Neptunium Isotopes: 234, 235, 236

Neptunium isotopes of mass numbers 234, 235, and 236 were produced by an 18-Mev deuteron bombardment of uranium highly enriched in ${\mathrm{U}}^{235}$. The radiations of ${\mathrm{Np}}^{235}$ were studied with scintillation and proportional counters in singles and coincidence. The $\frac{L}{K}$ and $\frac{M}{L}$ electron capture branching ratios of ${\mathrm{Np}}^{235}$ were measured to be 36.7 and 0.46, respectively. From the $\frac{L}{K}$ ratio, the calculated decay energy of ${\mathrm{Np}}^{235}$ is 123 kev. An upper limit of 2.1% of the ${\mathrm{Np}}^{235}$ electron capture disintegrations populate excited states of ${\mathrm{U}}^{235}$. The experimental data favor a spin of $\frac{5}{2}$ for ${\mathrm{Np}}^{235}$. The alpha to electron capture ratio for ${\mathrm{Np}}^{235}$ is 1.59×${10}^{-5\mathrm{}}$, giving a partial alpha half-life of 7.0×${10}^4$ years. The long-lived isomer of ${\mathrm{Np}}^{236}$ was produced in the bombardment, but no isomer of ${\mathrm{Np}}^{234}$ with half-life longer than two months was observed in a mass spectrometer analysis of the neptunium sample.