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Register a new userEvidence for the Strong Dominance of Proton-Neutron Correlations in Nuclei
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We analyze recent data from high-momentum-transfer $(p,pp)$ and $(p,ppn)$ reactions on Carbon. For this analysis, the two-nucleon short-range correlation (NN-SRC) model for backward nucleon emission is extended to include the motion of the NN-pair in the mean field. The model is found to describe major characteristics of the data. Our analysis demonstrates that the removal of a proton from the nucleus with initial momentum 275-550 MeV/c is $92^{+8}_{-18}%$ of the time accompanied by the emission of a correlated neutron that carries momentum roughly equal and opposite to the initial proton momentum. Within the NN-SRC dominance assumption the data indicate that the probabilities of $pp$ or $nn$ SRCs in the nucleus are at least a factor of six smaller than that of $pn$ SRCs. Our result is the first estimate of the isospin structure of NN-SRCs in nuclei, and may have important implication for modeling the equation of state of asymmetric nuclear matter.
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A regular pattern, revealing the leading role of the light-fragment nuclear charge, is found to emerge from a consistent analysis of the experimental information collected recently on low-energy asymmetric fission of neutron-deficient nuclei around lead. The observation is corroborated by a theoretical investigation within a microscopic framework, suggesting the importance of proton configurations driven by quadrupole-octupole correlations. This is in contrast to the earlier theoretical interpretations in terms of dominant neutron shells. The survey of a wider area of the nuclear chart by a semi-empirical approach points to the lack of understanding of the competition between the different underlying macroscopic and microscopic forces in a quantitative manner. Combined with previously identified stabilizing forces, the present finding shows a striking connection between the old (actinide) and new (pre-actinide) islands of asymmetric fission which could steer the strive for an unified theory of fission.
We present expressions for the matrix elements of the spin--spin operator $vec S_{rm n}cdotvec S_{rm p}$ in a variety of coupling schemes. These results are then applied to calculate the expectation value $langlevec S_{rm n}cdotvec S_{rm p}rangle$ in eigenstates of a schematic Hamiltonian describing neutrons and protons interacting in a single-$l$ shell through a Surface Delta Interaction. The model allows us to trace $langlevec S_{rm n}cdotvec S_{rm p}rangle$ as a function of the competition between the isovector and isoscalar interaction strengths and the spin--orbit splitting of the $j=lpm frac{1}{2}$ shells. We find negative $langlevec S_{rm n}cdotvec S_{rm p}rangle$ values in the ground state of all even--even $N=Z$ nuclei, contrary to what has been observed in hadronic inelastic scattering at medium energies. We discuss the possible origin of this discrepancy and indicate directions for future theoretical and experimental studies related to neutron--proton spin--spin correlations.
A detailed analysis of the effect of tensor correlations on one- and two-body densities and momentum distributions of complex nuclei is presented within a linked cluster expansion providing reliable results for the ground state properties of nuclei calculated with realistic interactions.
It is shown that the renormalized nuclear deformations in different mass regions can be globally scaled by two probability partition factors of Boltzmann-like distribution, which are derived from the competing valence $np$ and like-nucleon interactions. The partition factors are simply related to the probabilities of anti-parallel and fully-aligned orientations of the angular momenta of the neutrons and protons in the valence $np$ pairs, responsible for spherical- and deformed-shape phases, respectively. The partition factors derived from the renormalized deformations are also present in the new scaling law for the energies of the first $2^+$ states. A striking concordance between the distributions of the renormalized deformations and of the newly introduced parameter for the energies of the first $2^+$ states over the extended mass region from Ge to Cf is achieved, giving strong support to the existence of two phases: anti-aligned and fully-aligned subsets of $np$ pairs.
We discuss the sensitivity of fission barrier for heavy neutron-rich nuclei to fission paths in the two dimensional neutron-proton quadrupole plane. To this end, we use the constrained Skyrme-Hartree-Fock + BCS method, and examine the difference of fission barriers obtained with three constraining operators, that is, the neutron, proton, and mass quadrupole operators. We investigate $^{220}$U, $^{236}$U, and $^{266}$U, %from proton-rich to neutron-rich uranium isotopes, that is relevant to r-process nucleosynthesis. We find that the fission barrier heights are almost the same among the three constraining operators even for neutron-rich nuclei, indicating that the usual way to calculate fission barriers with the mass quadrupole operator is well justified. We also discuss the difference between proton and neutron deformation parameters along the fission paths.
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