Search for the population of the low-energy continuum of a tetraneutron system was performed for reactions of a 8He beam on a deuterium target. These studies are based on the data [I. A. Muzalevskii et al., Phys. Rev. C 103, 044313 (2021)] previously used for the studies of 7H and 6H in the 2H(8He, 3He)7H and 2H(8He, 4He)6H reactions. Evidence for a hump in the 4n continuum at 3.5 +/- 0.7 and 3.2 +/- 0.8 MeV was observed in the 2H(8He, 6Li)4n and 2H(8He, 3He)7H-* 3H+4n reactions, respectively. The observed statistics is quite low (6 events and up to 40 events) corresponding to very low cross sections of few microbarns or tens of microbarns. The background conditions for the 2H(8He, 6Li)4n reaction are shown to be good, favoring the physical nature of the observed events. The 2H(8He, 3He)7H-* 3H +4n process transforms to the 2H(8He, 6Li & lowast;)4n reaction in the limit of the highest 7H decay energies. The population of the low-energy region in the 4n spectrum is found to be perfectly correlated with the population of the lowest 6Li state in the 3He+3H continuum with E & lowast; = 18 MeV. Theoretical calculations of 8He in five-body alpha + 4n and of 4n in four-body hyperspherical models are presented. The 8He wave function is shown to contain strong specific correlations, which may give rise to very low-energy structures in the tetraneutron continuum in extreme-peripheral reaction scenarios.

Neutron scattering on exotic nuclides is a class of processes which can not be studied directly now and in any observable future. Resonance proton scattering of exotic nuclide on a thick target in inverse kinematics can be used to infer the properties of the low-energy neutron scattering of this nuclide assuming the isobaric symmetry. However, the results of such resonance proton scattering reactions are so far analyzed in theoretical approaches (optical, R-matrix models), which are missing important aspects of isospin dynamics, isospin violation in continuum and threshold dynamics. The isospin conserving coupled-channel model (ICM) is proposed, which provides a more reliable basis for understanding of such experimental studies. Qualitatively different phase shifts for the ^8He+p T=5/2 and T=3/2 resonances are predicted by ICM with quite unusual profile for the T=5/2 states. Alternative interpretation of the existing ^8He+p data is proposed. The observable properties of the T=5/2 resonances may be strongly affected by the isobaric-partner T=3/2 states. Crucial importance of studies of the neutron-emission channel for disentangling this possible influence is demonstrated.

This paper presents a systematic discussion of the design of all key subsystems of the ACCULINNA-2 facility: components of the primary beam line and beam diagnostics equipment, production target, and primary beam absorbers; equipment for the transport, purification, and diagnostics of the secondary beam; a RF-filter for additional beam purification; the cryogenic physical target; the forward spectrometer; and the automated control system. The characteristics of secondary beams are presented, such as angular and spatial distribution in the final focus; the quality of identification of ions by the Δ E-ToF method; and the yields of the certain isotopes obtained during experiments in 2018-2020 with primary beams ^11 B (33.5 AMeV), ^15 N (49.3 AMeV), and 32S (52.7 AMeV) while tuning to secondary ^8 He, ^9 Li, ^11 Li, ^10 Be, ^27 S, and ^26 P beams with different energies. A comparison of secondary beam transport with the calculated one has been performed. The possibility is substantiated to carry out experiments with light exotic ions at high accuracy of detection and measuring parameters of the incident ion on condensed targets of hydrogen and helium isotopes.

The proton and deuteron pickup reactions ^2H(^10Be,^3He)^9Li and ^2H(^10Be,^4He)^8Li were studied with the ^10Be radioactive beam produced by the new fragment separator ACCULINNA-2 at FLNR, JINR. These measurements were initially motivated as the test reactions intended for the elucidation of results obtained in the study of the extremely neutron-rich ^7 H and ^6 H systems created in the ^2H(^8He,^3He)^7H and ^2H(^8He,^4He)^6H reactions with the use of the same setup. In the ^2H(^10Be,^3He)^9Li reaction the ^9 Li ground-state ( 3/2^- ) and its first excited state (2.69 MeV, 1/2^- ) were identified in the low-energy region of its excitation spectrum. The differential cross sections for the ^9Li_g.s. population were extracted at the forward center-of-mass angles ( 3^∘-13^∘ ) and compared with the FRESCO calculations. Spectroscopic factor of ∼1.7 , derived by a model suggesting the ^10Be=p+^9Li_g.s. clustering was found in accord with the experimental data. The energy spectrum of ^8 Li populated in the ^2H(^10Be,^4He)^8Li reaction shows the strong peak which corresponds to the excitation of the second excited state of ^8 Li (2.25 MeV, 3^+ ). The fact that the ground and the first excited states of ^8 Li were not observed in this reaction is consistent with the shell-model structure of the nuclei involved.

In the present paper momentum distributions of nuclei produced in the heavy ion beam fragmentation at the relatively low energies (below 100 A·MeV) are studied. For this study, a new theoretical approach is developed on the basis of the Glauber model modified for taking into account the energy and momentum conservation laws. In this approach, the longitudinal momentum of the most neutron rich nuclei, ^10Be, ^9Li, ^8He, produced in a few neutron removal reactions in the ^11B fragmentation in the Be target at a beam energy of 35 A·MeV are calculated. The region of applicability of the new approach is discussed. This approach gives the asymmetric longitudinal momentum distributions at low energies, and the asymmetry is defined by the kinematical locus and geometry of the reaction (central of peripheral reactions). We analyze the changes of the phase volume and the longitudinal momentum distributions with the beam energy and number of the removed nucleons. The results of the calculations are compared to the parametrizations widely used for estimates of nuclear production in fragmentation for planning of nuclear experiments.

Analysis of the nuclear physics experiment often requires Monte Carlo simulation of the detector setup. Such kind of simulations often requires a quite advanced model of nuclear interaction. In this work we describe application of Monte Carlo simulation in the nuclear physic experiment, and discuss on example of elastic scattering how to apply well known High Energy Approximation (or Glauber model) to Monte Carlo simulation of the experimental setup.

The extremely neutron-rich systems 7H, 6H were studied in the 2H(8He, 3He)7H and 2H(8He, 4He)6H proton and deuteron pickup reactions with a 26 AMeV secondary 8He beam produced at the new ACCULINNA-2 fragment separator. In addition, the same proton and deuteron pickup reactions were generated using the 42 AMeV 10Be beam, and the population of low-lying 9Li and 8Li states was measured in reactions 2H(10Be,3He)9Li and 2H (10Be,4He)8Li, respectively. The latter were used as reference measurements in order to check the setup cali-bration over the excitation energy of 7,6H and to determine the real experimental energy resolution which was compared with Monte Carlo calculations. The corresponding results obtained for the superheavy hydrogen systems 7H, 6H are presented and discussed. Typical excitation spectra of the 9Li and 8Li nuclei are also shown.

Setup fitting the requirements for the detailed study of the five-body decay of the 7 H nucleus obtained as a result of the proton transfer from the 8 He projectiles to the deuterium target nuclei is being built at the radioactive beam line of ACCULINNA-2 separator in the G.N. Flerov Laboratory of Nuclear Reactions. Described here is the assembly of 100 BC-404 plastic scintillators, intended for neutron detection, the annular Si detector telescope for the 3 He recoils, and the detector array providing the Δ E – E -TOF registration of 3 H nuclei emitted at the 7 H decay. Results obtained by the Monte Carlo simulations made for the energy values and flight passes of all these particles are given together with the luminosity expected for the discussed experiments.

Studies of the properties of nuclei remote from the "valley of nuclear stability" make it possible to predict the properties of new nuclides based on systematic accumulations of data on the structure of nuclear matter. New phenomena in the behavior of nuclear matter are also being discovered. Such cores are called "exotic". The production of exotic nuclei is a multi-stage process, during which various approaches of theoretical and experimental physics are applied. One of the ways to obtain nuclei are fragmentation reactions of relatively light nuclei with high energy (more than 100 MeV), as a result of which exotic nuclei with different A and Z can be obtained. In this article, a study of the applicability of the high-energy approximation (HEA) in modeling such direct nuclear reactions was conducted and the results of comparing this approach with the exact solution of the Schrodinger equation using the example of a rectangular potential barrier and a Gaussian potential barrier are presented. Comparison of different approaches provides an understanding of the limitations of their applicability for further study of the properties of nuclei in interaction with each other and for solving the Schrodinger equation with similar potentials considered.

The extremely neutron-rich system $^{6}$H was studied in the direct $^2\text{H}(^8\text{He},{^4\text{He}})^{6}$H transfer reaction with a $26 A$ MeV secondary $^{8}$He beam. The measured missing mass spectrum shows a broad bump at $\sim 4-8$ MeV above the $^3$H+$3n$ decay threshold. This bump can be interpreted as a broad resonant state in $^{6}$H at $6.8(5)$ MeV. The population cross section of such a presumably $p$-wave state (or may be few overlapping states) in the energy range from 4 to 8 MeV is $d\sigma/d\Omega_{\text{c.m.}} \simeq 190^{+40}_{-80}$ $\mu$b/sr in the angular range $5^{\circ}<\theta_{\text{c.m.}}<16^{\circ}$. The obtained missing mass spectrum is practically free of the $^{6}$H events below 3.5 MeV ($d\sigma/d\Omega_{\text{c.m.}} \lesssim 5$ $\mu$b/sr in the same angular range). The steep rise of the $^{6}$H missing mass spectrum at $\sim 3$ MeV allows to derive the lower limit for the possible resonant-state energy in $^{6}$H to be $4.5(3)$ MeV. According to the paring energy estimates, such a $4.5(3)$ MeV resonance is a realistic candidate for the $^{6}$H ground state (g.s.). The obtained results confirm that the decay mechanism of the $^{7}$H g.s.\ (located at 2.2 MeV above the $^{3}$H+$4n$ threshold) is the "true" (or simultaneous) $4n$ emission. The resonance energy profiles and the momentum distributions of fragments of the sequential $^{6}$H$ \,\rightarrow \, ^5$H(g.s.)+$n\, \rightarrow \, ^3$H+$3n$ decay were analyzed by the theoretically-updated direct four-body-decay and sequential-emission mechanisms. The measured momentum distributions of the $^{3}$H fragments in the $^{6}$H rest frame indicate very strong "dineutron-type" correlations in the $^{5}$H ground state decay.

The extremely neutron-rich system 6 H was studied in the direct 2 H( 8 He , 4 He) 6 H transfer reaction with a 26 A MeV secondary 8 He beam. The measured missing mass spectrum shows a broad bump at ∼ 4 − 8 MeV above the 3 H+3 n decay threshold. This bump can be interpreted as a broad resonant state in 6 H at 6 . 8(5) MeV. The population cross section of such a presumably p -wave state (or may be few overlapping states) in the energy range from 4 to 8 MeV is dσ/d Ω c.m. ≃ 190 +40 − 80 µ b/sr in the angular range 5 ◦ < θ c.m. < 16 ◦ . The obtained missing mass spectrum is practically free of the 6 H events below 3.5 MeV ( dσ/d Ω c.m. . 5 µ b/sr in the same angular range). The steep rise of the 6 H missing mass spectrum at ∼ 3 MeV allows to derive the lower limit for the possible resonant-state energy in 6 H to be 4 . 5(3) MeV. According to the paring energy estimates, such a 4 . 5(3) MeV resonance is a realistic candidate for the 6 H ground state (g.s.). The obtained results confirm that the decay mechanism of the 7 H g.s. (located at 2.2 MeV above the 3 H+4 n threshold) is the “true” (or simultaneous) 4 n emission. The resonance energy profiles and the momentum distributions of fragments of the sequential 6 H → 5 H(g.s.)+ n → 3 H+3 n decay were analyzed by the theoretically-updated direct four-body-decay and sequential-emission mechanisms. The measured momentum distributions of the 3 H fragments in the 6 H rest frame indicate very strong “dineutron-type” correlations in the 5 H ground state decay.

Mass measurements of the $^{69}$As, $^{70,71}$Se and $^{71}$Br isotopes, produced via fragmentation of a $^{124}$Xe primary beam at the FRS at GSI, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher with an unprecedented mass resolving power of almost 1,000,000. For the $^{69}$As isotope, this is the first direct mass measurement. A mass uncertainty of 22 keV was achieved with only 10 events. For the $^{70}$Se isotope, a mass uncertainty of 2.6 keV was obtained, corresponding to a relative accuracy of $\delta$m/m = 4.0$\times 10^{-8}$, with less than 500 events. The masses of the $^{71}$Se and $^{71}$Br isotopes were measured with an uncertainty of 23 and 16 keV, respectively. Our results for the $^{70,71}$Se and $^{71}$Br isotopes agree with the 2016 Atomic Mass Evaluation, and our result for the $^{69}$As isotope resolves the discrepancy between previous indirect measurements. We measured also the mass of $^{14}$N$^{15}$N$^{40}$Ar (A=69) with a relative accuracy of $\delta$m/m = 1.7$\times 10^{-8}$, the highest yet achieved with a MR-TOF-MS. Our results show that the measured restrengthening of the proton-neutron interaction ($\delta$V$_{pn}$) for odd-odd nuclei at the N=Z line above Z=29 (recently extended to Z=37) is hardly evident at N-Z=2, and not evident at N-Z=4. Nevertheless, detailed structure of $\delta$V$_{pn}$ along the N-Z=2 and N-Z=4 lines, confirmed by our mass measurements, may provide a hint regarding the ongoing $\approx$500 keV discrepancy in the mass value of the $^{70}$Br isotope, which prevents including it in the world average of ${Ft}$-value for superallowed 0$^+\rightarrow$ 0$^+$ $\beta$ decays. The reported work sets the stage for mass measurements with the FRS Ion Catcher of nuclei at and beyond the N=Z line in the same region of the nuclear chart, including the $^{70}$Br isotope.

The extremely neutron-rich system $^{7}$H was studied in the direct $^2$H($^8$He,$^3$He)$^7$H transfer reaction with a 26 AMeV secondary $^{8}$He beam [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502]. The missing mass spectrum and center-of-mass (c.m.) angular distributions of $^{7}$H, as well as the momentum distribution of the $^{3}$H fragment in the $^{7}$H frame, were constructed. In addition to the investigation reported in Ref. [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502], we carried out another experiment with the same beam but a modified setup, which was cross-checked by the study of the $^2$H($^{10}$Be,$^3$He$)^{9}$Li reaction. A solid experimental evidence is provided that two resonant states of $^{7}$H are located in its spectrum at 2.2(5) and 5.5(3) MeV relative to the $^3$H+4$n$ decay threshold. Also, there are indications that the resonant states at 7.5(3) and 11.0(3) MeV are present in the measured $^{7}$H spectrum. Based on the energy and angular distributions, obtained for the studied $^2$H($^8$He,$^3$He)$^7$H reaction, the weakly populated 2.2(5) MeV peak is ascribed to the $^7$H ground state. It is highly plausible that the firmly ascertained 5.5(3) MeV state is the $5/2^+$ member of the $^7$H excitation $5/2^+$-$3/2^+$ doublet, built on the $2^+$ configuration of valence neutrons. The supposed 7.5 MeV state can be another member of this doublet, which could not be resolved in Ref. [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502]. Consequently, the two doublet members appeared in the spectrum of $^{7}$H in [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502] as a single broad 6.5 MeV peak.

The mechanism of simultaneous non-sequential four-neutron emission (or "true" four-neutron decay) has been considered in the phenomenological five-body approach. It is demonstrated that four-neutron decay fragments should have specific energy and angular correlations reflecting strong spatial correlations of "valence" nucleons orbiting in their four-neutron precursors. Due to the Pauli exclusion principle, the valence neutrons are pushed to the symmetry-allowed configurations in the four-neutron precursor structure, which causes a "Pauli focusing" effect. Prospects of the observation of the Pauli focusing have been considered for the hydrogen-7 nucleus. Fingerprints of its nuclear structure or/and decay dynamics are predicted.

Investigation of the 7H-system in the experiment conducted at the fragment separator ACCULINNA-2 in the 8He(2H,3He)7H reaction requires to detect the recoil 3He ions with energy down to 6 MeV. For this purpose two $$\Delta E{\text{—}}E$$ particle telescopes are used, with each telescope having in front a thin (20-μm) Si strip detector ($$\Delta E{\text{—}}SSD$$). The maps of thickness heterogeneity of the thin detectors were determined by measuring the energy losses of the 226Ra α-particles. The adopted thickness normalization method provides a good identification of the 3He nuclei being recorded in the presence of a high 4He background. Two approaches were used for calculating the energy losses of the identified 3He and 4He reaction ejectiles and reconstructing their energy values available at the exit from the deuterium target. The developed techniques were applied for the 7H missing-mass reconstruction.

The one-neutron transfer 2H(9Li, p)10Li reaction has been investigated at 29 A MeV incident energy at the ACCULINNA-2 facility (Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research). The setup has been used in order to detect the recoil protons at backward angles in coincidences with the outgoing 9Li and neutrons from the 10Li decay at forward angles. This setup allows to study the 10Li emitted in the crucial region at forward angles in the center of mass. The preliminary results in the part of the analysis of double proton–9Li coincidence are reported.

The H-7 system was populated in the H-2(He-8, He-3) H-7 reaction with a 26 AMeV He-8 beam. The H-7 missing mass energy spectrum, the H-3 energy and angular distributions in the H-7 decay frame were reconstructed. The H-7 missing mass spectrum shows a peak, which can be interpreted either as unresolved 5/2(+) and 3/2(+) doublet or one of these states at 6.5(5) MeV. The data also provide indications of the 1/2(+) ground state of H-7 located at 1.8(5) MeV with quite a low population cross section of similar to 25 mu b/sr within angular range theta(c.m) similar or equal to (17 degrees-27 degrees).