Measurement of Radii in the Triplet 6He-6Li-6Be and the Possibility of Neutron-Proton Halo

1. Possible existence of neutron-proton halo in 6Li A.S.Demyanova, A.A.Ogloblin, A.N.Danilov, T.L.Belyaeva, P.N. Terehin The 3rd international conference on particle physics and astrophysics

2. The present report is devoted to the investigation of the properties of the states of the triplet 6He - 6Li - 6Be. The main thing is to measure the radii of triplet states.

3. MDM method for determining radii with halo We have developed the Modified diffraction model of MDM, as a method allowing to determine the radii of excited states. It was successfully used to analyze alpha cluster states and states with a neutron halo. The anomalously large radius of the valence neutron (neutrons) is the primary and most important characteristic of the halo. The use of MDM to determine the radii of states with a halo was verified by independent methods and proved to be particularly effective in observing halos in unstable excited states R*rms = R0.0 + [R*(dif) – R0.0(dif)] R(dif) are determined from positions of diffraction min/max in angular distributions Modified diffraction model (MDM)of inelastic scattering A.N. Danilov et al., Phys.Rev. C(2009)

4. One of the most recent results was the development of an MDM analog for the charge exchange reactions (3He, t), the use of which made it possible to determine the proton halo in the first excited state of the 13N nucleus. It turned out that this state has the same radius as the mirror state 1/2 +, 3.09 MeV in 13C, in spite of the fact that one of them lies under the neutron emission threshold, and the second one is above the proton emission threshold [A.S. Demyanova et al., Physics of Atomic Nuclei, 2017, Vol. 80, No. 5, pp. 831–837.]. This observation allows us to take the next step and try to apply this approach to measure the radii of isobar - analog states.

5. The study of triplets and mirror states in doublets makes it possible to significantly expand knowledge of the halo In many cases, the left-handed triplet members have a neutron halo in the ground or excited states. Therefore, there is every reason to believe that right-handed members can also have a proton halo. However, the wave function of the valence proton due to the presence of the Coulomb barrier can differ greatly from the wave function of the neutron analog, and the appearance of a proton halo is not obvious. Moreover, the right-hand members of triplets and doublets are often unstable. Until now, the theory has not considered a halo in the continuum or, at least, it was thought to be of a different type. Comparing the right and left terms, one can obtain an inaccessible wound material about the halo in a discrete and continuous spectrum (there is already an example of A = 13). A special place is occupied by the average members of triplets with the same number of neutrons and protons. In them, a new type of halo (pn) is possible, which makes it possible to study the transition from the Borromean halo to tango (A = 6).

6. To the question of tango-halo 2n The first studies of the neutron halo led to the discovery of completely new nuclear configurations. In particular, this statement applies to three-particle systems near the neutron stability boundary. The most popular structure here was the so-called. The Borromean scheme in which each pair of particles does not form a bound cluster (for example, in the nucleus 11Li, considered as 9Li + n + n, 10Li and the dineutron are not connected), and together they form a nucleus stable with respect to neutron emission. n n 10Li 10Li 9Li 11Li A large number of papers have been devoted to the study of the Borromean structures), but other schemes were also discussed along with them, for example, those in which one of the three pairs turned out to be loosely related. To emphasize the difference between the latter and other three-particle halos, it was suggested to call such configurations, including similar objects in atomic and molecular physics, "tango-halo"[F. Robicheaux, Phys. Rev. A 60, 1706 (1999)]. In nuclear physics, there were few candidates for belonging to the tango (see the review [AS Jensen, Rev. Mod Phys]). For example, they referred to the nucleus 8B, considered as a 7Be cluster (the binding energy of 3He + 4He is 1.59 MeV) plus the proton-forming proton, whose binding energy is only 137 keV. The question to what extent the correlated motion of 3He and 4He in composition 8B determines the halo properties and whether it is impossible to consider this problem simply as a two-body 7Be + p remained open. Nevertheless, the term "tango - halo" has been preserved in some recent articles [Izosimov I., Yad. Fiz. (in press).

7. The present paper is devoted to the measurement of the radii of states of the triplet 6He-6Li-6Be Among these triplet states, the neutron halo in 6He is well known. A proton-neutron halo is predicted in the excited state of 0+, 3.56 MeV in 6Li, which lies only 137 keV below the 6Li 4He + p + n threshold. Its radius is not known, but it is predicted by about 0.25 Fm larger than the radius of 6He [Arai]. We can expect the appearance of a two-proton halo in the ground state of 6Be. The table shows the known radii of states shown in Fig.

8. The extrema for 2.19 MeV are shifted toward smaller q with respect to the extrema for elastic scattering The radius for 2.19 MeV is increased by ~ 0.46 Fm in comparison with the ground state radius 6Li This increase is due to centrifugal energy

9. 6Li(3He, 3He’) 6Li, E(3He) = 24.6 & 27 MeV MDM: <R(3.56MeV)> = 2.49 ± 0.16 Fm The resulting radius equals within errors to the radius of the g.s. 6He <R (6He, g.s.)> = 2.50 ± 0.05 Fm

10. The diffraction radii of elastic scattering are 3He + 6Li and inelastic with the excitation of 2.19 and 3.56 MeV states in the 6Li nucleus as a function of energy. For comparison, the diffraction radii of elastic scattering 6Li + 12С are also shown. The energy dependence of the diffraction radii for lithium data has the following peculiarity: in the low-energy region the diffraction radii decrease rather steeply, but in the medium-energy region they already reach the plateau. Moreover, experimental data for 3.56 MeV exist only at two close low energies of 24.6 and 27 MeV. Therefore, it is advisable to carry out an experiment at medium energies in the plateau region

11. Conclusions • The resulting radius for the state 0+, 3.56 MeV in 6Li practically equals to the radius of the g.s. of its "Borromean" isobar analogue 6He – 2.49±0.16 . It can be said that 0+, 3.56 MeV in 6Li is "Borromean" also and is neutron-proton halo. • 2) We can not yet confirm the increase in the radius predicted in [Arai]. • 3) There is a suggestion that 0+, 3.56 MeV in 6Li is a tango-halo. In this case pair p-n should be connected. The explanation is absent. • Thus, we can not confirm the transition from the Borromean to the tango structure. • We recall that the spatial structure of the 6He nucleus was predicted to be quite complex, in which correlations of two types appeared: "cigar" and "dineutron." In this connection, the question arises: does the structure of the state change so strongly when passing from 6He to the isobar analog in 6Li, which requires the introduction of a special type of tango-halo? The neutron and proton density distributions for the 3.56 MeV 0+ state of 6Li and the ground state of 6He are presented. Enlarged radius for the 3.56 MeV state was obtained due to more spatially extended proton distribution. It is reasonable because of the Coulomb repulsion.

12. Prospects • 1) Measurements of the cross sections for the reaction 6Li (3He, 3 He ') 6Li at other large energies of 3He ions, which would allow us to determine the radius of the 3.56 MeV state in 6Li with a smaller error. • 2) The increased radii in the first excited states of the 6He-6Li-6Be triplet, which may also have a halo structure, are not excluded. 3) To determine the radius 6Be, a new method with the aid of the reaction (3He, t). It is also necessary to show the universality of the method. 4) Consideration of other isobar-analog triplets.

13. Thank you for the attention

14. KURCHATOV NUCLEAR PHYSICAL COMPLEX The proton halo in the 13N nucleusA.S. Demyanova, AA Ogloblin, AN Danilov, TL. Belyaeva, S.A. Goncharov The discovery of the neutron halo at the end of the last century was one of the most important achievements of nuclear physics. In the present work, a much more rare phenomenon, the proton halo, was discovered. It was observed in the first excited state of the 13N nucleus. The proton in the halo is on the average from the center of the 12C core at a distance twice as large (7.2 fm) as the radius of the latter. This is the third case of observation of the proton halo in the history of nuclear physics and the first case of its observation in a state located in a continuous spectrum. The figure shows the mirror states of the 13C and 13N nuclei, which have, respectively, the neutron and proton halo. 13N: ground state 13N in the state proton halo The work was published in the journal "Letters in JETF", 104, 547 (2016) and reported at three international conferences in 2016.

15. The differential cross-sections of the 13C(3He,3He)13C elastic scattering at E(3He) = 39.6 MeV (squares)[1], of the 13C(3He,3He’)13C(3.09 MeV) inelastic scattering (triangles)[2] and of the 13C(3He,t)13N(2.37 MeV) reaction (open circules)[3] at E(3He) = 43.6 MeV as a function of the linear momentum transfer q. The lines connect the minima/maxima used for determining diffraction radii are shown. R(3.09 MeV) = 2.92 ± 0.07 fm R(2.37 MeV) = 2.83 ± 0.13 fm MDM