromean three-body structure. Such systems are modelled effectively as a well-bound core nucleus plus two weakly-bound valence neutrons, where none of the two-body subsystems forms a bound state. It is now known that the heaviest particle-bound carbon isotope, 22C, shares these properties. We discuss a development of four-body reaction model calculations, using the fast adiabatic approximation, that is particularly well-suited for a quantitative analysis of reactions of such neutron-rich nuclei with a target nucleus at beam energies of order 100-300 MeV per nucleon; energies available at new and future radioactive ion beam (RIB) facilities. The 22C projectile wave function is calculated using the 20C core plus two-valence neutron three-body description.
    Journal of Physics Conference Series 09/2012; 381(1):2109-.

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