R. Kalpakchieva

Spectroscopy of exotic nuclei with multi-nucleon transfer reactions

Structure investigations of neutronrich helium isotopes and of10Li and11Be have been performed using multi-nucleon transfer reactions. New results have been obtained for7He He, where we have evidence for the observation of the first excited state, i.e. the 1/2-resonance, which is of interest for the spin-orbit splitting in these light neutronrich nuclei. Our results on8He,9He and10He are briefly summarized. Reactions of distinct selectivity have been chosen to study the states of10Li. The structure of the eight states found in these reactions is discussed, as well as their relevance for the decay of11Li resonances. The isotopes10Be and11Be have been investigated to study the a-cluster structure and high-lying states. Nine states of the observed11Be-resonances can be characterized as a molecular rotational band extending up to 25 MeV excitation energy, with a probable maximum spin of 19/2. The reaction mechanism is studied in terms of two-step calculations, where protons are picked-up from the target and neutrons are transferred to the target. Using measured strength amplitudes for the single steps results in a quantitative description of the observed two-step cross sections.

Spectroscopy of 10He

Abstract The mass of 10 He has been measured with the double charge exchange reaction 10 Be( 14 C, 14 O) 10 He at E Lab = 334.4 MeV and a mass excess M.E. = 48.81 (7) MeV is obtained. It follows that 10 He is particle unstable against 2 n - emission by S 2n = −1.07(7) MeV . Two excited states are observed: a broad resonance at an excitation energy of E x = 3.24(20) MeV , and a strong resonance at E x = 6.80(7) MeV .

Isomeric ratios for ^{196,198}{\rm Tl} and ^{196,198}{\rm Au} from fusion and transfer in the interaction of 6He with 197Au

Excitation functions and isomeric cross-section ratios have been measured for the isomeric pairs 198m,gTl, 196m,gTl, 198m,gAu and 196m,gAu, formed in fusion–evaporation and transfer processes in reactions of 6He with 197Au in the energy range 7–60 MeV. The population probability of high- and low-spin states in the fusion reaction with the formation of 198Tl and 196Tl (Jπm = 7+, Jπg = 2−), as well as of the transfer products 198Au and 196Au (Jπm = 12−, Jπg = 2−) is compared with calculations within the statistical model and with similar results, obtained in reactions induced by other nuclei.

Spectroscopy of 13Be

Abstract Six quasi-stationary states of 13 Be populated in the 14 C( 11 B, 12 N) 13 Be reaction at E lab = 190 MeV are reported. A Q -value = −39.60(9) MeV and a mass excess, M.E.= 33.95(9) MeV, have been found for the lowest observed spectral line. The ground state is unstable with respect to one-neutron emission by 0.80(9) MeV. Excitation energies of 1.22(10), 2.10(16), 4.14(12), 5.09(14) and 7.0(2) MeV have been obtained for the observed spectral lines.

Stud of weakly bound and unbound states of exotic nuclei with binary reactions

Abstract Binary reactions using neutron-rich beams (and targets) give the best approach to study the level structure of exotic nuclei. Systematic cross section dependencies of multi-step transfer reactions are discussed. Some rections, for example the proton-neutron exchange ( 12 C, 12 N) and ( 13 C, 13 N) , have distinct selectivity for the population of unnatural or natural parity states, respectively. This selectivity has been applied to new spectroscopic studies of 10 Li using the 10 Be( 12 C, 12 N) and 9 Be)(P 13 C, 12 N) reactions. The lowest resonance of 10 Li observed at 0.24(6) MeV above the neutron threshold. The results are discussed together with other available data.

Mass spectroscopy of13Be

The mass of13Be has been measured with the reaction13C(14C,14O)13Be at ELab=337 MeV. A Q-value of Q0=−37.02(5) MeV was obtained and the mass excess is M.E.=35.16(5) MeV. If the observed line corresponds to the ground state,13 Be is particle unstable with respect to the oneneutron emission by 2.01 MeV. The observed line width of 0.3(2) MeV supports an assignment ofJπ=5/2+ or 1/2−, but excludesJπ=1/2+. An excited state is seen at 3.12(7) MeV; there are indications of a second excited state at 6.5(2) MeV.

Spectroscopy of 13B, 14B, 15B and 16B using multi-nucleon transfer reactions

Abstract: New information has been obtained on excited states of the neutron-rich boron isotopes 14B, 15B and 16B, using the reactions 12C(14C,12N)14B, 13C(14C,12N)15B and 14C(14C,12N)16B at about 24 MeV/A. The mass excess of 16B has been measured for the first time, it is 37.08(6) MeV. This means that 16B is unbound by only 0.04(6) MeV. Furthermore, the nucleus 13B has been investigated with the four reactions 16O(14C,17F), 12C(14C,13N), 12C(13C,12N) and 12C(15N,14O). Choosing different target-projectile combinations, it was possible to populate states with different selectivity. New states are observed in 13B at excitation energies above the threshold for two-neutron decay.

Complete and incomplete fusion of 6Li ions with Bi and Pt

Experimental excitation functions are presented for fusion–evaporation and transfer reactions in the interaction of 6Li with Bi and Pt targets. The data on the cross sections obtained for the produced evaporation residues are in agreement with calculations within the framework of the statistical model using the ALICE-MP code. Enhancement of the cross section for deuteron transfer reactions is observed at energies below the Coulomb barrier. The results are discussed from the point of view of how the nuclear cluster structure influences the probability of interaction at near-barrier energies.

Search for 4H, 5H and 6H nuclei in the 11B-induced reaction on 9Be

Abstract In the reaction 11 B(88.0 MeV) + 9 Be the energy spectra of the 14 O, 15 O and 16 O nuclei have been measured to obtain some information about their partners in the exit channel — the neutron-rich hydrogen isotopes 4 H, 5 H and 6 H. The unbound levels in the 4 H and 6 H systems have been observed at excitation energies of 3.5 ± 0.5 MeV ( Γ ~1 MeV) and 2.6 ± 0.5 MeV ( Γ = 1.5 ± 0.3 MeV), respectively. No evidence for the existence of any bound or unbound state in 5 H has been found.

Spectroscopy of excited states of11Li

The10Be(14C,13N)11 Lireaction and the14C(14C,17F)11 Lireaction have been used to study the levels of11Li. Three excited states have been found, in both reactions, at excitation energies of 2.47(7) MeV, 4.85(7) MeVand 6.22(8) MeV