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.

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.

To carry out the experiments with radioactive beams at the ACCULINNA-2 fragment separator in the Flerov Laboratory of Nuclear Reactions, a complex of cryogenic thin physical targets with isotopes H 2 , D 2 , T 2 , 3 He, and 4 He is being created for the gas, liquid, and solid phase (for hydrogen). One of the three types of cryotargets created for nonhazardous gases in any phase at a temperature of 11–30 K is described. Requirements to the targets are formulated. A variety of available cells are presented, a concept of safe operation with the solid phase of hydrogen is given, and the gas–vacuum test bench for its implementation is described.

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.

The 7He nucleus was studied using the 6He[Formula: see text]He reaction in inverse kinematics at 29 [Formula: see text]MeV 6He beam delivered by the ACCULINNA-2 fragment separator (FLNR, JINR). The registration of neutrons from [Formula: see text] decay made it possible to derive the 7He ground state parameters, the decay energy of 0.38(2)[Formula: see text]MeV and width of 0.11(3)[Formula: see text]MeV.

The (7) He nucleus was studied by the ( d , p ) reaction at 29 A MeV beam energy. The He-7 spectrum was measured up to 8 MeV above the (6) He + n threshold. The forward-backward asymmetry in the neutron emission from unbound states of (7) He has been found. That implies the presence of a positive parity partial wave in the (7) He spectrum.

In this paper, we model the fission process by assuming that at certain elongation, after crossing the fission barrier, a fissile nucleus can be treated as a superposition of dinuclear systems (DNS). The distribution of primary fission fragments is described as a result of competition between evolution of initially formed DNS and its decay in relative distance. The level densities required for the calculations were microscopically derived accounting for deformation and excitation energy effects. The calculations performed for even 244-260Fm isotopes give overall good description of mass and neutron multiplicity distributions. To describe sudden onset of symmetric fission in 258Fm, the fissile nucleus is treated as superposition of DNS at smaller elongations than for lighter Fm isotopes, which is in line with significant reduction of half-life for 258Fm. Our results indicate the presence of bimodality due to coexistence of spherical and deformed mass symmetric fission modes.

In the recent work [Nikolskii et al., Phys. Rev. C 105, 064605 (2022)] the 2H(8He,4He)6H reaction was used for the study of the extreme neutron-rich 6H isotope. A broad bump was observed in the measured 6H spectrum interpreted as the broad overlapping ground and some low-lying states of this nuclide. There could be certain doubts in the interpretation of this work: in conditions of the limited phase space it is not impossible that the structure in the missing mass spectrum of 6H is actually induced by the resonant states populated by some other channels opened in the 8He+2H interaction. This work provides a body of the evidence for the correct channel identification and for the absence of the 6H resonances at energy ET = 0 − 3.5 MeV above the 3H+3n decay threshold. In addition the first strong experimental evidence is given that the 6H → 5H*+n → 3H+3n sequential decay is the dominating 6H decay channel.

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.

The secondary beam diagnostics at the ACCULINNA-2 fragment separator is implemented event-by-event by the beam particle detector, allowing one to identify the projectile ion with the dE-ToF method and determine the ion’s velocity vector and localization on the target. The detector comprises two ToF stations and a pair of MWPCs. Two identical ToF stations located on the straight section of the ACCULINNA-2 fragment separator form a base of 12.35 m. In each station, light from a thin organic scintillator is detected with four compact PMTs directed symmetrically inside a closed volume. MWPCs are stationary shells designed to work in vacuum at atmospheric pressure of gas CF 4 . Each chamber provides XY coordinates with a pitch of 0.125 cm. This work studies the purpose and scope of application of the beam particles detector and its characteristics, layout, operation logic, as well as techniques of setting and calibration.

Background: Calculations of the structure of the low-lying states of nuclei with Z = 97-109 play an important role in understanding the properties of nuclei belonging to the new region of the nuclide chart, which is available now for experimental study.Purpose: To calculate quasiparticle-phonon structure and the reduced gamma-transition probabilities for the excited states with excitation energies below 1 MeV for odd-proton nuclei with Z = 97-109.Methods: The quasiparticle-phonon model, which takes into account the quasiparticle-phonon interaction of different multipolarities, is used as a basis for the calculations.Results: The quasiparticle-phonon structure and the gamma-reduced transition probabilities of odd-proton nuclei 263,265,267,269Mt, 259,261,263,265Bh, 255,257,259,261,263Db, 251,253,255,257,259,261Lr, 249,251,253,255Md, 245,247,249,251Es, and 243,245,247Bk are calculated. The alpha-decay chains starting from 263,265,267,269Mt are analyzed.Conclusion: The structure of the nuclear states with excitation energies below 1 MeV in the considered nuclei is mainly exhausted by the one-quasiparticle component. However, in some isotopes the quasiparticle-phonon admixtures plays an important role to destroy the smooth isotopic dependence of energy of the states. The nuclei in the alpha-decay chains starting from 263,265,267,269Mt have up to two alpha-decay lines. The number of alpha-decay lines could be different in the alpha-decay chain and in the direct production of the nucleus.

In order to follow relative yields of ^209 Bi( γ,xn ) reactions, samples of natural bismuth were exposed in LINAC-200 bremsstrahlung beam at four different energies. Activities of eight obtained photonuclear reaction products with neutron multiplicity up to ( γ,9n ) were observed. Reaction yields normalized to the ^206 Bi yield were calculated. The measured relative yields of ^20x Bi isotopes were compared with the theoretical evaporation model based on the microscopically calculated level densities and with GEANT4 simulation.

Using the microscopic-macroscopic approach based on the modified two-center shell model, ground-state shell corrections for even Z superheavy nuclei in the alpha-decay chains containing of the element $$^{{295 - 300,302,304}}$$ 120 isotops. The calculations reveal quite strong shell effects at $$Z = 120 - 126$$ and $$N = 184$$ .

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.

A short review of the results on the structure of superheavy nuclei is presented.