## Publications

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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.Physical Review C 04/2015; 91(4). DOI:10.1103/PhysRevC.91.041302 · 3.88 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Direct reaction techniques are powerful tools to study the single-particle nature of nuclei. Performing direct reactions on short-lived nuclei requires radioactive ion beams produced either via fragmentation or the Isotope Separation OnLine (ISOL) method. Some of the most interesting regions to study with direct reactions are close to the magic numbers where changes in shell structure can be tracked. These changes can impact the final abundances of explosive nucleosynthesis. The structure of the chain of tin isotopes is strongly influenced by the Z = 50 proton shell closure, as well as the neutron shell closures lying in the neutron-rich, N = 82, and neutron-deficient, N = 50, regions. Here, we present two examples of direct reactions on exotic tin isotopes. The first uses a one-neutron transfer reaction and a low-energy reaccelerated ISOL beam to study states in 131Sn from across the N = 82 shell closure. The second example utilizes a one-neutron knockout reaction on fragmentation beams of neutron-deficient 106,108Sn. In both cases, measurements of γ rays in coincidence with charged particles proved to be invaluable.Acta Physica Polonica Series B 03/2015; 46(3):537. DOI:10.5506/APhysPolB.46.537 · 1.00 Impact Factor - [Show abstract] [Hide abstract]

**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.Physical Review C 12/2014; 90(6). DOI:10.1103/PhysRevC.90.064615 · 3.88 Impact Factor - [Show abstract] [Hide abstract]

**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.Physical Review C 09/2014; 90(5). DOI:10.1103/PhysRevC.90.057602 · 3.88 Impact Factor - [Show abstract] [Hide abstract]

**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 -
##### Article: Mg 19 two-proton decay lifetime

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**ABSTRACT:**The ground state two-proton decay lifetime of $^{19}\mathrm{Mg}$, populated by the one-neutron knockout of an intermediate-energy $^{20}\mathrm{Mg}$ radioactive beam, was measured utilizing a new experimental technique. A thin silicon detector positioned at varying distances (0.0\char21{}1.0 mm) downstream of the reaction target measured the energy loss of $^{19}\mathrm{Mg}$ and the two-proton decay product $^{17}\mathrm{Ne}$. The lifetime was deduced from fits to the measured energy-loss line shapes and depended upon the contribution of prompt reaction processes to the yield of $^{17}\mathrm{Ne}$. For relative $^{17}\mathrm{Ne}$ prompt contributions from 82% to 92%, the extracted lifetimes ranged from $1.{75}_{$-${}0.42}^{+0.43}$ to $6.{4}_{$-${}2.7}^{+2.4}$ ps. The results are consistent with the previously reported $^{19}\mathrm{Mg}$ lifetime measurement and serve as both an important complementary study and a validation of this new technique, which can provide lifetime information for short-lived states beyond the proton drip line.Physical Review C 07/2014; 90(1). DOI:10.1103/PhysRevC.90.014301 · 3.88 Impact Factor - [Show abstract] [Hide abstract]

**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. · 7.51 Impact Factor - [Show abstract] [Hide abstract]

**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. DOI:10.1103/PhysRevLett.112.242501 · 7.51 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**A measurement of the direct two-proton removal from $^{42}\mathrm{Si}$ has provided the first structural information on the $N=28$ isotone $^{40}\mathrm{Mg}$. The value for the inclusive cross section for two-proton removal from $^{42}\mathrm{Si}$ of 40${}_{\ensuremath{-}17}^{+27}$ $\ensuremath{\mu}$b is significantly lower than that predicted by structure calculations using the recent SDPF-MU shell-model effective interaction combined with eikonal reaction theory. This observed discrepancy is consistent with the interpretation that only one of the predicted low-lying 0${}^{+}$ states in $^{40}\mathrm{Mg}$ is bound. A two-state mixing analysis describing two-proton knockout cross sections along $N=28$ provides support for the interpretation of a prolate-deformed $^{40}\mathrm{Mg}$ ground state.Physical Review C 04/2014; 89(4). DOI:10.1103/PhysRevC.89.041303 · 3.88 Impact Factor - [Show abstract] [Hide abstract]

**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. DOI:10.1103/PhysRevLett.112.142501 · 7.51 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The (p,t) transfer reaction is being studied for its potential use in surrogate reaction analyses. A theoretical model has been developed to predict spin-parity distributions of final states excited in the reaction. The model, after comparisons with experimental data, may provide a predictive capability to identify candidate isotopes for measurement. Preliminary results are presented for the Zr-92(p,t)Zr-90 reaction at incident proton energy E-p = 28.5 MeV. New experimental data for this reaction at a similar energy, and for several other stable Zr isotopes, will soon be available.Nuclear Data Sheets 04/2014; 118:295-297. DOI:10.1016/j.nds.2014.04.062 · 3.35 Impact Factor - [Show abstract] [Hide abstract]

**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. · 7.51 Impact Factor - [Show abstract] [Hide abstract]

**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}.Physical Review Letters 02/2014; 112(11). DOI:10.1103/PhysRevLett.112.112503 · 7.51 Impact Factor - [Show abstract] [Hide abstract]

**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.Physical Review C 12/2013; 89(3). DOI:10.1103/PhysRevC.89.034607 · 3.88 Impact Factor - [Show abstract] [Hide abstract]

**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. - [Show abstract] [Hide abstract]

**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. · 7.51 Impact Factor - [Show abstract] [Hide abstract]

**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. DOI:10.1103/PhysRevLett.111.072501 · 7.51 Impact Factor - [Show abstract] [Hide abstract]

**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). DOI:10.1103/PhysRevC.87.064613 · 3.88 Impact Factor - [Show abstract] [Hide abstract]

**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; 87(2):27601-. DOI:10.1103/PhysRevC.87.027601 · 3.88 Impact Factor -
##### Article: One-neutron knockout from 51–55Sc

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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. DOI:10.1140/epja/i2012-12191-5 · 2.42 Impact Factor

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