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±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±3-min, half-life; it has an alpha-decay half-life roughly determined to be $\mathrm{ca}$. 10 yr. corresponding to a $\frac{K}{\alpha }$-branching ratio of 1.5×${10}^5$ and the alpha-particles have an energy of 5.53 Mev. A 13±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 ${10}^{-26\mathrm{}}$ ${\mathrm{cm}}^2$, possibly because most of the excited nuclei undergo fission.