Search

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.

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

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.

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