From 1937 to 1938, he worked as a radiological officer at the Christie Hospital, a centre that specialised in the treatment of cancer, based in Manchester. In 1938, he returned to the University of Cambridge where he joined the Department of Medicine as an assistant to J. A. Ryle, the then Regius Professor of Physic. In 1939, with the outbreak of World War II, he became radiotherapist with the Emergency Medical Service in Cambridge. In 1943, he joined the newly opened Radiotherapeutic Centre at Addenbrooke's Hospital. In 1944, he joined the Chalk River Laboratories, Canada, as head of medical investigations for the joint British-Canadian atomic energy project. He ensured the safety of the workers and undertook research into the biological effects of radiation. During this research he discovered the possibility of using the radioactive isotope cobalt-60 to treat cancer.
