John Cockcroft became director of the Montreal Laboratory in 1944. For safety reasons, he decided to build the reactor at the remote Chalk River Laboratories. With an eye on a post-war nuclear program, he had Pontecorvo and Allan Nunn May "debrief" Manhattan Project scientists who visited Canada, in practice spying for Britain. Unfortunately, Nunn May was also a Soviet spy. Pontecorvo's second son was born on 20 March 1944, and was named Tito after the Yugoslavian communist leader. A third son, Antonio, was born in July 1945. With heavy water supplied by the United States, the reactor at Chalk River, known as ZEEP went critical on 5 September 1945. In addition to reactor design, Pontecorvo also looked into cosmic rays, the decay of muons, and what would become his obsession, neutrinos. He wrote 25 papers related to reactor design, although only two were published. He also did some prospecting with his old firm, searching for uranium deposits near Port Radium in the Northwest Territories.