The previously-unknown nucleus 20Al has been observed for the first time by detecting its in-flight decays. Tracking trajectories of all decay products with silicon micro-strip detectors allowed for a conclusion that 20Al is unbound with respect to three-proton (3p) emission. The 3p-decay energy of 20Al ground state has been determined to be 1.93(+0.11,-0.09) MeV through a detailed study of angular correlations of its decay products, 17Ne+p+p+p. This value is much smaller in comparison with the predictions inferred from the isospin symmetry by using the known energy of its mirror nucleus 20N, which indicates a possible mirror symmetry violation in the structure of 3p emitters. Such an isospin symmetry breaking is supported by the calculations of the continuum embedded theoretical frameworks, describing the observed 20Al ground state as an 1p s-wave state with a spin-parity of 1-, which contradicts to the spin-parity (2-) of the 20N ground state. The 20Al ground state decays by sequential 1p-2p emission via intermediate ground state of 19Mg, which is the first observed case of daughter two-proton radioactivity following 1p decay of the parent state.

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.

We report on the observation of previously-unknown isotope ^21Al, the first unbound aluminum isotope located beyond the proton dripline. The ^21Al nucleus decays by one-proton (1p) emission, and its in-flight decays were detected by tracking trajectories of all decay products with micro-strip silicon detectors. The 1p-emission processes were studied by analyses of the measured angular correlations of decay products ^20Mg+p. The 1p-decay energies of ground and low-lying excited states of ^21Al, its mass excess and proton separation energy value S_p=-1.1(1) MeV were determined.

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.

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.

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.

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.