Exploring manifestation and nature of a dineutron in two-neutron emission using a dynamical dineutron model

Emission of two neutrons or two protons in reactions and decays is often discussed in terms of "dineutron" or "diproton" emission. The discussion often leans intuitively on something described by Migdal-Watson approximation. In this work we propose a way to formalize situations of dineutron emission. It is demonstrated that properly formally defined dineutron emission may reveal properties which are drastically different from those traditionally expected, and properties which are actually observed in three-body decays.Comment: 11 pages, 11 Figure

Thomas-Ehrman effect in a three-body model: 16^{16}Ne case

The dynamic mechanism of the Thomas-Ehrman shift is studied in three-cluster systems by example of $^{16}$Ne and $^{16}$C isobaric mirror partners. We predict configuration mixings for $0^+$ and $2^+$ states in $^{16}$Ne and $^{16}$C. Large isospin symmetry breaking on the level of wave function component weights is demonstrated for these states and discussed as three-body mechanism of Thomas-Ehrman shift. It is shown that the description of the Coulomb displacement energies requires a consistency among three parameters: the $^{16}$Ne decay energy $E_T$, the $^{15}$F ground state energy $E_r$, and the configuration mixing parameters for the $^{16}$Ne/$^{16}$C $0^+$ and $2^+$ states. Basing on this analysis we infer the $^{15}$F $1/2^+$ ground state energy to be $E_r=1.39-1.42$ MeV.Comment: 10 pages 8 figure

High precision studies of soft dipole mode in two-neutron halo nuclei: 6^{6}He case

The "soft dipole" E1 strength function is calculated for the transition from the $^{6}$He $0^+$ ground state to the $1^-$ continuum $^{4}$He+$n$+$n$. The calculations were performed within the hyperspherical harmonics formalism. The sensitivity of the results to the $^{6}$He ground state structure and to final state interactions, are analyzed. The large-basis calculations show the reliably converged results for soft dipole strength function and for momentum correlations of the ^{6}\mbox{He} \rightarrow \, ^{4}He+$n$+$n$ dissociation products. Transition mechanisms are analyzed based on the momentum correlations. The comparison with experimental data is provided.Comment: 16 pages, 17 figure

Pauli-principle driven correlations in four-neutron nuclear decays

Mechanism of simultaneous non-sequential four-neutron ($4n$) emission (or `true' $4n$-decay) has been considered in phenomenological five-body approach. This approach is analogous to the model of the direct decay to the continuum often applied to $2n$- and $2p$-decays. It is demonstrated that $4n$-decay fragments should have specific energy and angular correlations reflecting strong spatial correlations of `valence' nucleons orbiting in their $4n$-precursors. Due to the Pauli exclusion principle, the valence neutrons are pushed to the symmetry-allowed configurations in the $4n$-precursor structure, which causes a `Pauli focusing' effect. Prospects of the observation of the Pauli focusing have been considered for the $4n$-precursors $^7$H and $^{28}$O. Fingerprints of their nuclear structure or/and decay dynamics are predicted

Anomalous population of 10^{10}He states in reactions with 11^{11}Li

Structure with the lowest energy observed in the $^{10}$He spectrum populated in the proton knockout reaction with $^{11}$Li beam has a peak at $1.2-1.5$ MeV. This peak is usually interpreted as a resonant $0^+$ ground state of $^{10}$He. Our theoretical calculations indicate that this peak is likely to be a pileup of $1^-$, $0^+$, and $2^+$ excitations with very similar shapes. %We predict a very specific nature of the $1^-$ excitation in $^{10}$He. Moreover, the ``soft'' $1^-$ excitation appears to be the lowest one in energy. Such an anomalous continuum response is traced to the halo structure of $^{11}$Li providing extreme low energy shift to all the expected continuum excitations. Competitions of the initial state structure (ISS) and the final state interaction (FSI) effects on the spectrum and three-body correlations in $^{10}$He are discussed. Analogous effect of the extreme low-energy shift could also be expected in other cases of $2n$ emitters populated in reactions with halo nuclei. Simplified example of the $^{10}$He spectrum in $\alpha$ knockout from $^{14}$Be, is given. We also discuss limits on the properties of $^{9}$He stemming from the observed $^{10}$He spectrum.Comment: 10 pages, 13 figure

Possibility to study a two-proton halo in 17^{17}Ne

The nuclide $^{17}$Ne is studied theoretically in a three-body $^{15}$O+$p$+$p$ model. We demonstrate that the experimental condition for existence of a proton halo in $^{17}$Ne can be reasonably quantified in terms of $s/d$ configuration mixing. We discuss experimental evidences for a proton halo in $^{17}$Ne. We define which kind of experimental data could elucidate this issue.Comment: 5 pages, 5 figure

Decay mechanism and lifetime of 67^{67}Kr

The lifetime of the recently discovered $2p$ emitter $^{67}$Kr was recently found considerably below the lower limit predicted theoretically. This communication addresses this issue.Different separation energy systematics are analyzed and different mechanisms for $2p$ emission are evaluated. It is found that the most plausible reason for this disagreement is a decay mechanism of $^{67}$Kr, which is not "true $2p$" emission, but "transition dynamics" on the borderline between true $2p$ and sequential $2p$ decay mechanisms. If this is true, this imposes stringent limits $E_r=1.35-1.42$ MeV on the ground state energy of $^{66}$Br relative to the $^{65}$Se-$p$ threshold.Comment: 7 pages, 8 figure

Two-proton radioactivity and three-body decay. V. Improved momentum distributions

Nowadays quantum-mechanical theory allows one to reliably calculate the processes of 2p radioactivity (true three-body decays) and the corresponding energy and angular correlations up to distances of the order of 1000 fm. However, the precision of modern experiments has now become sufficient to indicate some deficiency of the predicted theoretical distributions. In this paper we discuss the extrapolation along the classical trajectories as a method to improve the convergence of the theoretical energy and angular correlations at very large distances (of the order of atomic distances), where only the long-range Coulomb forces are still operating. The precision of this approach is demonstrated using the "exactly" solvable semianalytical models with simplified three-body Hamiltonians. It is also demonstrated that for heavy 2p emitters, the 2p decay momentum distributions can be sensitive to the effect of the screening by atomic electrons. We compare theoretical results with available experimental data.Comment: 13 pages, 18 figure

Two-proton radioactivity and three-body decay. IV. Connection to quasiclassical formulation

We derive quasiclassical expressions for the three-body decay width and define the ``preexponential'' coefficients for them. The derivation is based on the integral formulae for the three-body width obtained in the semianalytical approach with simplified three-body Hamiltonian [L.V. Grigorenko and M.V.\ Zhukov, arXiv:0704.0920v1]. The model is applied to the decays of the first excited $3/2^{-}$ state of $^{17}$Ne and $3/2^{-}$ ground state of $^{45}$Fe. Various qualitative aspects of the model and relations with the other simplified approaches to the three-body decays are discussed.Comment: 9 Pages, 2 figure