Search for K isomers in 252,254 No and 260 Sg and investigation of their nuclear structure
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Abstract
Study of K isomerism in the transfermium
region around the
deformed shells at N=152, Z=102, and N=162, Z=108 provides
important information on the structure of heavy nuclei. Recent
calculations suggest that the K-isomerism can enhance the
stability of such nuclei against alpha emission and spontaneous
fission. Nuclei showing K isomerism have neutron and proton
orbitals with large spin projections on the symmetry axis which is due to multi quasiparticle states with aligned spins K.
Quasi-particle states are formed by breaking pairs of nucleons and raising one or two nucleons in orbitals near the Fermi surface above the gap, forming high K (multi)quasi-particle states mainly at low excitation energies. Experimental examples are the recently studied two quasi-particle K isomers in 250,256-Fm, 254-No, and 270-Ds. Nuclei in this region, are produced with cross sections ranging from several nb up to µb, which are high enough for a detailed decay study. In this work, K isomerism in Sg and No isotopes was studied at the velocity filter
SHIP of GSI, Darmstadt. The data were obtained by using a new data acquisition system which was developed and installed during this work. 252,254-No and 260-Sg were produced in fusion
evaporation reactions of 48-Ca and 54-Cr projectiles with 206,208-Pb targets at beam energies close to the Coulomb
barrier. A new K isomer was discovered in 252-No at excitation
energy of 1.25 MeV, which decays to the ground state rotational
band via gamma emission. It has a half-life of about 100 ms. The
population of the isomeric state was about 20% of the ground
state population. Detailed investigations were performed on
254-No in which two isomeric states (275 ms and 198 µs)
were already discovered by R.-D. Herzberg, but due to the higher
number of observed gamma decays more detailed information about
the decay path of the isomers was obtained in the present work. In 260-Sg, we observed no statistically significant component
with a half life different from that of the ground state. A
ncomparison between experimental results and theoretical
calculations of the single particle energies shows a fair
agreement. The structure of the here studied nuclei is in
particular important as single particle levels are involved which are relevant for the next shell closure expected to form the region of the shell stabilized superheavy elements at proton
numbers 114, 120, or 126 and neutron number 184. K isomers, in
particular, could be an ideal tool for the synthesis and study of these isotopes due to enhanced spontaneous fission life times
which could result in higher alpha to spontaneous fission
branching ratios and longer half lifes.