Isomeric states (long-lived excited states of nuclei) are valuable sources of information on nuclear structure in the region of transfermium nuclei (Z>100). The electromagnetic transitions often compete with alpha decay and bear information about the change of the spin and parity between the isomeric and lower-lying states. This "mixed" (alpha vs electromagnetic transition) type of decay is characteristic for heavier N=153 isotones, where alpha decay of their long-lived isomeric states (11/2-[725]) was observed (257Rf [1], 259Sg [2]). However, the situation is substantially different for lighter N=153 isotones (249Cm, 251Cf). Their isomeric states decay only via electromagnetic transition. The reasons are half-lives of low-energy isomeric states ranging from ns 10-9 s to 10-6 s for lighter N =153 isotones (249Cm [3], 251Cf [4], 253Fm [5]) up to seconds in the case of 257Rf [1]. This behavior is presumably connected with the energy decrease of the 11/2-[725] Nilsson level. The energy of this level declines significantly between 253Fm and 257Rf. Isotope 255No is the connecting link between the lighter and heavier N=153 isotones. Thus localization of its 11/2-[725] Nilsson level is crucial for understanding the level ordering in this region. Furthermore, it can settle the validity of theoretical macroscopic – microscopic models.
Decay data were obtained at the velocity filter SHIP [6] in GSI Darmstadt. Isotope 255No was produced in the fusion-evaporation reaction 48Ca+208Pb. Decays of 255No isomeric states were registered by the detection set-up placed at the focal plane of the SHIP filter. By using the evaporation residue-conversion electron-conversion electron correlation technique, two new isomeric states of 255No were identified.
Interpretation of the long-lived states in 255No will be shown. Emphasis will be given to the 11/2-[725] Nilsson level in the single-particle level systematic of N=153 isotones. A preliminary scheme of excited states populated in 255No will be presented. In addition, the high-excitation energies and high-spin projection (K) values for two of these excited levels with suppressed deexcitation will be proposed. This specific mechanism of their hindered decay is based on the significant change in the K value. Similar high-energy high-K isomers were observed in other nobelium isotopes [5,7-9].
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