Z. Phys. A 356, 239-240 (1996) ZBTSCHRIFT FOR R'P/SIK A Q Springer-Verlag1996 Short note In-beam study of l°2Sn M. Lipoglav,~ek 1'2, M, G6rska 3, J. Nyberg 1, A. Ata.c l, A. Axelsson 1, R.A. Bark 4, J. Blomqvist 5, J. Cederk~ill 5, B. Cederwall 5, G. de Angelis 6, C. Fahlander 1'6, H. Grawe 3, A. Johnson 5, S. Leoni 4, A. Likar 2, M. Matiuzzi 4, S. Mitarai 7, L.-O. Norlin 5, M. Palacz s, J. Persson 1, H.A. Roth 9, R. Schubart 3, D. Seweryniak 1°, T. Shizuma 7, O. Skeppstedt 9, D. Sohler z, G. Sletten4, W.B, Waiters 11, M. Weiszflog 1 ! The SvedbergLaboratoryand Departmentof Radiation Sciences, Uppsala University,Box 533, S-75121 Uppsala, Sweden 2 j. Stefan Institute, Ljubljana, Slovenia 3 Gesellschaftffir Schwerionenforschung,Darmstadt, Germany 4 Niels Bohr Institute, Universityof Copenhagen, Copenhagen, Denmark 5 Departmentof Physics, Royal Institute of Technology, Stockholm,Sweden 6 INFN, LaboratoriNazionalidi Legnaro (Padova), Italy 7 Departmentof Physics, Faculty of Science, Kyushu University,Japan 8 Soltan Institutefor Nuclear Studies, Swierk, Poland 9 ChalmersUniversityof Technology,Gothenburg, Sweden l0 ArgonneNational Laboratory,Argonne, IL, USA II Departmentof Chemistry, Universityof Maryland, College Park, MD, USA Received: 13 September Communicated by B. Herskind Abstract. Excited states in l°2Sn have been identified for the first time, in an in-beam ~/-ray spectroscopic experi- ment. Two 7-ray transitions with energies 1472 and 497 keV following the decay of the seniority 6 + isomer with h/2 = 1.0(5) #s were unambiguously assigned to l°2Sn. Due to the very low cross section of about 2#b for producing J°2Sn in the reaction 5°Cr(SSNi, la2n), a highly selective detector setup utilizing NORDBALL ancillary detectors and a recoil catcher device was used. High "),-ray detection efficiency was achieved with two EUROBALL Ge cluster detectors. PACS: 21.10.Tg; 23.20.Lv; 25.70.Gh; 27,60.+j In order to extend our current knowledge of the structure of nuclei in the l°°Sn region we have recently performed two experiments aiming at studies of excited states in the two proton-hole nucleus 98Cd [1,2] and in its two neutron- particle counterpart 1°2Sn. This work reports on our results concerning 1°2Sn. The experiment was performed at the Tandem Acceler- ator Laboratory of the Niels Bohr Institute in Denmark. A 225 MeV 58Ni beam was used to bombard a 1 mg/cm 2 thick 5°Cr target enriched to 99.0 %. Since there were indications for the existence of a long lived isomeric state in l°ZSn [3], a special detector setup made of two parts, was assembled. The first part, which was placed around the target, consisted of a 47r Silicon ball with 31 elements, a 27r Neutron wall with 15 elements and 5 large BaF2 detectors. The ancillary detectors were used for identification of the reaction channel and for obtaining a precise time reference signal for each event. The second part of the setup consisted of an exchangeable recoil catcher foil placed 60 cm downstream from the target and surrounded by two EUROBALL Ge cluster detectors. Only "y-ray decays, that occurred after the recoils were stopped in the catcher foil, were detected. The flight time of the recoils was about 50 ns. Additional experimental details are given in [1]. A part of a lc~2n gated "),-ray spectrum is shown in Fig. lb. A 7-ray line at 1472keV can be seen among other lines in this spectrum. This line is also present in the lain gated spectrum (Fig. la) due to detection of only one of the two neutrons, but absent in the lplcdn (Fig. lc) and 2aln (Fig. ld) gated spectra. Thus, the 1472 keV line does not be- long to a reaction channel with a proton multiplicity > 0 nor to one with an c~-particle multiplicity > 1. The intensity ratio of the 1472keV line in the lc~2n and the lcdn gated spectra has been compared to the intensity ratio of transi- tions in l°°Cd, which was populated in the 2pl~2n channel, The result indicates that the 1472 KeV line is in a nucleus populated in a reaction with neutron multiplicity 2. There- fore, we assign this 7 ray to the lc~2n reaction channel. A considerable build up of carbon and oxygen on the target took place during the experiment. By Studying both the in- tensity of the 1472 keV q, ray as a function of beam time as well as the angular distribution of the coincident c~ particle it was found that this ")'-ray line is not due to reactions on C or O but originates from reactions on 5°Cr. Thus we can unambiguously assign it to l°ZSn. The cross section for the lO2 reaction leading to Sn was estimated to be about 2 #b [ 1]. In addition to the 1472 keV 7 ray, we found in the lc~2n gated spectrum a ~/-ray line. at 497 keV that behaved in the