Earlier, in late 1938, Otto Hahn and Fritz Strassman in Germany had discovered nuclear fission. The DTM laboratory was converted immediately to confirmatory experiments, which were successful. More importantly, Richard Roberts discovered the delayed emission of neutrons from fission products. This discovery provided the basis for the laboratory control of nuclear fission in all subsequently developed nuclear power plants.
My own work at DTM during 1939-40 was the measurement of the absolute cross section for photodisintegration of the deuteron by 6.2 MeV gamma rays from protons on fluorine. This was done in collaboration with Nicholas Smith, another Carnegie fellow, formerly at the University of Chicago. Also Norman Ramsey, still another Carnegie fellow, and I measured neutron-proton cross sections using a small proportional counter which I had devised for observing the recoil protons.
Of much greater importance to my future career was my crossing of the culture gap at DTM from nuclear physics to the department's traditional research in geomagnetism, cosmic rays, auroral physics, and ionospheric physics. I was impressed especially by the work of Scott Forbush and Harry Vestine. Also there were occasional visits by Sydney Chapman and Julius Bartels who were then completing their great two-volume treatise Geomagnetism. As a result, my interest in low energy nuclear physics dwindled and I resolved to make geomagnetism, cosmic rays, and solar-terrestrial physics my fields of research -- at some unidentified future date.