8 , ▁he ▁returned ▁to ▁the ▁University ▁of ▁Cambridge ▁where ▁he ▁joined ▁the ▁Department ▁of ▁Medicine ▁as ▁an ▁assistant ▁to ▁J . ▁A . ▁R yle , ▁the ▁then ▁Reg ius ▁Professor ▁of ▁Phys ic . ▁In ▁ 1 9 3 9 , ▁with ▁the ▁out break ▁of ▁World ▁War ▁II , ▁he ▁became ▁radi other ap ist ▁with ▁the ▁Emer gency ▁Medical ▁Service ▁in ▁Cambridge . ▁In ▁ 1 9 4 3 , ▁he ▁joined ▁the ▁newly ▁opened ▁Rad i other ape ut ic ▁Centre ▁at ▁Ad den bro oke ' s ▁Hospital . ▁In ▁ 1 9 4 4 , ▁he ▁joined ▁the ▁Ch alk ▁River ▁Labor ator ies , ▁Canada , ▁as ▁head ▁of ▁medical ▁investig ations ▁for ▁the ▁joint ▁British - Can ad ian ▁atomic ▁energy ▁project . ▁He ▁ens ured ▁the ▁safety ▁of ▁the ▁workers ▁and ▁under to ok ▁research ▁into ▁the ▁bi ological ▁effects ▁of ▁radiation . ▁During ▁this ▁research ▁he ▁discovered ▁the ▁possibility ▁of ▁using ▁the ▁radio active ▁is ot ope ▁c ob alt - 6 0 ▁to ▁treat ▁cancer . ▁ ▁In ▁ 1 9 4 6 , ▁he ▁returned ▁to ▁England ▁and ▁became ▁Professor ▁of ▁Rad i other ape ut ics ▁at ▁the ▁University ▁of ▁Cambridge . ▁He ▁was ▁also ▁appointed ▁Director ▁of ▁the ▁Rad i other ape ut ic ▁Centre ▁at ▁Ad den bro oke ' s ▁Hospital . ▁During ▁this ▁appointment ▁he ▁became ▁intern ation ally ▁known ▁for ▁his ▁work ▁on ▁the ▁treatment ▁of ▁cancer ▁by ▁ir rad iation . ▁In ▁