Samples of einsteinium and fermium have been separated from neutron-irradiated plutonium and carefully purified. Four isotopes of einsteinium were found: 20.03-day ${\mathrm{E}}^{253}$, emitting 6.636-Mev and 6.24-Mev alpha particles and 42- and 393-kev gamma rays, both magnetic dipole transitions, and with a spontaneous fission half-life of 7×${10}^5$ years; 38.5-hr ${\mathrm{E}}^{254m}$, a beta emitter with 1.04-Mev beta rays and 0.680-Mev gamma rays; ∼320-day ${\mathrm{E}}^{254}$, an alpha emitter with 6.42-Mev alpha particles; and 24-day ${\mathrm{E}}^{255}$, a beta emitter. Two fermium isotopes were found: 3.24-hr ${\mathrm{Fm}}^{254}$ emitting 7.20-Mev alpha particles and with a spontaneous fission half-life of 246 days; and 21.5-hr ${\mathrm{Fm}}^{255}$ emitting 7.08-Mev alpha particles.

• Received 30 December 1955

DOI:https://doi.org/10.1103/PhysRev.102.203