The nuclear decay properties of heavy nuclides ($A\le 257$) produced in two low-yield thermonuclear explosions, the Par and Barbel events, have been studied with the following results: The $\alpha$-decay branching of ${\mathrm{Cf}}^{253}$ has been observed, $E_{\alpha }=5.978\mathrm{}\pm 0.005$ MeV, $\frac{\alpha }{(\alpha +{\beta }^{-})}=(0.31\mathrm{}\pm 0.04)\%$. The $\alpha$-decay branching of ${\mathrm{Es}}^{255}$ has been observed, $E_{\alpha }=6.300\mathrm{}\pm 0.003$ MeV, $\frac{\alpha }{(\alpha +{\beta }^{-})}=(8.5\mathrm{}\pm 0.3)\%$. The spontaneous fission half-life of ${\mathrm{Cm}}^{250}$ was remeasured and found to be (1.74±0.24) × ${10}^4$ years. Upper limits for the half-lives of ${\mathrm{Cm}}^{252}$ and ${\mathrm{Bk}}^{251}$ have been set at 2 and 3 days, respectively. The existence of 80-day ${\mathrm{Fm}}^{257}$ was confirmed; a sample of ${\mathrm{Fm}}^{257}$ from the Par event decayed with a half-life of 94±10 days. Attempts to produce and detect ${\mathrm{Fm}}^{258}$ by irradiating Par ${\mathrm{Fm}}^{257}$ in a reactor neutron flux were unsuccessful. Upper limits for the half-life of ${\mathrm{Fm}}^{258}$ were determined.

• Received 28 March 1966

DOI:https://doi.org/10.1103/PhysRev.148.1192