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.

An Erratum to this paper has been published: https://doi.org/10.1134/S1547477125020025

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.

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 separation efficiency and extraction time of mass separator MASHA, which is a combination of solid ISOL method and classic mass spectroscopy, were obtained for mercury and radon isotopes produced in complete fusion reactions 40 Ar + 144 Sm = 184– xn Hg and 40 Ar + 166 Er = 206– xn Rn. The extraction time and separation efficiency at the MASHA setup for both reactions are determined by the ratio of cross sections measured with mass separator to absolute cross sections obtained in our previous experiments [1].

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.

The complete-fusion excitation functions of xn-evaporation channels for the reactions Sm-144(Ar-40, xn)Hg184-x, Sm-148(Ar-36, xn)Hg184-x, Nd-144(Ca-40, xn)Hg184-x, Nd-142(Ca-48, xn)Hg190-x, and Er-166(Ar-40, xn)Rn206-x have been measured by using the catcher foil technique. Its modified version as well as a corresponding software of data processing, including a deconvolution procedure to take into account effects caused by spreading the energy of the beam at its passing through absorbing foils, have been described. The measured excitation functions have been compared with ones theoretically calculated with the coupled-channel model.

A modified catcher foil technique is used to measure the absolute cross sections of xn -evaporation channels in complete fusion reactions 144 Sm( 40 Ar, xn ) 184− x Hg, 148 Sm( 36 Ar, xn ) 184− x Hg, 144 Nd( 40 Ca, xn ) 184− x Hg, 142 Nd( 48 Ca, xn ) 190− x Hg, and 166 Er( 40 Ar, xn ) 206− x Rn. The effect the spread of the beam’s energy has on the excitation functions as it moves through absorbing foils and the target is addressed by means of deconvolution. The measured excitation functions are compared to ones calculated theoretically using the coupled-channel model.

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.

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 MASHA facility [1-3] was developed as a high precision mass-spectrometer for heavy and super heavy elements with masses up to 450 a.m.u. It uses ISOL (Isotope Separation On-Line) method. Its unique properties opens great prospective for the investigation of neutron-rich nuclei produced in multinucleon transfer reactions. Mainly nuclei near the neutron N = 126 and N = 152 shell closures are of great interest. This region of nuclei is not so far thoroughly enough investigated while its research has direct relation to the synthesis of super heavy elements. As is known the island of stability close to super heavy elements (Z = 112-118) exists due to the shell effects in nuclei. More detailed investigation of these shell effects can greatly help in the synthesis of next super heavy elements. Heavy neutron rich radon isotopes were produced in the multinucleon transfer reaction Ar-40 + Th-232 at Flerov Laboratory of Nuclear Reactions, Dubna. Radon isotopes with given masses were detected using two types of detectors: a multi-strip well-type detector (made in Canberra) and a position-sensitive quantum counting hybrid pixel detector of the Timepix type [4]. The latter detector has an array of 256x256 square pixels each with a pitch size of 55 mu m for the full sensitive area 14x14mm(2). Radon isotopes implanted into the detector emit then alpha and beta particles until they reach the stable or long-lived isotopes at the end of their decay chains. The positions of radon isotopes, the tracks, times and energies of the beta particles were measured and analyzed. New software for the particle recognition and data analysis of the results obtained from the experiment was developed and used. It has been proven that MASHA + Timepix setup is a powerful instrument for investigation of neutron-rich isotopes far from stability limits.

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.

Experiments on measuring the separation efficiency of evaporation residues produced in complete fusion reactions with heavy ions have been performed on the MASHA mass separator. A new design has been developed for a hot catcher based on the use of thin paper from graphite nanotubes and graphene. The catcher is to be used for the synthesis of nuclei at a high intensity of the primary beam to increase the efficiency of separation. A 16-strip silicon detector was used for continuous monitoring during an experiment to measure the mass spectrometer’s efficiency of separation in the intermediate focal plane.