Physicists were in great demand after the war ended in August 1945, and Pontecorvo received attractive and lucrative offers from several universities in the United States. Instead, on 21 February 1946, he accepted an offer from Cockcroft to join the British Atomic Energy Research Establishment (AERE). For the time being, he remained at Chalk River for the commissioning of the new NRX reactor in 1947. He was one of four physicists present in the control room when the NRX was started up on the night of 21–22 July 1947. At the time, it had five times the neutron flux of any other reactor, and was the most powerful research reactor in the world. He acquired the nickname "Ramon Novarro" after the actor of that name following an adventure in which he made a trip to Boston with two women, which culminated in Marianne clearing out the bank account and departing for Banff with the children; but they were reconciled. Although he had previously taken steps to become a United States citizen, he instead became a British subject on 7 February 1948. He finally departed Chalk River for the United Kingdom on 24 January 1949.