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

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

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

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

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

Assessment of cognitive abilities in multiethnic countries: The case of the Wolof and Mandinka in the Gambia

Background: The use of cognitive tests is increasing in Africa but little is known about how such tests are affected by the great ethnic and linguistic diversity on the continent. Aim: To assess ethnic and linguistic group differences in cognitive test performance in the West African country of the Gambia and to investigate the sources of these differences. Samples: Study 1 included 579 participants aged 14–19 years from the Wolof and Mandinka ethnic groups of the Gambia. Study 2 included 41 participants aged 12–18 years from the two ethnic groups. Methods: Study 1 assessed performance on six cognitive tests. Participants were also asked about their history of education, residence in the city, parental education, and family socio-economic status. Study 2 assessed performance on two versions of the digit span test. Recall of the numbers 1–5 were compared with recall of numbers 1–9 for both the Wolof (who count in base 5) and the Mandinka (who count in base 10). Results: Study 1 established that Wolof performance was lower than that of the Mandinka on five out of six cognitive tests. In four of these tests, group differences were partially mediated by participation in primary school and migration to the city. Group differences were substantial for the digit span test and were not attenuated by mediating variables. Study 2 found that digit span among the Wolof was shorter than that of the Mandinka for numbers 1–9 but not for numbers 1–5. Conclusions: Several suggestions are made on how to consider the ethnicity, language, education, and residence (urban vs. rural) of groups when conducting comparative cognitive assessments or collecting normative data

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

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