In a paper written shortly after they arrived in England, Halban and Kowarski theorised that slow neutrons could be absorbed by uranium-238, forming uranium-239. A letter by Edwin McMillan and Philip Abelson published in the "Physical Review" on 15 June 1940 stated that this decayed to an element with an atomic number of 93, and then to one with an atomic number of 94 and mass of 239, which, while still radioactive, was fairly long-lived. That a letter on such a sensitive subject could still be published irked Chadwick, and he asked for an official protest to be sent to the Americans, which was done.