Publications

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ABSTRACT: Shell evolution is studied in the neutronrich silicon isotopes 36,38,40 Si using neutron singleparticle strengths deduced from oneneutron 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 crossshell excitations provide the first quantitative support for repulsive contributions to the crossshell T = 1 interaction arising from threenucleon forces. 
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ABSTRACT: The 9Be(28Mg,27Na) oneproton 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 weaklybound protons from lighter nuclei. The role of transitions of the proton between different bound singleparticle configurations upon the elastic breakup cross section is also quantified in this wellbound 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 
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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 intermediateenergy beam of neutron or protonrich nuclei. In each such measurement, one obtains the inclusive cross section for the population of all bound final states of the mass A1 reaction residue. These data, from different regions of the nuclear chart, and that involve weakly and stronglybound nucleons, are compared with theoretical expectations. These calculations include an approximate treatment of the reaction dynamics and shellmodel descriptions of the projectile initial state, the bound final states of the residues, and the singleparticle 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 
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ABSTRACT: The structure of the neutronrich silicon isotopes $^{36,38,40}\mathrm{Si}$ was studied by oneneutron and oneproton 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 largescale shellmodel 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 lowmomentum 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 twoproton decay lifetime
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ABSTRACT: The ground state twoproton decay lifetime of $^{19}\mathrm{Mg}$, populated by the oneneutron knockout of an intermediateenergy $^{20}\mathrm{Mg}$ 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 twoproton decay product $^{17}\mathrm{Ne}$. The lifetime was deduced from fits to the measured energyloss 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 shortlived states beyond the proton drip line.Physical Review C 07/2014; 90(1). DOI:10.1103/PhysRevC.90.014301 · 3.88 Impact Factor 
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ABSTRACT: Cross sections of 1nremoval reactions from the neutronrich 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 Coulombdominated 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 pwave neutron singleparticle strength. These results confirm that ^{37}Mg lies near the edge of the "island of inversion" and has a sizable pwave 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 1nremoval reactions from the neutronrich 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 Coulombdominated 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 pwave neutron singleparticle strength. These results confirm that ^{37}Mg lies near the edge of the "island of inversion" and has a sizable pwave neutron halo component, the heaviest such system identified to date.Physical Review Letters 06/2014; 112(24):242501. DOI:10.1103/PhysRevLett.112.242501 · 7.73 Impact Factor 
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ABSTRACT: The halo structure of Ne31 is studied using 1nremoval 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 groundstate reveals Ne31 to have a small neutron separation energy, 0.150.10+0.16 MeV, and spinparity 3/2. Consistency of the data with reaction and largescale shellmodel calculations identifies Ne31 as deformed and having a significant pwave 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 · 7.73 Impact Factor 
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ABSTRACT: Excited states in the neutronrich N=38, 36 nuclei Ti60 and Ti58 were populated in nucleonremoval reactions from V61 projectiles at 90 MeV/nucleon. The γray transitions from such states in these Ti isotopes were detected with the advanced γray tracking array GRETINA and were corrected event by event for large Doppler shifts (v/c∼0.4) using the γray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutronrich N=36, 38 Fe and Cr toward the Ti and Ca isotones. In fact, Ti58,60 provide some of the most neutronrich 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 neutronrich N=38,36 nuclei \nuc{60}{Ti} and \nuc{58}{Ti} were populated in nucleonremoval reactions from \nuc{61}{V} projectiles at 90~MeV/nucleon. The \gammaray transitions from such states in these Ti isotopes were detected with the advanced \gammaray tracking array GRETINA and were corrected eventbyevent for large Doppler shifts (v/c \sim 0.4) using the \gammaray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutronrich N=36,38 Fe and Cr toward the Ti and Ca isotones. In fact, \nuc{58,60}{Ti} provide some of the most neutronrich benchmarks accessible today for calculations attempting to determine the structure of the potentially doublymagic nucleus \nuc{60}{Ca}. 
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ABSTRACT: The heaviest particlebound carbon isotope, $^{22}$C, is thought to have a Borromean threebody structure. We discuss and compare fourbody, i.e. threebody projectile plus target, reaction model calculations of reaction cross sections for such systems that use the fast adiabatic approximation. These methods are efficient and wellsuited for quantitative analyses of reactions of neutronrich 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 twovalence neutron threebody 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. 
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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, protonrich isotopes cannot be formed during those processes, because they are shielded from the sprocess flow and rprocess betadecay 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 timereversed 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. 
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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 twoneutron 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 largescale shellmodel 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 twoneutron 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 shellmodel 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 · 7.73 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 threebody 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 threebody 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 
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ABSTRACT: Newly published 42Si γray spectra and a finalstateinclusive 42Si production cross section value, obtained in a higherstatistics intermediateenergy twoproton removal experiment from 44S, are considered in terms of the finalstateexclusive cross sections computed using proposed shellmodel effective interactions for nuclei near N=28. Specifically, we give cross section predictions when using the two nucleon amplitudes of the twoproton overlaps <42Si(Jπ)44S> computed using the newly proposed sdpfmu shellmodel Hamiltonian. We show that these partial cross sections or their longitudinal momentum distributions should enable a lesstentative interpretation of the measured gammaray spectra and provide a more quantitative assessment of proposed shellmodel 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: Oneneutron knockout from 51–55Sc
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ABSTRACT: Results are presented from a oneneutron knockout experiment at relativistic energies of ~ 420 A MeV on 5155Sc using the GSI Fragment Separator as a twostage magnetic spectrometer and the MINIBALL array for gammaray detection. Inclusive longitudinal momentum distributions and crosssections 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 crosssections using eikonal knockout theory and spectroscopic factors from shell model calculations using the GXPF1A interaction. The measured inclusive knockout crosssections generally follow the trends expected theoretically and given by the spectroscopic strength predicted from the shell model calculations. However, the deduced L = 1 crosssections are generally 3040% 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 lowmomentum interactions V_lowk obtained by evolving a chiral N3LO nucleonnucleon potential.European Physical Journal A 12/2012; 48(12):191 110. DOI:10.1140/epja/i2012121915 · 2.42 Impact Factor 
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ABSTRACT: Newlypublished $^{42}$Si gammaray spectra and a finalstateinclusive $^{42}$Si production cross section value, obtained in a higherstatistics intermediateenergy twoproton removal experiment from $^{44}$S, are considered in terms of the finalstateexclusive cross sections computed using proposed shellmodel effective interactions for nuclei near N=28. Specifically, we give cross section predictions when using the two nucleon amplitudes of the twoproton overlaps $<^{42}$Si$ (J^\pi)\,\,^{44}$S$>$ computed using the newlyproposed {\sc sdpfmu} shellmodel Hamiltonian. We show that these partial cross sections or their longitudinal momentum distributions should enable a lesstentative interpretation of the measured gammaray spectra and provide a more quantitative assessment of proposed shellmodel Hamiltonians in this interesting and challenging region of the chart of nuclides. 
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ABSTRACT: We discuss intermediateenergy singlenucleon removal reactions from deformed projectile nuclei. The removed nucleon is assumed to originate from a given Nilsson model singleparticle 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 prolatelike Nilsson states, having large values for the asymptotic quantum number nz, are reduced. For oblatelike 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). DOI:10.1103/PhysRevC.86.054603 · 3.88 Impact Factor 
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ABSTRACT: We report finalstateexclusive measurements of the light charged fragments in coincidence with ^{26}Ne residual nuclei following the direct twoproton removal from a neutronrich ^{28}Mg secondary beam. A Dalitzplot analysis and comparisons with simulations show that a majority of the triplecoincidence events with two protons display phasespace correlations consistent with the (twobody) kinematics of a spatially correlated pairremoval 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 twoproton configurations in the entrancechannel (shellmodel) ^{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(20):202505. DOI:10.1103/PhysRevLett.109.202505 · 7.73 Impact Factor

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