Isomer shifts in neptunium compounds

Abstract

Characteristic isomer shift ranges have been measured for several valence states of neptunium, using the Mössbauer resonance of the 59.54-keV gamma ray of ²³⁷Np. Isomer shifts were observed (a) for various neptunium compounds as absorbers, with an ²⁴¹Am—Th alloy as a single-line source, and (b) for the charge states produced by alpha decay of various ²⁴¹Am compounds as sources, with a single-line NpO₂ absorber. Approximate shifts for the 0, +3, +4, +5, and +6 valences of neptunium are, respectively, 0.0, +4.1, +0.2, –1.5 and –3.5 cm/sec, with respect to NpO₂. These data show that the fractional change in nuclear deformation is negative and that the excited- and ground-state quadrupole moments are equal to within 1%. The unusually large isomer shifts observed, spanning more than 2000 natural line-widths, offer an extremely sensitive method for investigating chemical bonding in neptunium-containing systems.