Search

The one-neutron transfer 2H(9Li, p)10Li reaction has been investigated at 29 A MeV incident energy at the ACCULINNA-2 facility (Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research). The setup has been used in order to detect the recoil protons at backward angles in coincidences with the outgoing 9Li and neutrons from the 10Li decay at forward angles. This setup allows to study the 10Li emitted in the crucial region at forward angles in the center of mass. The preliminary results in the part of the analysis of double proton–9Li coincidence are reported.

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.

The start of operation of a new separator ACCULINNA-2 makes an important upgrade for the Radioactive-Ion Beam (RIB) research done at the Flerov Laboratory of Nuclear Reactions (FLNR, JINR). Test results indicate that the separator meets the project specifications. Intensities obtained for the 6,8He, 9,11Li, 12Be RIBs are 15 times higher in comparison with the results achieved at the old separator ACCULINNA. An overview of the design, construction and commissioning studies of the ACCULINNA-2 separator is presented. The separator will be equipped with some key facilities: a cryogenic tritium target, zero degree spectrometer following the physical target bombarded by the RIBs, and with a neutron detector array, and the Time Projection Chamber (TPC). This opens a wide range of experimental possibilities. Overview is presented on the two first experiments devoted to the study of d + 6He elastic scattering and search for a 7H low-lying resonance state populated in the 2H(8He,3He)7H reaction.

Studies of radioactive ions (RIs) are the most thriving field of low-energy nuclear physics. In this paper, the concept and the scientific agenda of the prospective accelerator and storage ring facility for RI beam (RIB) research are proposed for a large-scale international project based at the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research. The motivation for the new facility is discussed and its characteristics are briefly presented and shown to be comparable to those of advanced world centers, the so-called “RIB factories”. In the project, the emphasis is made on studies with short-lived RIBs in storage rings. A unique feature of the project is the possibility of studying electron–RI interactions in a collider experiment to determine the fundamental properties of nuclear matter, in particular, electromagnetic form factors of exotic nuclei.

Characteristics of a new separator of radioactive ion beams (RIBs) are described. It was recently commissioned at the Flerov Laboratory of Nuclear Reactions (FLNR JINR) and is in operation at the primary beam line of the U-400M heavy ion cyclotron providing RIBs in energy range of 10–50 MeV/nucleon. Preliminary results obtained in the first experiments performed with this separator in 2018 are presented. A medium-term program of scientific research is discussed.

Three-body continuum p-p-α for the charge exchange reaction 1H(6Li,6Be)n is studied in detail. The data are obtained with a high volume of statistics (approximately 4.7 million events), making it possible to analyze three-particle correlations. Detailed information about the structure of 6Ве and the reaction is obtained as a result.

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°).

A neutron spectrometer based on stilbene crystals has been developed by the Flerov Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research (Dubna, Russia). The timing resolution is determined as a function of the signal amplitude (σT = 0.18 ns at an amplitude of 1 MeV in the electron equivalent). The measured energy resolution of the detecting modules for γ rays is σE/E = 4.5% at E = 1 MeV. The quality of the n–γ discrimination is investigated. It is shown that reliable discrimination is possible, beginning with a deposited energy of 100 keV in the electron equivalent, which corresponds to the kinetic energy of recoil protons of ∼700 keV. The neutron spectrometer helps to significantly expand the experimental capabilities and to carry out correlation experiments with radioactive beams on the ACCULINNA-2 fragment separator at a new level of quality.

New experimental measurements of the total reaction cross sections for the 6He + natSi and 9Li + natSi processes in the energy range of 5 to 40 A MeV are presented. A modified transmission method based on high-efficiency detection of prompt n–γ radiation has been used in the experiment. A bump is observed for the first time in the energy dependence σR(E) at E ∼ 10–30 A MeV for the 9Li + natSi reaction, and existence of the bump in σR(E) at E ∼ 10–20 A MeV first observed in the standard transmission experiments is experimentally confirmed for the 6He + natSi reaction. Theoretical analysis of the measured 6He + natSi and 9Li + natSi reaction cross sections is performed within the microscopic double folding model. Disagreement is observed between the experimental and theoretical cross sections in the region of the bump at the energies of 10 to 20 A MeV, which requires further study.

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 of the 1/2^{+} ground state of ^{7}H located at 1.8(5) MeV with quite a low population cross section of ∼25 μb/sr within angular range θ_{c.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.

The spectrum of low-lying states in the 10He nucleus is investigated for the two-neutron transfer reaction 3H(8He, p)10He. The secondary beam of 8He nuclei with the energy 21.5 MeV/nucleon and a cryogenic tritium target are used in the experiment. The 10He ground state is observed in the missing mass spectrum at the energy of 2.1 MeV (Γ ∼ 2 MeV) above the decay threshold. Analysis of the angular correlations of the 10He decay products yields the spin and parity of two excited 10He states, J π = 1− in the energy range from 4 to 6 MeV and J π = 2+ at energies above 6 MeV.

The ACCULINNA-2 separator is intended to provide beams of exotic nuclei with Z = 1−30, energies of 10–50 MeV/amu, and intensities that allow one to study excitation spectra and decay dynamics of neutron- and proton-drip nuclei. Examples from studies of the 10He and 6Be nuclei are presented.

The VMEbased data acquisition system designed for physics experiments involving measure� ments of several hundreds of parameters is described. The equipment has been tested in the experiments with the 6 He, 8 He, 18 Ne, and 27 S beams at the ACCULINNA fragment separator (http://aculina.jinr.ru/) in the energy range of 20-35 MeV/nucleon. The system is composed of the RIO�3 processor module, which is used to read the VME bus, communicate with the CAMAC and FASTBUS crates, and transmit data to the per� sonal computer; the TRIVA�5 master trigger module; st andard analogtodigital, chargetodigital, and timetodigital converters from CAEN (V785, V792 , and V775 models); and related software. This data acquisition system is faster (by a factor of 10) than a CAMACbased system and is capable of operating with new types of electronic modules, e.g., with digitizers.

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

Momentum correlations inherent to the 6He constituents in the ground state of this nucleus were studied in the quasi-free scattering (QFS) reaction 4He(6He, 2α)2n at 6He beam energy of 25 MeV/nucleon. A detailed study of nucleus structure was performed for the first time in QFS reactions with an unbound spectator. The Plane Wave Impulse Approximation was used in analyzing the experimental data. It was shown that the experimental data are described by model calculations in which the two neutron final state interaction is taken into account. t + t and t + d + n configurations were also studied in the 4He(6He, tα)t and 4He(6He, tα)dn QFS reactions.

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 spectrum of 9He was studied by means of the 8He(d,p)9He 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 9He decay products by complete kinematical reconstruction. The data analysis was done in the assumption of a direct one-neutron transfer reaction mechanism. The 9He structure was analysed in the framework of a simple single-particle model. The lowest resonant state of 9He is found at about 2 MeV with a width of ∼2MeV 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+ (a limit on the scattering length is obtained as a > -20fm), and with a 5/2+ resonance at energy ≥4.2MeV.

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 $0.07\ifmmode\pm\else\textpm\fi{}0.02$%.

Spectroscopy of neutron rich exotic isotopes via their isobaric analog states (IAS) in less exotic nuclei is discussed. Several different experimental techniques, which can be applied to search for IAS of exotic isotopes, are described. Successful application of these techniques to the studies of heavy helium isotopes 7 He and 9 He led to the observation of unknown IAS in 7 Li and 9 Li. Spectroscopic information for these states were obtained, and implication of these findings to the structure of 7,9 He is considered.

The heavy helium isotopes 7,9 He were studied via their isobaric analog states (IAS) in 7,9 Li. The IAS were populated via resonance reactions of protons with radioactive beams of 6,8 He. The isospin-conserving neutron decay of T=3/2 resonances in 7 Li and proton decay of T=5/2 resonances in 9 Li were measured. New spectroscopic information on these states were obtained, and compared with the properties of levels in 7,9 He.

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.

A cryogenic tritium target and computer-controlled equipment providing for feeding/recovering tritium into/from the target cell and radiation monitoring in technological lines and in workrooms is described. The tritium target set-up showed reliable operation in experiments done with a 58.3 MeV triton beam focused on a 5 mm spot at intensity approaching 3×107 s−1. The t+t elastic scattering cross-section was obtained in a wide angular range.

To investigate 5H resonance states with a better instrumental resolution, we utilized the two-neutron transfer reaction 3H(t, p)5H accomplished with the use of a cryogenic liquid-tritium target and 57.5-MeV triton beam. As a result of this study, a valuable fraction of protons detected at θlab = 18°-32° in ptn coincidence events was attributed to the states of the 5H nucleus. Two resonance states situated at 1.8 plusmn; 0.1 and 2.7 ± 0.1 MeV above the t + n + n decay threshold were obtained in the missing mass energy spectrum of the 5H nucleus. The peak located close to E5H = 1.8 MeV was clearly seen in the H spectrum obtained from the energy distributions of 3H nuclei emitted in the reaction 2H(6He, 5H)3He at θ lab = 17°-32°. The width (Γobs ≤ 0.5 MeV) obtained for the two 5H resonance states is surprisingly small. A state of 4H with Eres = 3.3 MeV and γ2 = 2.3 MeV was obtained in the reaction 2H(t, p)4H from the spectra of protons leaving the target at θlab = 18°-32° and detected in coincidence with neutrons emitted in the decay of 4H nuclei. © 2003 MAIK "Nauka/Interperiodica".

Experimental search for 4 H and 5 H has been performed in the reactions 3 H(t,p) 5 H, 2 H( 6 He, 3 He) 5 H and 2 H(t,p) 4 H. A resonance state situated at 1.8±0.1 MeV above the t+n+n decay threshold is obtained in the missing mass energy spectrum of the 5 H nucleus. Also, there is an indication that another resonance located at 2.7±0.1 MeV presents in this spectrum. Visible peak widths are governed mostly by the instrumental resolution, which came to about 0.4 MeV. We set an upper limit of 0.5 MeV on the true width of any of the two states. The resonance state of 4 H with E res = 3.3 ± 0.2 MeV and Γ res = 4.1 ± 0.3 Mev was obtained in the 2 H(t,p) 4 H reaction.

TheseparatorACCULINNAwasupgradedtoachievenewexperimentalrequirements.Thebeamlinewasextendedbynewion-opticalelementsbeyondthecyclotronhall.Thenewarrangementsyieldmuchbetterbackgroundcondi-tions.Theintensitiesof 6 Heand 8 Heradioactivebeamsproducedinfragmentationof35AMeV 11 Bionswereincreaseduptoafactorof10.Theupgradedbeamlinewasusedinexperimentstostudythe 5 Hresonancestatespopulatedinthet þt reaction.Acryogenicliquidtritiumtargetwasdesignedandinstalledattheseparatorbeamline. 2002ElsevierScienceB.V.Allrightsreserved. PACS:25.70.Mn;29.25.RmKeywords:Radioactive ion beams;Productiontarget;Profileddegrader;Liquidtritiumtarget 1.IntroductionTheseparatorACCULINNA[1]isinoperationsince1996.Highintensityprimarybeamsof 7 Li, 11 B, 13 C, 15 Nand 18 Owithenergyvaluesrangingbetween32and50AMeVaredeliveredbytheU-400Mcyclotrontotheproductiontargetoftheseparator.Thesecondarybeamsof 6 Heand 8 Henucleiwithenergiesofabout25AMeVallowedustogainnewinsightsintothestructureofneutronhalonucleithroughthestudyofone-andtwo-neutrontransferreactionsmadewithhydrogenandheliumtargets[2–5].Suchbeamsofferfavor-ableconditionsfortheproductionofsofarun-knownparticle-unstablenucleibeyondtheneutrondripline.Forthispurposeonecoulduseone-stepone-proton and two-neutron transfer reactionsoccurringwheneither

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