Observation of 18 new microsecond isomers among fission products from in-flight fission of 345 MeV/nucleon 238U by Dr.Daisuke Kameda

Tuesday 26 Feb 2013, 13:30 → 14:30 Asia/Tokyo

RIBF Hall, RIKEN

Lecturer: Dr.Daisuke Kameda, RIKEN Nishina Center BigLIPS Title: Observation of 18 new microsecond isomers among fission products from in-flight fission of 345 MeV/nucleon 238U Abstract: In-flight fission with a high-energy 238U beam provides the efficient means to produce neutron-rich nuclei far from stability over a wide range of atomic number roughly ranging from 30 to 60. Taking this unique advantage, we performed a comprehensive search for new isomers [1] as well as new isotopes [2] by the in-flight fission of 345 MeV/nucleon 238U at RIKEN RIBF. Fission fragments were separated and identified using the BigRIPS in-flight separator and were implanted in an aluminum stopper at the focal plane. Delayed  rays were detected within a time window of 20 s following the implantation using three clover-type high-purity germanium detectors. We identified a total of 54 isomers with half-lives of ~ 0.1 - 10 s, including the discovery of 18 new isomers: 59mTi, 90mAs, 92mSe, 93mSe, 94mBr, 95mBr, 96mBr, 97mRb, 108mNb, 109mMo, 117mRu, 119mRu, 120mRh, 122mRh, 121mPd, 124mPd, 124mAg, and 126mAg. A wealth of spectroscopic information on observed isomeric decays allowed us to propose 15 new level schemes for the 12 new isomers and 3 known isomers, 82mGa, 92mBr, and 98mRb, as well as 2 revised level schemes for the known ones, 108mZr and 125mAg. We investigated the nature of nuclear isomerism in relation to the evolution of nuclear structure far from stability, e.g., the evolutions of the N=34 subshell gap and shell structure at N ~ 51, a sudden onset of nuclear deformation at N ~ 60 with shape coexistence, emergence of a variety of nuclear shapes at N ~ 68 with their coexistence. We also found several new isomers, such as 117m,119mRu and 120m,122mRu, in the unexplored region at N ~ 75. Shape isomerism was suggested from the hindered transition probabilities for 117m,119mRu. The ETFSI-Q mass model [3] shows the occurrence of large deformation at this region in the systematic behavior of two neutron separation energies. We proposed that these new isomers are generated by shape coexistence in a new well-deformed region of N ~ 75, similarly to the case of N ~ 60. [1] D. Kameda, T. Kubo et al., Phys. Rev. C 86, 054319 (2012). [2] T. Ohnishi, T. Kubo et al., J. Phys. Soc. Jpn. 79, 073201 (2010). [3] J. M. Pearson et al., Phys. Lett. B 387, 455 (1996).