Search

We report on the observation of previously-unknown isotope $^{21}$Al, the first unbound aluminum isotope located beyond the proton dripline. The $^{21}$Al 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 $^{20}$Mg+p. The 1p-decay energies of ground and low-lying excited states of $^{21}$Al, its mass excess and proton separation energy value $S_p$=$-1.1(1)$ MeV were determined., Comment: 6 pages, 7 figures

Search for the population of the low-energy continuum of a tetraneutron system was performed for reactions of the $^{8}$He beam on a deuterium target. These studies are based on the data [I.A. Muzalevskii \textit{et al.}, Phys.\ Rev.\ C \textbf{103}, 044313 (2021)], previously used for the studies of $^{7}$H and $^{6}$H in the $^2\text{H}(^8\text{He},{^3\text{He}})^{7}$H and $^2\text{H}(^8\text{He},{^4\text{He}})^{6}$H reactions. Evidence for a hump in the $^4$n continuum at $3.5 \pm 0.7$ and $3.2 \pm 0.8$ MeV was observed in the $^2$H($^8$He,$^6$Li)$^4$n and $^2$H($^8$He,$^3$He)$^7$H$\rightarrow ^3$H+$^4$n 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 $^2$H($^8$He,$^6$Li)$^4$n reaction are shown to be good, favoring the physical nature of the observed events. The $^2$H($^8$He,$^3$He)$^7$H$\rightarrow ^3$H+$^4$n process transforms to the $^2$H($^8$He,$^6$Li$^{\ast})^4n$ reaction in the limit of the highest $^7$H decay energies. The population of the low-energy region in the $^{4}$n spectrum is found to be perfectly correlated with the population of the lowest $^{6}$Li state in the $^{3}$He+$^{3}$H continuum with $E^*=18$ MeV. Theoretical calculations of $^{8}$He in a five-body $\alpha$+$4n$ and of $^{4}$n in a four-body hyperspherical models are presented. The $^{8}$He wave function is shown to contain strong specific correlations, which may give rise to very low-energy structures in tetraneutron continuum in extreme-peripheral reaction scenarios., Comment: 14 pages, 9 figures

Recent measurements indicate that the previously established upper limit for the $\gamma$-decay branch of the $3_{1}^{-}$ resonance in $^{12}\textrm{C}$ at $E_{x} = 9.641(5)$ MeV may be incorrect. As a result, the $3_{1}^{-}$ resonance has been suggested as a significant resonance for mediating the triple-$\alpha$ reaction at high temperatures above 2 GK. Accurate estimations of the $3_{1}^{-}$ contribution to the triple-$\alpha$ reaction rate require accurate knowledge of not only the radiative width, but also the total width. In anticipation of future measurements to more accurately determine the $\gamma$-decay branch of the $3_{1}^{-}$ resonance, the objective of this work is to accurately determine the total width of the $3_{1}^{-}$ resonance. An evaluation was performed on all previous results considered in the current ENSDF average of 46(3) keV for the physical total width (FWHM) of the $3_{1}^{-}$ resonance in $^{12}\textrm{C}$. Significant unaccounted-for uncertainties and a misstated result were discovered in these previous results, leading to an invalid ENSDF average. In this work, the new global \textbf{R}-matrix analysis performed on direct-reaction data yields a formal total width of $\Gamma(E_{r}) = 46(2)$ keV and an observed total width of $\Gamma_{\textrm{obs}}(E_{r}) = 38(2)$ keV for the $3_{1}^{-}$ resonance. An observed total width of $\Gamma_{\textrm{obs}}(E_{r}) = 38(2)$ keV is recommended for the $3_{1}^{-}$ resonance in $\mathrm{^{12}C}$. This observed total width should be employed for future evaluations of the observed total radiative width for the $3_{1}^{-}$ resonance and its contribution to the high-temperature triple-$\alpha$ reaction rate.

Setup fitting the requirements for the detailed study of the five-body decay of the 7H nucleus obtained as a result of the proton transfer from the 8He 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 3He recoils, and the detector array providing the $\Delta E$-$E$-TOF registration of 3H nuclei emitted at the 7H 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 proton and deuteron pickup reactions 2H(10Be,3He)9Li and 2H(10Be,4He)8Li radioactive beam produced by the new fragment separator ACCULINNA-2 at FLNR, JINR\@. These measurements were initially motivated as test reactions intended for the elucidation of results obtained in the study of the extremely neutron-rich 7H and 6H systems created in the 2H(10Be,3He)9Li and 2H(10Be,4He)8Li reactions using the same setup. In the 2H(10Be,3He)9Li reaction the 9Li ground-state ($3/2^-$) and its first excited state (2.69MeV, $1/2^-$) were identified in the low-energy region of its excitation spectrum. The differential cross sections for the 9Li g.~s.) population were extracted at forward center-of-mass angles ($3^\circ-13^\circ$) and compared with the FRESCO calculations. Spectroscopic factor of $\sim 1.7$, derived by a model for the 10Be$ = p +$9Li(g.s.) clustering was found in accord with the experimental data. The energy spectrum of 8Li populated in the 2H(10Be,4He)8Li reaction shows the strong peak which corresponds to excitation of the second excited state of 8Li (2.25 MeV, $3^+$). The fact that the ground and the first excited states of 8Li were not observed is fully consistent with Shell-Model calculations carried out for the 10Be g.\,s. and 8Li level structure applying momentum selection rules.

Knowledge of the low-lying monopole strength in $\mathrm{^{12}C}$ $-$ the Hoyle state in particular $-$ is crucial for our understanding of both the astrophysically important $3\alpha$ reaction and of $\alpha$-particle clustering. Multiple theoretical models have predicted a breathing mode of the Hoyle State at $E_{x} \approx 9$ MeV, corresponding to a radial in-phase oscillation of the underlying $\alpha$ clusters. The $\mathrm{^{12}C}(\alpha, \alpha^{\prime})\mathrm{^{12}C}$ and $\mathrm{^{14}C}(p, t)\mathrm{^{12}C}$ reactions were employed to populate states in $^{12}$C in order to search for this predicted breathing mode. A self-consistent, simultaneous analysis of the inclusive spectra with R-matrix lineshapes, together with angular distributions of charged-particle decay, yielded clear evidence for excess monopole strength at $E_{x} \approx 9$ MeV which is highly collective. Reproduction of the experimentally observed inclusive yields using a fit, with consistent population ratios for the various broad states, required an additional source of monopole strength. The interpretation of this additional monopole resonance as the breathing-mode excitation of the Hoyle state would provide evidence supporting a $\mathcal{D}_{3h}$ symmetry for the Hoyle state itself. The excess monopole strength may complicate analysis of the properties of the Hoyle state, modifying the temperature dependence of the $3\alpha$ rate at $T_{9} \gtrsim 2$ and ultimately, the predicted nucleosynthesis in explosive stars., Comment: 8 pages, 5 figures

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., Comment: 16 pages, 19 figures

Knowledge of the low-lying monopole strength in $\mathrm{^{12}C}$, the Hoyle state in particular, is crucial for our understanding of both the astrophysically important $3\alpha$ reaction and of $\alpha$-particle clustering. The $\mathrm{^{12}C}(\alpha, \alpha^{\prime})\mathrm{^{12}C}$ and $\mathrm{^{14}C}(p, t)\mathrm{^{12}C}$ reactions were employed to populate states in $^{12}$C. A self-consistent, simultaneous analysis of the inclusive spectra with lineshapes was performed, which accounted for distortion due to nuclear dynamics and experimental effects. Clear evidence was found for excess monopole strength at $E_{x} \sim 9$ MeV, particularly in the $\mathrm{^{12}C}(\alpha, \alpha^{\prime})\mathrm{^{12}C}$ reaction at $0^{\circ}$. This additional strength cannot be reproduced by the previously established monopole states between $E_{x} = 7$ and 13 MeV. An additional $0^{+}$ state at $E_{x} \sim 9$ MeV yielded a significantly improved fit of the data and is the leading candidate for the predicted breathing-mode excitation of the Hoyle state. Alternatively, the results may suggest that a more sophisticated, physically motivated parameterization of the astrophysically important monopole strengths in $\mathrm{^{12}C}$ is required., Comment: 25 pages (including appendix and bibliography), 16 figures

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., Comment: 16 pages, 15 figures

The $^7$H system was populated in the $^2$H($^8$He,$^3$He)$^7$H reaction with a 26 AMeV $^8$He beam. The $^{7}$H missing mass energy spectrum, the $^{3}$H energy and angular distributions in the $^7$H decay frame were reconstructed. The $^7$H 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 on the $1/2^+$ ground state of $^7$H located at 2.0(5) MeV with quite a low population cross section of $\sim 10$ $\mu$b/sr within angular range $\theta_{\text{cm}} \simeq 6^{\circ} - 30^{\circ}$.

Prospects of experimental studies of argon and chlorine isotopes located far beyond the proton dripline are studied by using systematics and cluster models. The deviations from the widespread systematics observed in $^{28,29}$Cl and $^{29,30}$Ar have been theoretically substantiated, and analogous deviations predicted for the lighter chlorine and argon isotopes. The limits of nuclear structure existence are predicted for Ar and Cl isotopic chains, with $^{26}$Ar and $^{25}$Cl found to be the lightest sufficiently long-living nuclear systems. By simultaneous measurements of protons and $\gamma$-rays following decays of such systems as well as their $\beta$-delayed emission, an interesting synergy effect may be achieved, which is demonstrated by the example of $^{30}$Cl and $^{31}$Ar ground state studies. Such synergy effect may be provided by the new EXPERT setup (EXotic Particle Emission and Radioactivity by Tracking), being operated inside the fragment separator and spectrometer facility at GSI, Darmstadt., Comment: 10 pages, 9 figures. Chapter added, language and some figures corrected

The emission of neutron pairs from the neutron-rich $N\!=\!12$ isotones $^{18}$C and $^{20}$O has been studied by high-energy nucleon knockout from $^{19}$N and $^{21}$O secondary beams, populating unbound states of the two isotones up to 15~MeV above their two-neutron emission thresholds. The analysis of triple fragment-$n$-$n$ correlations shows that the decay $^{19}$N$(-1p)^{18}$C$^*\!\rightarrow^{16}$C+$n$+$n$ is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a $^{14}$C core surrounded by four valence neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay $^{21}$O$(-1n)^{20}$O$^*\!\rightarrow^{18}$O+$n$+$n$, attributed to its formation through the knockout of a deeply-bound neutron that breaks the $^{16}$O core and reduces the number of pairs., Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev. Lett

The $^{6}$Be continuum states were populated in the charge-exchange reaction $^1$H($^{6}$Li,$^{6}$Be)$n$ collecting very high statistics data ($\sim 5 \times 10^6$ events) on the three-body $\alpha$+$p$+$p$ correlations. The $^{6}$Be excitation energy region below $\sim 3$ MeV is considered, where the data are dominated by contributions from the $0^+$ and $2^+$ states. It is demonstrated how the high-statistics few-body correlation data can be used to extract detailed information on the reaction mechanism. Such a derivation is based on the fact that highly spin-aligned states are typically populated in the direct reactions., Comment: submitted to Physical Review C

Background: Odd-odd nuclei, around doubly closed shells, have been extensively used to study proton-neutron interactions. However, the evolution of these interactions as a function of the binding energy, ultimately when nuclei become unbound, is poorly known. The $^{26}$F nucleus, composed of a deeply bound $\pi0d\_{5/2}$ proton and an unbound $\nu0d\_{3/2}$ neutron on top of an $^{24}$O core, is particularly adapted for this purpose. The coupling of this proton and neutron results in a $J^{\pi} = 1^{+}\_1 - 4^{+}\_1$ multiplet, whose energies must be determined to study the influence of the proximity of the continuum on the corresponding proton-neutron interaction. The $J^{\pi} = 1^{+}\_1, 2^{+}\_1,4^{+}\_1$ bound states have been determined, and only a clear identification of the $J^{\pi} =3^{+}\_1$ is missing.Purpose: We wish to complete the study of the $J^{\pi} = 1^{+}\_1 - 4^{+}\_1$ multiplet in $^{26}$F, by studying the energy and width of the $J^{\pi} =3^{+}\_1$ unbound state. The method was firstly validated by the study of unbound states in $^{25}$F, for which resonances were already observed in a previous experiment.Method: Radioactive beams of $^{26}$Ne and $^{27}$Ne, produced at about $440A$\,MeV by the FRagment Separator at the GSI facility, were used to populate unbound states in $^{25}$F and $^{26}$F via one-proton knockout reactions on a CH$\_2$ target, located at the object focal point of the R$^3$B/LAND setup. The detection of emitted $\gamma$-rays and neutrons, added to the reconstruction of the momentum vector of the $A-1$ nuclei, allowed the determination of the energy of three unbound states in $^{25}$F and two in $^{26}$F. Results: Based on its width and decay properties, the first unbound state in $^{25}$F is proposed to be a $J^{\pi} = 1/2^-$ arising from a $p\_{1/2}$ proton-hole state. In $^{26}$F, the first resonance at 323(33)~keV is proposed to be the $J^{\pi} =3^{+}\_1$ member of the $J^{\pi} = 1^{+}\_1 - 4^{+}\_1$ multiplet. Energies of observed states in $^{25,26}$F have been compared to calculations using the independent-particle shell model, a phenomenological shell-model, and the ab initio valence-space in-medium similarity renormalization group method.Conclusions: The deduced effective proton-neutron interaction is weakened by about 30-40\% in comparison to the models, pointing to the need of implementing the role of the continuum in theoretical descriptions, or to a wrong determination of the atomic mass of $^{26}$F., Comment: 14 pages, 7 figures, 3 tables, submitted to PRC

Two-proton decay of the $^{17}$Ne low-lying states populated in the $^1$H($^{18}$Ne,$d$)$^{17}$Ne transfer reaction was studied. The two-proton width $\Gamma_{2p}$ of the $^{17}$Ne first excited $3/2^-$ state at $E^*=1.288$ MeV is of importance for the two-proton radioactivity theory and nuclear-astrophysics applications. A dedicated search for the two-proton emission of this state was performed leading to the new upper limit obtained for the width ratio $\Gamma_{2p}/\Gamma_{\gamma} < 1.6(3) \times 10^{-4}$. A novel, "combined mass" method is suggested and tested capable to improve the resolution of the experiment which is a prime significance for the study of nuclear states with extremely small particle-to-gamma width ratios $\Gamma_{\mathrm{part}}/\Gamma_{\gamma}$. The condition $\Gamma_{\mathrm{part}} \ll \Gamma_{\gamma}$ is quite common for the states of astrophysical interest which makes the proposed approach promising in this field.

Fragment separator ACCULINNA-2 has been built in the Flerov Laboratory of Nuclear reactions (JINR, Dubna). It is timely now to choose meaningful and challenging objectives for experiments dedicated to the study of light drip-line nuclei. Considerable interest makes the search for the minor $2p$-decay branch of the first excited state of $^{17}$Ne. The knowledge on the $\Gamma_{2p}/\Gamma_{\gamma}$ width ratio for this excited state is of considerable interest because the reverse process of simultaneous two-proton radiative capture could be a bypass for the $^{15}$O "waiting point" occurring in the rp-process of nucleosynthesis. Accumulation of high-statistics data for the $^{10}$He excitation spectrum populated in the $^2$H($^8$He,$p$)$^9$He and $^3$H($^8$He,$p$)$^{10}$He reactions, as well as the study of cross-check reactions made with the $^{11}$Li and $^{14}$Be beams, will make an effective way to clarify the succession of $^{10}$He excited states. A hot topic beyond the neutron drip line makes the observation of results capable to elucidate the low-energy resonance states anticipated for the $4n$ decay of $^{7}$H. The RIB beams provided by ACCULINNA-2 will allow one to perform experiments where luminosity coming to a level of more than $2 \times 10^{26}$ cm$^{-2}$ s$^{-1}$ will be achievable in experiments aimed study of the $^2$H($^8$He,$^3$He)$^7$H and $^2$H($^{11}$Li,$^6$Li)$^7$H reactions., Comment: 9 pages, 3 figures, submitted to Proceedings of Int. Symp. on Exotic Nuclei, EXON-2016, 4-10 September 2016, Kazan, Russia

The 0+ ground state of the 10He nucleus produced in the 3H(8He,p)10He reaction was found at about $2.1\pm0.2$ MeV (\Gamma ~ 2 MeV) above the three-body 8He+n+n breakup threshold. Angular correlations observed for 10He decay products show prominent interference patterns allowing to draw conclusions about the structure of low-energy excited states. We interpret the observed correlations as a coherent superposition of the broad 1- state having a maximum at energy 4-6 MeV and the 2+ state above 6 MeV, setting both on top of the 0+ state "tail". This anomalous level ordering indicates that the breakdown of the N=8 shell known in 12Be thus extends also to the 10He system., Comment: 5 pages, 5 figures

The low-lying spectra of $^8$He and $^{10}$He nuclei were studied in the $^3$H($^6$He,$p$)$^8$He and $^3$H($^8$He,$p$)$^{10}$He transfer reactions. The $0^+$ ground state (g.s.) of $^8$He and excited states, $2^+$ at $3.6-3.9$ MeV and $(1^+)$ at $5.3-5.5$ MeV, were populated with cross sections of 200, 100-250, and 90-125 $\mu$b/sr, respectively. Some evidence for $^8$He state at about 7.5 MeV is obtained. We discuss a possible nature of the near-threshold anomaly above 2.14 MeV in $^8$He and relate it to the population of a $1^-$ continuum (soft dipole excitation) with peak value at about 3 MeV. The lowest energy group of events in the $^{10}$He spectrum was observed at $\sim 3$ MeV with a cross section of $\sim 140$ $\mu$b/sr. We argue that this result is possibly consistent with the previously reported observation of $^{10}$He, in that case providing a new g.s. position for $^{10}$He at about 3 MeV., Comment: 18 pages, 8 figures, submitted to Phys. Lett. B

The spectrum of $^9$He was studied by means of the $^8$He($d$,$p$)$^9$He reaction at a lab energy of 25 MeV/n and small center of mass (c.m.) angles. Energy and angular correlations were obtained for the $^9$He decay products by complete kinematical reconstruction. The data do not show narrow states at $\sim $1.3 and $\sim $2.4 MeV reported before for $^9$He. The lowest resonant state of $^9$He is found at about 2 MeV with a width of $\sim $2 MeV and is identified as $1/2^-$. The observed angular correlation pattern is uniquely explained by the interference of the $1/2^-$ resonance with a virtual state $1/2^+$ (limit on the scattering length is obtained as $a > -20$ fm), and with the $5/2^+$ resonance at energy $\geq 4.2$ MeV., Comment: 5 pages, 4 figures, 2 tables