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.

• Received 1 September 1954

DOI:https://doi.org/10.1103/PhysRev.96.1576