A high-fluence proton irradiation of neptunium was the last experiment in PSIs programme ATHENA related to accelerator-based transmutation. The principal aim of the programme has been to provide experimental data for the validation of theoretical models in nucleon–meson transport codes, with emphasis on the mass yield distribution of fission and spallation products. An improved mass spectrometry method has allowed the direct derivation of isobaric production cross sections with only minor corrections and an estimate of the fission cross section by integration in the fission hump. In a second sample position of the irradiation head, a repetition of the previous ATHENA experiment with thorium was possible, profiting from the improved mass spectrometry technique. The experimental results are better predicted by the code FUSSPOT than by HETC/RAL, both used at PSI.