New Isotopes of Neptunium

Abstract

Evidence is presented for three new, light neptunium isotopes formed by deuteron bombardments of ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{235}$, and ${\mathrm{U}}^{238}$. The isotope ${\mathrm{Np}}^{231}$ has a half-life of 50\ifmmode\pm\else\textpm\fi{}3 min. and emits alpha-particles of 6.28-Mev energy; the proportion of decay by electron-capture has not been determined. The isotope ${\mathrm{Np}}^{233}$ decays predominantly by electron-capture with a 35\ifmmode\pm\else\textpm\fi{}3-min, half-life; it has an alpha-decay half-life roughly determined to be $\mathrm{ca}$. 10 yr. corresponding to a $\frac{K}{\ensuremath{\alpha}}$-branching ratio of 1.5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ and the alpha-particles have an energy of 5.53 Mev. A 13\ifmmode\pm\else\textpm\fi{}3-min. period with electromagnetic radiation, indicating orbital electroncapture, is attributed to ${\mathrm{Np}}^{232}$. Approximate cross sections for formation of various neptunium isotopes by deuterons of energy from 15 to 100 Mev have been determined. The cross sections are low, of the order of ${10}^{\ensuremath{-}26}$ ${\mathrm{cm}}^{2}$, possibly because most of the excited nuclei undergo fission.