New Isotopes of Neptunium

Evidence is presented for three new, light neptunium isotopes formed by deuteron bombardments of U²³³, U²³⁵, and U²³⁸. The isotope Np²³¹ 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 Np²³³ decays predominantly by electron-capture with a 35±{}3-min, half-life; it has an alpha-decay half-life roughly determined to be ca. 10 yr. corresponding to a Kα-branching ratio of 1.5×{}10⁵ 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 Np²³². 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 cm², possibly because most of the excited nuclei undergo fission.