Alpha-decay properties of ²⁶¹Bh

Abstract

The isotope ²⁶¹Bh was produced in the reaction ²⁰⁹Bi(⁵⁴Cr, 2n)²⁶¹Bh and its $\alpha$ decay has been remeasured. It was found that it populates by an unhindered transition of $\approx$ 10 MeV an excited level at E ^* > 350 keV in the daughter nucleus ²⁵⁷Db . The latter decays by internal transitions either into the isomeric state or the ground state. A somewhat improved half-life value of T _1/2 = 11.8^+3.9 _-2.4 ms was obtained for ²⁶¹Bh . The data support the previous assignment of the $\alpha$ activities ²⁵⁷Db (1) and ²⁵⁷Db (2) to the isomer and to the ground state, respectively. No evidence for an isomeric state in ²⁶¹Bh decaying by $\alpha$ emission was found. Based on the experimental results and theoretical calculations a partial decay scheme of ²⁶¹Bh including spin and parity assignments of the ground-state and excited levels in the daughter nucleus ²⁵⁷Db populated by the $\alpha$ decay and succeeding internal transitions have been suggested. ²⁶¹Bh represents so far the heaviest nucleus for which such an attempt has been made. No spontaneous fission (SF) events that could be attributed to ²⁶¹Bh were observed, resulting in an SF branching b _SF < 0.05 . The measured production cross-section is (64±15) pb at E ^* = 22 MeV.