Search

The H7 system was populated in the H2(He8,He3)H7 reaction with a 26 AMeV He8 beam. The H7 missing mass energy spectrum, the H3 energy and angular distributions in the H7 decay frame were reconstructed. The H7 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 H7 located at 1.8(5) MeV with quite a low population cross section of ∼25 μb/sr within angular range θc.m.≃(17°-27°).

The most remote isotope from the proton dripline (by 4 atomic mass units) has been observed: ^{31}K. It is unbound with respect to three-proton (3p) emission, and its decays have been detected in flight by measuring the trajectories of all decay products using microstrip detectors. The 3p emission processes have been studied by the means of angular correlations of ^{28}S+3p and the respective decay vertices. The energies of the previously unknown ground and excited states of ^{31}K have been determined. This provides its 3p separation energy value S_{3p} of -4.6(2) MeV. Upper half-life limits of 10 ps of the observed ^{31}K states have been derived from distributions of the measured decay vertices.

The proton-unbound argon and chlorine isotopes have been studied by measuring trajectories of their decay-in-flight products by using a tracking technique with micro-strip detectors. The proton ($1p$) and two-proton ($2p$) emission processes have been detected in the measured angular correlations "heavy-fragment"+$p$ and "heavy-fragment"+$p$+$p$, respectively. The ground states of the previously unknown isotopes $^{30}$Cl and $^{28}$Cl have been observed for the first time, providing the $1p$-separation energies $S_p$ of $-0.48(2)$ and $-1.60(8)$ MeV, respectively. The relevant systematics of $1p$ and $2p$ separations energies have been studied theoretically in the core+$p$ and core+$p$+$p$ cluster models. The first-time observed excited states of $^{31}$Ar allow to infer the $2p$-separation energy $S_{2p}$ of 6(34) keV for its ground state. The first-time observed state in $^{29}$Ar with $S_{2p} = -5.50(18)$ MeV can be identified either as a ground or an excited state according to different systematics., 13 pages, 14 figures. Name changed, some affiliations changed and figures corrections added

This work was supported in part by the Helmholtz International Center for FAIR (HIC for FAIR), the Helmholtz Association (grant IK-RU-002), the Russian Ministry of Education and Science (grant No. NSh- 932.2014.2), the Russian Science Foundation (grant No. 17-12-01367), the Polish National Science Center (Contract No. UMO-2011/01/B/ST2/01943), the Polish Ministry of Science and Higher Education (Grant No. 0079/DIA/2014/43,Grant Diamentowy), the Helmholtz- CAS Joint Research Group (grant HCJRG-108), the FPA2009-08848 contract (MICINN, Spain), the Justus- Liebig-Universit¨at Giesen (JLU) and GSI under the JLUGSI strategic Helmholtz partnership agreement. This article is a part of PhD thesis of X.-D. Xu. The authors acknowledge the help of D. Kostyleva in the preparation of the manuscript.

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

Two-proton decay of the $^{17}\mathrm{Ne}$ low-lying states populated in the $^{1}\mathrm{H}(^{18}\mathrm{Ne},d)^{17}\mathrm{Ne}$ transfer reaction is studied. The two-proton width ${\mathrm{\ensuremath{\Gamma}}}_{2p}$ of the $^{17}\mathrm{Ne}$ first excited $3/{2}^{\ensuremath{-}}$ 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 ${\mathrm{\ensuremath{\Gamma}}}_{2p}/{\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}l1.6(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. An original, ``combined mass'' method is suggested and tested as capable of improving the resolution of the experiment, which is a prime significance for the study of nuclear states with extremely small particle-to-gamma width ratios ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{part}}/{\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}$. The condition ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{part}}\ensuremath{\ll}{\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}$ is quite common for the states of astrophysical interest, which makes the proposed approach promising in this field.

Previously unknown isotopes Ar-30 and Cl-29 have been identified by measurement of the trajectories of their in-flight decay products S-28 + p + p and S-28 + p, respectively. The analysis of angular correlations of the fragments provided information on decay energies and the structure of the parent states. The ground states of Ar-30 and Cl-29 were found at 2.25(-0.10)(+0.15) and 1.8 +/- 0.1 MeV above the two-and one-proton thresholds, respectively. The lowest states in Ar-30 and Cl-29 point to a violation of isobaric symmetry in the structure of these unbound nuclei. The two-proton decay has been identified in a transition region between simultaneous two-proton and sequential proton emissions from the Ar-30 ground state, which is characterized by an interplay of three-body and two-body decay mechanisms. The first hint of a fine structure of the two-proton decay of Ar-30*(2(+)) has been obtained by detecting two decay branches into the ground and first-excited states of the S-28 fragment.

Structure of nuclei located near and beyond the drip-lines plays important role in the explosive astrophysical processes. The problem of two-proton decay of the 17Ne first excited state is a good example of such situation. The two-proton radiative capture is a possible bypass of the 15O waiting point in the rp-process. The rate of this process drastically depends on the value of the weak 2

The low-lying spectra of 8He and 10He nuclei were studied in the 3H(6He, p)8He and 3H(8He, p)10He transfer reactions. The 0+ ground state (g.s.) of 8He 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 μb/sr, respectively. Some evidence for a 8He state at about 7.5 MeV was obtained. We discuss a possible nature of the nearthreshold anomaly above 2.14 MeV in 8He and relate it to the population of a 1− continuum (soft dipole excitation) with a peak value at about 3 MeV. The lowest energy group of events in the 10He spectrum was observed at ∼ 3 MeV with a cross section of ∼ 140 μb/sr. We argue that this result is consistent with the previously reported observation of 10He providing the new 10He g.s. position at about 3 MeV

The $\ensuremath{\beta}$ decay of $^{31}\mathrm{Ar}$, produced by fragmentation of an $^{36}\mathrm{Ar}$ beam at 880 MeV/nucleon, was investigated. Identified ions of $^{31}\mathrm{Ar}$ were stopped in a gaseous time projection chamber with optical readout allowing us to record decay events with emission of protons. In addition to $\ensuremath{\beta}$-delayed emission of one and two protons we clearly observed the $\ensuremath{\beta}$-delayed three-proton branch. The branching ratio for this channel in $^{31}\mathrm{Ar}$ is found to be $0.07\ifmmode\pm\else\textpm\fi{}0.02$%.

The 7H nucleus most likely possesses an unique decay dynamics: the four-neutron emission (or 5-body decay). We estimated theoretically the 7H width as a function of its decay energy. Uncertainty inherent to the estimated decay energy leaves a room for a possible very small width of 7H. In an attempt to observe a long living quasi stable 7H nucleus, produced in the reaction 2H(8He, 7H)3He, we set an upper limit of 3 nb/sr for the reaction cross section. We utilized a 20.5 MeV/amu 8He beam bombarding a 5.6 cm thick liquid deuterium target, and searched for 7H within 0°–50° CM angles. The obtained cross section limit corresponds to a 7H lifetime less than 1 ns, which allows us to estimate a lower limit of 50–100 keV for the 7H energy above the 3 H +4n breakup threshold.

The structure of 6 He is explored with proton elastic and inelastic scattering to the first 2 + excited state (Q =− 1. 8M eV) of 6 He in the inverse kinematics using a 24.5 A MeV 6 He beam. The distorted wave approximation (DWA) with an optical potential obtained by folding an effective nucleon–nucleon (NN) g-matrix with the 6 He density matrix elements as given by a large basis shell model is used to analyse the data. The comparison of predictions with the data confirms the view that 6 He has

The fragment separator ACCULINNA in the G. N. Flerov Laboratory of Nuclear Reactions of JINR was used to expose a nuclear track emulsion to a beam of radioactive $^{8}$He nuclei of energy of 60 MeV and enrichment of about 80%. Measurements of decays of $^{8}$He nuclei stopped in the emulsion allow one to evaluate possibilities of $\alpha$-spectrometry and to observe a thermal drift of $^{8}$He atoms in matter. Knowledge of the energy and emission angles of $\alpha$-particles allows one to derive the energy distribution of $\alpha$-decays Q$_{2\alpha}$. The presence of a "tail" of large values Q$_{2\alpha}$ is established. The physical reason for the appearance of this "tail" in the distribution Q$_{2\alpha}$ is not clear. Its shape could allow one to verify calculations of spatial structure of nucleon ensembles emerging as $\alpha$-pairs of decays via the state $^8$Be$_{2+}$., Comment: 9 pages, 4 figures, conference: The 22nd European Conference on Few Body Problems in Physics, Krakow, Poland, 9 - 13 September

Nuclear track emulsion is exposed to a beam of radioactive $^8$He nuclei with an energy of 60 MeV and enrichment of about 80% at the ACCULINNA separator. Measurements of 278 decays of the $^8$He nuclei stopped in the emulsion allow the potential of the $\alpha$ spectrometry to be estimated and the thermal drift of $^8$He atoms in matter to be observed for the first time., Comment: ISSN 1547-4771. 7 pages, 13 figures

The ${0}^{+}$ ground state of the $^{10}\mathrm{He}$ nucleus produced in the $^{3}\mathrm{H}(^{8}\mathrm{He},p)^{10}\mathrm{He}$ reaction was found at about $2.1\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{ }\mathrm{MeV}$ ($\ensuremath{\Gamma}\ensuremath{\sim}2\text{ }\text{ }\mathrm{MeV}$) above the three-body $^{8}\mathrm{He}+n+n$ breakup threshold. Angular correlations observed for $^{10}\mathrm{He}$ decay products show prominent interference patterns allowing us to draw conclusions about the structure of low-energy excited states. We interpret the observed correlations as a coherent superposition of a broad ${1}^{\ensuremath{-}}$ state having a maximum at energy 4--6 MeV and a ${2}^{+}$ 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 $^{12}\mathrm{Be}$ thus extends also to the $^{10}\mathrm{He}$ system.

By using the 1H(6Li,6Be)n charge-exchange reaction, continuum states in 6Be were populated up to E_t=16 MeV, E_t being the 6Be energy above its three-body decay threshold. In kinematically complete measurements performed by detecting alpha+p+p coincidences, an E_t spectrum of high statistics was obtained, containing approximately ~5x10^6 events. The spectrum provides detailed correlation information about the well-known 0^+ ground state of 6Be at E_t=1.37 MeV and its 2^+ state at E_t=3.05 MeV. Moreover, a broad structure extending from 4 to 16 MeV was observed. It contains negative parity states populated by Delta L=1 angular momentum transfer without other significant contributions. This structure can be interpreted as a novel phenomenon, i.e. the isovector soft dipole mode associated with the 6Li ground state. The population of this mode in the charge-exchange reaction is a dominant phenomenon for this reaction, being responsible for about 60% of the cross section obtained in the measured energy range., 8 pages, 7 figures

Project of a new in‐flight fragment separator is proposed as a part of the third generation DRIBs facilities in Dubna. As compared to the existing separator ACCULINNA, beam intensity should be increased by a factor 10–15, the beam quality improved and the RIB assortment should broaden considerably at ACCULINNA‐2. Research program and structure are outlined for the new instrument.

Unknown isotope 26S, expected to decay by two-proton (2p) emission, was studied theoretically and was searched experimentally. The structure of this nucleus was examined within the relativistic mean field (RMF) approach. A method for taking into account the many-body structure in the three-body decay calculations was developed. The results of the RMF calculations were used as an input for the three-cluster decay model worked out to study a possible 2p decay branch of this nucleus. The experimental search for 26S was performed in fragmentation reactions of a 50.3 A MeV 32S beam. No events of 26S or 25P (a presumably proton-unstable subsystem of 26S) were observed. Based on the obtained production systematics an upper half-life limit of T_{1/2}640 keV for 26S. Analogous limits for 25P are found as T_{1/2}110 keV. In the case that the one-proton emission is the main branch of the 26S decay a limit Q_{2p}>230 keV would follow for this nucleus. It is likely that 26S resides in the picosecond lifetime range and the further search for this isotope is prospective for the decay-in-flight technique., 15 pages, 7 figures

International audience; The t(6He,8He)p t(8He,10He)p reactions were studied in experiments with 25A MeV 6He beam and 27.4A MeV 8He beams and an unique tritium gas target. Results on the 8He and 10He excitation energy spectra are presented. The 8He spectrum exhibits 3 states, apart of the ground state and known (2+) state at 3.6 MeV, the third state is seen at 5.4 MeV. A tentative assignment 1+ is proposed for this structure. The 10He spectrum shows no event at the excitation energy below 3 MeV what is in contradiction with the earlier reports on this nucleus. A high statistics of α-n coincidence events collected for the 8He beam run has been subjected to analysis in order search for the bound tetraneutron. The search produced a negative result.

The spectrum of $^{9}\mathrm{He}$ was studied by means of the $^{2}\mathrm{H}$($^{8}\mathrm{He}$,$p$)$^{9}\mathrm{He}$ reaction at a laboratory energy of 25 MeV/nucleon and small center-of-mass angles. Energy and angular correlations were obtained for the $^{9}\mathrm{He}$ decay products by complete kinematical reconstruction. The lowest resonant state of $^{9}\mathrm{He}$ is found at 2.0 \ifmmode\pm\else\textpm\fi{} 0.2 MeV with a width of $~2$ MeV and is identified as $1/{2}^{\ensuremath{-}}$. The observed angular correlation pattern is uniquely explained by the interference of the $1/{2}^{\ensuremath{-}}$ resonance with a virtual $1/{2}^{+}$ state (a limit $ag\ensuremath{-}20$ fm is obtained for the scattering length), and with a $5/{2}^{+}$ resonance at energy $\ensuremath{\geqslant}4.2$ MeV.

The H-4 resonance was investigated in the transfer reactions H-2(t,p)H-4 and H-3(t,d)H-4 using cryogenic liquid targets of deuterium and tritium and 58 MeV triton beam. The multidetector system with a large angular acceptance was used to measure the recoil protons and deuterons in coincidences with tritons or neutrons from the decay of H-4. Data analysis involved careful considerations made for the competing processes of final state interaction and quasifree scattering. The experimental data show a peak with resonance parameters E-res = 3.05 +/- 0.19 MeV and Gamma(obs) = 4.18 +/- 1.02 MeV. The pole of S-matrix corresponding to the physical values of energy and width, E-0 = 1.99 +/- 0.37 MeV and Gamma(0) = 2.85 +/- 10.3 MeV, has been extracted. (C) 2004 Elsevier B.V. All rights reserved.

Resonance states of 5H were investigated through the two-neutron transfer reaction t(t, p)5 H. A triton beam at 57.5 MeV and a cryogenic liquid tritium target were used. The 5H missing mass spectrum in triple coincidence, proton + triton + neutron, shows a resonance at 1.8 ± 0.1 MeV above the t + 2n decay threshold. This energy is in good agreement with the result reported in Phys. Rev. Lett. 87 (2001) 092501. The resonance width, Γintr ≤ 0.5 MeV, is surprisingly small and difficult to reconcile with theory predictions. © 2003 Published by Elsevier B.V., SCOPUS: ar.j, info:eu-repo/semantics/published

The new facility fragment separator ACCULINNA-2 will be put into operation at the beginning of 2015 in FLNR JINR. The new separator is destined to add considerably to the studies of drip-line nuclei performed with the use of variety of direct reactions known to be distinctive to the 15 - 50 MeV/amu exotic secondary RIBs. Intense beams provided by the U-400M cyclotron will ensure the achievement of this objective. In addition to the RIB separation accomplished by means of the dipole-wedge-dipole selection, the addition of a zero-degree dipole magnet setup is foreseen. A long (13 m) straight section will provide precise time-of-flight measurements. © Owned by the authors, published by EDP Sciences, 2014.

The project of a new and more powerful in-flight fragment separator ACCULINNA-2 at U-400M cyclotron in FLNR, JINR planned to build in addition to the existing separator ACCULINNA is presented. The new separator will provide high intensity RIBs in the lowest energy range (5÷50 MeV/nucleon) which is attainable for in-flight separators. The possibilities for the astrophysics studies at the proposed device are presented. ACCULINNA-2 separator is planned to be constructed in the years 2010-2015. The current status of the project is reported.

The 1H(6Li,6Be)n charge-exchange reaction and the 3H(8He, p)10He two-neutron transfer reaction were recently carried out at the ACCULINNA fragment separator (FLNR, Dubna) to populate the ground and excited states of 6Be and 10He nuclei, respectively. The 6Be ET spectrum (ET is the 6Be energy above its 4He+p+p decay threshold) was obtained with high statistics and described by the well-known 0+ ground state of 6Be at ET = 1.37 MeV, the 2+ state at ET = 3.05 MeV and a broad structure extending from 4 to 16 MeV which could be interpreted as the isovector soft dipole mode associated with the 6Li ground state. In the 10He case the 0+ ground state was found at about 2.1(2) MeV (T ∼ 2 MeV) above the 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: 1- (E T ∼ 5 MeV) and 2+ (ET > 6 MeV).