The new isotopes ${\mathrm{E}}^{249}$, ${\mathrm{E}}^{250}$, ${\mathrm{E}}^{251}$, and ${\mathrm{E}}^{252}$ were produced by bombardment of ${\mathrm{Bk}}^{249}$ with helium ions. A recoil method was used to separate the bombardment products from the target material, so that the target could be used repeatedly.

Approximate excitation functions for the formation of the einsteinium isotopes were measured as well as their half-lives, $\alpha$-particle energies, and electron-capture branching ratios.

• Received 13 August 1956

DOI:https://doi.org/10.1103/PhysRev.104.1315