A.G. Popeko, A.V. Yeremin, A.N. Andreyev
Flerov Laboratory of Nuclear Research, JINR, 141 980 Dubna, Russia

S. Saro
Deparment of Nuclear Physics, Comenius University, SK-84215 Bratislava, Slovakia

Spontaneous fission and alpha - decay of (257-253)104 and 258-106 were investigated in irraditions of Pb-204,206,208 with Ti-50 and Bi-209 with V-51, respectively. New spontaneous fission activities were identified and assigned to 253-104, 254-104 and 258-106. The half-lives were measured as T1/2 = (0.048 +0.017/-0.010) ms for 253-104, T1/2 = (0.023 +- 0.003) ms for 254-104 and T1/2 = (2.9 +1.3/-0.7) ms for 258-106. No indication for alpha decay of any of these isotopes was found. For the alpha - decay branching ratios b(alpha) limit corresponding to b(alpha) < 0.1 for 253-104, b(alpha) < 0.015 for 254-104, and b(alpha) = 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 half-lives for neutron numbers N < 152 up to element 104. Alpha - decay data of 257-104 and 255-104 have been improved. An isomeric state decaying by alpha - emission was identified in 257-104 and attributed to a low lying 11/2-[725] state. A small alpha - 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 Hs-264 (Z=108), the heaviest even - even nucleus identified so far.