Harold Elford Johns OC (4 July 1915 – 23 August 1998) was a Canadian medical physicist, noted for his extensive contributions to the use of ionizing radiation to treat cancer.
For the duration of the war, he taught physics, mathematics, radar systems, and radio navigation to newly recruited airplane pilots as part of the British Commonwealth Air Training Plan.
The first operational nuclear reactor outside the United States - the NRX - was located at Chalk River, and it provided a source of activated cobalt-60 for Johns's experiments.
Two groups - Johns's at the University of Saskatchewan, and another one in London, Ontario - designed and constructed external beam radiotherapy instruments using radioactive cobalt sources.
In early 1952, Maclean's magazine had dubbed the cobalt-source radiotherapy machine the cobalt bomb - a tongue-in-cheek tribute to this peaceful use of nuclear technology.