Lifetimes and structure of excited states of 115 Sb Yu. N. Lobach Institute for Nuclear Research, pr. Nauki 47, 252028 Kiev, Ukraine D. Bucurescu Horia Hulubei Institute of Physics and Nuclear Engineering, 76900 Bucharest, Romania Received 7 October 1997 Lifetimes of excited states of 115 Sb were measured by the Doppler shift attenuation method in the ( ,2n ) reaction at E = 27.2 MeV. The experimental level scheme and the electromagnetic transition probabilities have been interpreted in terms of the interacting boson-fermion model. A reasonable agreement with the experiment was obtained for the positive-parity states. The experimental data also show the applicability of the cluster-vibrational model for the mixing of two 9/2 + states having different intrinsic configurations. S0556-28139807205-7 PACS numbers: 21.10.Tg, 21.60.Fw, 23.20.Lv, 27.60.+j I. INTRODUCTION The level scheme of 115 Sb at low and medium excitation energies was previously investigated with the ( p ,2n ) 1, ( ,2n ) 2, and ( 6 Li,3n ) 3reactions, respectively, as well as through the decay of 115 Te 4. Recently, the high- spin region of this nucleus was investigated using heavy-ion reactions 5,6. As a result of these works a detailed level scheme has been obtained. Similarly to the other light odd antimony isotopes, the spectrum of the excited states of 115 Sb is determined by the valence proton beyond the Z =50 closed shell and its interaction with collective and qua- siparticle excitations of the Sn core. Moreover, a striking peculiarity inherent to these isotopes are collective bands arising from the excitation of a 2 p -1 h proton state 7–11. Such an interpretation has commonly been based on the level energies only, since the experimental data for electromag- netic transition probabilities is very poor and limited to three isomeric states of 115 Sb in the nanosecond region 2,3,12. Lifetime measurements in the picosecond region for low- lying states in 115 Sb have been reported recently, with the use of the ( p , n ) reaction 13. The aim of the present work is a further study of the lifetimes of excited states in this nucleus, by the ( ,2n ) reaction, and an appropriate inter- pretation of the structure of this nucleus on the basis of cur- rent nuclear models. II. EXPERIMENTAL PROCEDURE AND RESULTS The lifetime experiment was performed at the U-120 cy- clotron of the Institute for Nuclear Research in Kiev. A self- supported 113 In target of thickness 8.5 mg/cm 2 and 92% enrichment was used. The excited states of 115 Sb were popu- lated in the reaction 113 In( ,2n ) at a bombarding energy of 27.2 MeV. The initial recoil velocity of 0.54% of the veloc- ity of light was determined from the reaction kinematics. A high purity Germanium detector of 40% relative efficiency and 2.2 keV full width at half maximum FWHMenergy resolution at E = 1.3 MeV was located at a distance of 17 cm from the target. The -ray spectra were measured at angles of 30°, 60°, 90°, 120°, and 150° with respect to the beam direction. Figure 1 shows the spectrum obtained at 90°; spectra with similar statistics were obtained at the other angles. The lifetimes of the excited states were obtained from an analysis of the Doppler shift attenuated line shapes measured at different angles. Such an analysis was carried out using an updated version of the computer code described in Ref. 14. For the slowing-down process Lindhard’s cross sections 15 were used, with the correction factors f e =1.3 and f n =0.9 for the electronic and nuclear stopping power, respectively. These values are suggested from the analysis of the slowing- down process of Cd ions in a Cd target 16. The velocity distribution of the emitting nuclei was calculated from the simulation of 30 000 recoil histories by a Monte Carlo code which takes into account reactions at different depths of the target, the kinematics of the reaction, as well as the slowing down and the deflection of the recoils. The components of the velocity distributions seen at each of the five detector angles were stored in ‘‘shape-time’’ matrices containing 20 time steps over the range 0.01–10.0 ps. Each matrix contains the complete spectrum of line shapes from fully shifted to FIG. 1. Spectrum of the 113 In( ,2n ) reaction at 27.2 MeV incident energy, measured at 90°. Some of the 115 Sb -ray transi- tions of interest are marked by their energies in keV see also Fig. 2. All the other spectra at 30°, 60°, 120°, and 150°) have been measured with similar statistical accuracy. PHYSICAL REVIEW C JUNE 1998 VOLUME 57, NUMBER 6 57 0556-2813/98/576/28807/$15.00 2880 © 1998 The American Physical Society