Projectile deformation effects on single-nucleon removal reactions

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 n z, are reduced. For oblate-like Nilsson states, with small n z, 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. � 2012 American Physical Society

Two-proton removal from 44 S and the structure of 42 Si

Newly published 42Si ?-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(J?)| 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. � 2013 American Physical Society

Correlations in Intermediate Energy Two-Proton Removal Reactions

We report final-state-exclusive measurements of the light charged fragments in coincidence with Ne26 residual nuclei following the direct two-proton removal from a neutron-rich Mg28 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) Mg28 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. � 2012 American Physical Society

A Study of Polarization Effects in Subcoulomb (d,p) Reactions.

The accuracy of sub-coulomb (d,p) reaction tensor analysing powers, calculated using the conventional DWBA theory, is investigated. Two outstanding uncertainties in the DWBA method, relating to a) breakup of the deuteron in the coulomb field of the target nucleus, and b) the presence of T R tensor interactions in the deuteron target system, are studied quantitatively. Particular emphasis is placed upon the reaction 208Pb (d,p) 209Pb, at Ed = 9 MeV and Ed = 7 MeV, and its applicability to the measurement of the parameter of Johnson and Santos. Coulomb break up is treated in dipole approximation using perturbation theory and an adiabatic prescription. Special attention is paid to the use of a realistic state dependent interaction in the P-wave n-p continuum. Coulomb and strong tensor T[R] terms, obtained from both elastic scattering and break up models, are discussed and are included through a generalization of the conventional, Local Energy Approximation, DWBA stripping formalism. The resulting coupled differential equations are solved numerically. Results of calculations show that both effects studied must be accounted for in any analysis of precision experimental data at 9 MeV and/ or 7 MeV. Theoretical uncertainties in the strong interaction contribution to the T R potential, to which calculations at 9 MeV are sensitive, are found to be unimportant at an energy of 7 MeV. The effects of singlet ([1]S[0]) breakup of the deuteron are also shown to be completely negligible

A Study of Polarization Effects in Subcoulomb (d,p) Reactions.

The accuracy of sub-coulomb (d,p) reaction tensor analysing powers, calculated using the conventional DWBA theory, is investigated. Two outstanding uncertainties in the DWBA method, relating to a) breakup of the deuteron in the coulomb field of the target nucleus, and b) the presence of T R tensor interactions in the deuteron target system, are studied quantitatively. Particular emphasis is placed upon the reaction 208Pb (d,p) 209Pb, at Ed = 9 MeV and Ed = 7 MeV, and its applicability to the measurement of the parameter of Johnson and Santos. Coulomb break up is treated in dipole approximation using perturbation theory and an adiabatic prescription. Special attention is paid to the use of a realistic state dependent interaction in the P-wave n-p continuum. Coulomb and strong tensor T[R] terms, obtained from both elastic scattering and break up models, are discussed and are included through a generalization of the conventional, Local Energy Approximation, DWBA stripping formalism. The resulting coupled differential equations are solved numerically. Results of calculations show that both effects studied must be accounted for in any analysis of precision experimental data at 9 MeV and/ or 7 MeV. Theoretical uncertainties in the strong interaction contribution to the T R potential, to which calculations at 9 MeV are sensitive, are found to be unimportant at an energy of 7 MeV. The effects of singlet ([1]S[0]) breakup of the deuteron are also shown to be completely negligible

One- and two-neutron removal from the neutron-rich carbon isotopes

Reactions that involve the fast removal (knockout) of one and two neutrons from the neutron-rich carbon isotopes, C15-19, by light nuclear targets are studied within an eikonal reaction model. Shell model calculations are used to describe the excitation energy spectra and the structures of the carbon isotopes. The calculated one-neutron knockout cross sections from the CA isotopes, to particle-bound configurations of the CA-1 residues, are in agreement with the available experimental data. The two-neutron removal cross sections, producing CA-2 residues, receive contributions from both the direct, single-step two-neutron knockout and the indirect mechanism, involving single-neutron removal strength to neutron-unbound excited states in the CA-1 system followed by neutron emission. The latter two-step reaction mechanism is shown to be dominant. The empirical odd-even staggering of the single-neutron separation energies along the carbon isotopic chain is reflected in the two-neutron removal data. This staggering and the magnitudes of the two-neutron removal cross sections are reproduced qualitatively by the theoretical calculations. � 2009 The American Physical Society

Correlations probed in direct two-nucleon removal reactions

Final-state-exclusive momentum distributions of fast, forward-traveling residual nuclei, following two-nucleon removal from fast secondary radioactive beams of projectile nuclei, can and have now been measured. Assuming that the most important reaction mechanism is the sudden direct removal of a pair of nucleons from a set of relatively simple, active shell-model orbital configurations, such distributions were predicted to depend strongly on the total angular momentum I carried by the two nucleons-the final-state spin for spin 0+ projectiles. The sensitivity of these now-accessible observables to specific details of the (correlated) two-nucleon wave functions is of importance. We clarify that it is the total orbital angular momentum L of the two nucleons that is the primary factor in determining the shapes and widths of the calculated momentum distributions. It follows that, with accurate measurements, this dependence upon the L makeup of the two-nucleon wave functions could be used to assess the accuracy of (shell- or many-body-) model predictions of these two-nucleon configurations. By using several tailored examples, with specific combinations of active two-nucleon orbitals, we demonstrate that more-subtle structure aspects may be observed, allowing such reactions to probe and/or confirm the details of theoretical model wave functions. � 2010 The American Physical Society

Longitudinal momentum distributions of the reaction residues following fast two-nucleon knockout reactions

The longitudinal momentum distributions of the cross sections of heavy projectile-like residues after fast, direct two-like-nucleon knockout reactions are discussed. Both the two-nucleon inelastic breakup (stripping) and the stripping-diffraction remova

Population of low-seniority isomeric states of 206 Hg by two-proton knockout reactions at relativistic energies

We consider calculations of direct two-proton removal reactions from a relativistic-energy Pb208 beam incident on a Be9 target, leading to the population of two-proton-hole isomeric configurations in Hg206. The calculated 5- and 10+ isomeric state population ratios of 18.8% and 4.7%, respectively, are in fair agreement with the values deduced experimentally (21.9-2.9+1.2% and 3.1-1.2+1.0%); however, the high density of final states in this case complicates the prediction of isomeric ratios for those states fed by unobserved prompt ?-ray cascades. We also consider and quantify the effects of selection of events, according to the longitudinal momenta of the reaction residues, on the measured isomeric ratios for high-spin final states and, hence, of the potential to exploit such sensitivity to gain further insight into the reaction mechanism. � 2009 The American Physical Society

Differential isomeric ratios following two-proton knockout from 208 Pb

A theoretical analysis of experimental results for isomeric ratios of Hg206 residues, following direct two-proton knockout from Pb208, is extended to consider their dependence on the longitudinal momentum of the residues. Despite the significant degree of experimental (thick target) momentum broadening, the isomeric ratio retains a significant sensitivity to this momentum. The measured distribution is well reproduced by calculations that assume the direct two-proton removal mechanism. Adjustments to the isomeric residue momentum distribution due to possible additional (unobserved) prompt feeding of the 5- isomeric state are also discussed. � 2010 The American Physical Society