Probing the shell model using nucleon knockout reactions

Abstract

Abstract. We assess the sensitivity of the intermediate energy, direct two-nucleon knockout reaction mechanism to details of the correlated motions of the two removed nucleons -as might be revealed by measurements of partial cross sections to different final states of the mass A−2 reaction residues. Comparisons of new reaction calculations with recent data for exotic nuclei in the sd-shell suggest that this sensitivity is significant and that the coherence of the twonucleon configurations to the reaction allows small wave function components to be probed. The relative yields for the different J π transitions are well described by the USD shell model calculations, however the measured cross sections require an overall suppression of the shell model two-nucleon spectroscopic strengths. IntroductionOne-nucleon knockout reactions from intermediate-energy (projectile fragmentation) beams are an increasingly important tool for mapping the single-particle structures of nuclei with exotic neutron-and proton-number combinations. The single-nucleon removal cross sections are large (several tens of mb) and both the efficiency and selectivity of the associated reaction measurements are high. This allows an examination of orbitals of both the weakly-bound (excess) and strongly-bound (deficient) nucleon species at their respective (and very displaced) Fermi surfaces [1,2]. Exclusive (with respect to the knockout residue final states) single-nucleon knockout reaction analyses on light nuclear targets (usually 9 Be and 12 C), with energies from 40 MeV -1 GeV/nucleon, are now able to assess structure model predictions of level orderings and shell-gaps and of details of level occupations and spectroscopic strengths in some of the most rare, unstable species. Examples are discussed in [1,3,4] and in references therein.In contrast, two-nucleon removal reactions -and what they can teach us -are less well understood. The intrinsic two-nucleon removal cross sections are now smaller (of order 1 mb) and their theoretical interpretation is also more complicated. Different [j 1 ⊗ j 2 ] configurations of the two-nucleons now contribute coherently to the cross section [5] resulting in a less transparent connection between data and the underlying structures. An added complication is that the reactions will, in general, proceed by both (i) a sudden two-nucleon knockout component with direct population of the mass A−2 residue, and (ii) indirect, two-step mechanisms -involving one-nucleon knockout to excited, intermediate fragments of mass A−1, followed by nucleon

Two-proton knockout fromMg32: Intruder amplitudes inNe30Fallon1, Rodriguez‐Vieitez2, Macchiavelli3 et al. 2010Phys. Rev. C441231View full textAdd to dashboardCite

“…This is a robust quantity independent of unobserved higher lying transitions. This value is reproduced by all calculations and is substantially larger than the ∼25% value expected if the protons were uncorrelated [18], reflecting the enhanced spatial correlations associated with time-reversed J = 0 + pairs. The 1443 keV transition is placed in cascade with the 792 keV transition and could correspond to the decay from either of the J = 4 (Fig.…”

“…There, and more recently in [16,17] for the associated momentum spectra of the two-nucleon removal reaction residues, it is shown that the reaction dynamics restricts these direct removal events to grazing collisions between the projectile and target. This defines a (near cylindrical) volume through the projectile surface in which the collision with the target samples the joint position and momentum amplitudes of the wave functions of the two removed nucleons [18]. The shell model then provides this two-nucleon amplitude information.…”

Correlations probed in direct two-nucleon removal reactions

“…2 and 3 of Ref. [18], and will be emphasized here, the total angular momentum of the final state and the detailed TNA of the wave function can strongly affect the two-nucleon joint position probability, its projection and the magnitude of the removal cross sections. The (shell-model) structural correlations may also enhance or suppress particular total orbital angular momenta and so may affect the residue momentum distributions also.…”

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

“…The structure input now enters in the form of the initial-state to final-state two-nucleon overlap function, in which the two active (valence) nucleons are correlated and their configurations appear coherently. The sensitivity of two-nucleon knockout reaction yields to these two-nucleon correlations has been discussed in some detail elsewhere [21,23,29,30]. This is not our prime interest here.…”