Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

44 Biennial Report 2005-072.2.1.3 Sub-Barrier Coulomb Excitation of 110 Sn and its implications for the 100 Snshell closureWe had performed an experiment at CERN, REX-ISOLDE to measure B(E2; 0 + → 2 + ) of unstable106,108,110 Sn isotopes by ‘safe’ Coulomb excitation in inverse kinematics. The γ-rays were detectedby MINIBALL detector array in coincidence with 110 Sn detected in very forward angle (for safeCoulomb excitation) by double sided silicon strip detector. The B(E2; 0 + → 2 + ) of 110 Sn, obtainedfrom detailed data analysis is 0.22(0.022)e 2 b 2 . This strengthens the observation of the evoluationof the B(E2) values of neutron deficient Sn isotopes that has been observed recently in intermediateenergy Coulomb excitation of 108 Sn. It implies that the trend of these reduced transition probabilitiesin the even-even Sn isotopes is not symmetric with respect to midshell mass number A=116as 100 Sn (N=Z) is approached.J Cederkaell†, A Ekstrom†, C Fahlander†, A Hurst†, M Hjorth-Jensen†, F Ames†, A Banu†, PButler†, T Davinson†, U Datta Pramanik, J Eberth†, S Franchoo†, G Georgiev†, M Gorka† etalNAP2.2.1.4 Dipole strength measurements in the 130,132 Sn at GSI, Darmstadt, GermanyProperties of the Giant Dipole Resonance in the nuclei far off stability provide information on theisospin dependence of effective nuclear interaction. However, as theoretically predicted for neutronrichnuclei, a low-lying dipole strength is expected below normal GDR region. The existence ofsuch low-lying strength would have pronounced consequences for astrophysical r-process scenarios.Measurements of the dipole strength functions in neutron-rich nuclei around doubly magic nucleus130,132 Sn were performed with LAND-FRS facility at GSI. The beam of 132 Sn and about 20 otherisotopes of similar A/Z ratio were produced by in flight fission of 238 U primary beam. The secondarybeams with energies around 500 MeV/A were separated by FRS and transfer to cave-B whereprojectiles were excited by means of Coulomb excitation in a secondary lead target. The momentaof the neutrons and fragments after dissociation were measured by LAND-setup detectors. Inaddition to the giant dipole resonance, a resonance-like structure (pigmy resonance) is observed atlower excitation energy around 10 MeV which can be new dipole mode due to oscillation betweenexcess neutrons and core.P Adrich†, A Klimkiewicz†, M Fallot†, T Aumann†, K Boretzky†, U Datta Pramanik, H Emling†,H Geissel†, M Hellstroem†, KL Jones†, JV Kratz†, R Kulessa†, Y Leifels†, C Nociforo†, R Palit†,H Simon†, K Suemmerer†, W Walus†NAP2.2.1.5 Study of room background at the laboratory of SINP through γ-ray spectroscopyWe have studied the room background through γ-ray spectroscopy. The characteristic γ-rays ofthe nuclei were detected using semiconductor (HPGe) and scintillator (CsI, NaI) detectors. Wehave observed members of Thorium, Uranium series of natural background. In addition to thenaturally occurring activities, background also consists of some fission products activities whichoriginated with atmospheric fallout from past weapons testing; 137 Cs, 125 Sb, 95 Zr, 95 Nb etc. Weobserved that passive Pb shielding was not useful for low energy background reduction in gamma-