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 ${\mathrm{Bk}}^{249}$, ${\mathrm{Cf}}^{249}$, ${\mathrm{Cf}}^{250}$, ${\mathrm{Cf}}^{252}$, and ${\mathrm{Cf}}^{253}$ 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 ${\mathrm{Cf}}^{249}$, ${\mathrm{Cf}}^{250}$, and ${\mathrm{Cf}}^{252}$ 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 ${\mathrm{Cf}}^{252}$ is larger than the alpha energy of ${\mathrm{Cf}}^{250}$) indicating an irregularity in the nuclear energy surface. The alpha spectra of ${\mathrm{Cf}}^{250}$ and ${\mathrm{Cf}}^{252}$ exhibit characteristic even-even nuclide fine structure peaks. The beta-particle energy of ${\mathrm{Bk}}^{249}$ is 80±20 kev. The spontaneous fission half-lives of ${\mathrm{Cf}}^{250}$ and ${\mathrm{Cf}}^{252}$ are 1.5±0.5×${10}^4$ and 66±10 years, respectively. Lower limits of 2×${10}^8$ and 5×${10}^6$ years have been placed on the partial spontaneous fission half-lives of ${\mathrm{Bk}}^{249}$ and ${\mathrm{Cf}}^{249}$. The pile (Materials Testing Reactor at Arco, Idaho) neutron capture cross sections of ${\mathrm{Bk}}^{249}$, ${\mathrm{Cf}}^{250}$, ${\mathrm{Cf}}^{251}$, and ${\mathrm{Cf}}^{252}$ are 350, 1500, 3000, and 25 barns, respectively.