No abstract

Spontaneous fission and α-decay of 253−257 104 and 258 106 were investigated in irradiations of 204,206,208 Pb with 50 Ti and 209 Bi with 51 V, respectively. New spontaneous fission activities were identified and assigned to 253 104, 254 104, and 258 106. The half-lives were measured as T 1/2 = (48 +17 −10 ) µs for 253 104, T 1/2 = (23 ± 3) µs for 254 104, and T 1/2 = (2.9 +1.3 −0.7 ) ms for 258 106. No indication for α-decay of any of these isotopes was found. For the α-decay branching ratios b α limits corresponding to b α ≤ 0.1 for 253 104, b α ≤ 0.015 for 254 104, and b α ≤ 0.2 for 258 106 were obtained. These results prove a reduced fission probability for nuclei with neutron numbers N=152 up to Z=106 and a steep decrease of the fission halflives for neutron numbers N < 152 up to element 104.α-decay data of 257 104 and 255 104 have been improved. An isomeric state decaying by α-emission was identified in 257 104 and attributed to a low lying 11/2 − [725] state. A small α-decay branch for the even -even nucleus 256 104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even -even nuclei up to 264 Hs (Z=108), the heaviest even -even nucleus identified so far.