S. M. Grimes

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.

Evidence for the ground-state resonance of 26O.

Evidence for the ground state of the neutron-unbound nucleus (26)O was observed for the first time in the single proton-knockout reaction from a 82 MeV/u (27)F beam. Neutrons were measured in coincidence with (24)O fragments. (26)O was determined to be unbound by 150(-150)(+50) keV from the observation of low-energy neutrons. This result agrees with recent shell-model calculations based on microscopic two- and three-nucleon forces.

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 .

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.

Missing Gamow-Teller strength in mass 42

Abstract The reaction 42 Ca(p,n) 42 Sc at E p = 160 MeV is used to measure the Gamow-Teller (GT) strength function. Normalization of the dominant peak in the (p,n) spectrum to B (GT) determined from the analogous transition in the beta decay of 42 Ti renders the strength function absolute, and the total measured stregth is about half of the minimum value required for a T = 1 nucleus. Shell model calculations are presented which reproduce the shape of the strength function, but overpredict the absolute measured strength by about a factor of two. Evidently the missing strength has been moved out of the region of nucleon particle-hole excitations, and quenching, due possibly to Δ 33 coupling, is indicated. Symmetry implications of an observed strong suppression of the T > component of the GT strength are discussed.

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

Study of light neutron-rich nuclei with 14C-induced reactions

Abstract The 14C-beam has been used at HMI to study light neutron-rich nuclei with multi-nucleon transfer reactions. Spectroscopic results about the T=3 nuclei 10He, 14Be and 16B have been obtained, but also 11Li, 7He and the 3n-system have been investigated.

Structure studies of neutron-rich beryllium and carbon isotopes

The structure of neutron-rich beryllium isotopes has been investigated using different heavy-ion induced transfer reactions. A strong sensitivity to the chosen core nucleus is found for the cores of 9Be and 10Be, respectively. With a 9Be core, molecular rotational bands up to high excitation energies are observed due to the pronounced 2α-cluster structure, whereas only few states at low excitation energies are populated with a 10Be core. For 11Be a detailed investigation has been performed for the three states at 3.41 MeV, 3.89 MeV and 3.96 MeV, which resulted in the most probable spin-parity assignments 3/2+, 5/2− and 3/2−, respectively. Furthermore we have studied particlehole states of 16C using the 13C(12C,9C)16C reaction and found 14 previously unknown states.

Structure of neutron-rich Be and C isotopes

The structure of neutron-rich beryllium isotopes has been investigated using different heavy-ion-induced transfer reactions. In neutron transfer reactions, the population of final states shows a strong sensitivity to the chosen core nucleus, i.e., the target nuclei 9Be or 10Be, respectively. Molecular rotational bands up to high excitation energies are observed with 9Be as the core due to its pronounced 2α-cluster structure, whereas only a few states at low excitation energies are populated with 10Be as the core. For 11Be, a detailed investigation has been performed for the three states at 3.41, 3.89, and 3.96 MeV, which resulted in the most probable spin-parity assignments 3/2+, 5/2−, and 3/2−, respectively. Furthermore, we have studied particle-hole states of 16C using the 13C(12C, 9C)16C reaction and found 14 previously unknown states. Using the 12C(12C, 9C)15C reaction, five new states were observed for 15C.

Gamow-Teller strength function for 26Mg→26Al

Abstract A shell-model study of the total Gamow-Teller (GT) distribution for the 26 Mg → 26 Al transition has yielded good agreement with the 0° experimental (p, n) results when normalized to the β-decay rates of the two lowest-lyiong 1 + states in 26 Al. This normalization corresponds to a GT quenching of ≈40%, in good agreement with the Δ-isobar model prediction of Bohr and Mottelson