Decay properties of neutron-deficient isotopes ^{256, 257}Db, ²⁵⁵Rf, ^{252, 253}Lr

Abstract.

Isotopes of dubnium (element 105) with mass numbers A = 256, 257, and 258 were produced by the reaction ²⁰⁹Bi(⁵⁰Ti,xn) ^259-xDb (x = 1, 2, 3) at projectile energies of (4.59-5.08) AMeV. Excitation functions were measured for the 1n, 2n and 3n evaporation channels. The same position of the excitation function was observed for the 1n channel as for the previously measured 1n channel of the reaction ²⁰⁸Pb(⁵⁰Ti,1n)²⁵⁷Rf. The measured α-decay data of ²⁵⁷Db and its daughter products resulted in the identification of α-decaying isomeric states in ²⁵⁷Db and ²⁵³Lr. Two new isotopes, ²⁵⁶Db and ²⁵²Lr, were produced at the highest bombarding energies of 4.97 AMeV and 5.08 AMeV. They were identified by delayed α-α coincidences. The measured half-lives are (1.6^+0.5 _-0.3) s for ²⁵⁶Db and (0.36^+0.11 _-0.07) s for ²⁵²Lr. Besides α-decay, a spontaneous fission activity of T _1/2 = (2.3^+1.1 _-0.6) s was observed and attributed to an electron-capture branch of ²⁵⁶Db, which feeds the fissioning nucleus ²⁵⁶Rf. A branching ratio of 0.36±0.12 was obtained. The isotope ²⁵⁵Rf was produced by the reaction ²⁰⁷Pb(⁵⁰Ti,2n)²⁵⁵Rf. Improved decay data have been obtained by means of α- and α-γ spectroscopy.