Berkelium and Californium Isotopes Produced in Neutron Irradiation of Plutonium

Californium isotopes of mass numbers 249, 250, 251, and 252 have been identified by mass spectrometric analyses of californium samples produced from neutron-irradiated plutonium. The half-lives of Bk²⁴⁹, Cf²⁴⁹, Cf²⁵⁰, Cf²⁵², and Cf²⁵³ are 290+/-20 days, 470+/-100 years, 10.0+/-2.4 years, 2.2+/-0.2 years, and 18+/-3 days, respectively. The alpha-particle energies of the most prominent alpha peaks for Cf²⁴⁹, Cf²⁵⁰, and Cf²⁵² are 5.81+/-0.03, 6.033+/-0.010, and 6.117+/-0.010 Mev, respectively. The plot of alpha-particle energies vs mass number for the isotopes of californium shows a break (i.e., the alpha energy of Cf²⁵² is larger than the alpha energy of Cf²⁵⁰) indicating an irregularity in the nuclear energy surface. The alpha spectra of Cf²⁵⁰ and Cf²⁵² exhibit characteristic even-even nuclide fine structure peaks. The beta-particle energy of Bk²⁴⁹ is 80+/-20 kev. The spontaneous fission half-lives of Cf²⁵⁰ and Cf²⁵² are 1.5+/-0.5×10⁴ and 66+/-10 years, respectively. Lower limits of 2×10⁸ and 5×10⁶ years have been placed on the partial spontaneous fission half-lives of Bk²⁴⁹ and Cf²⁴⁹. The pile (Materials Testing Reactor at Arco, Idaho) neutron capture cross sections of Bk²⁴⁹, Cf²⁵⁰, Cf²⁵¹, and Cf²⁵² are 350, 1500, 3000, and 25 barns, respectively.