Conversation so far:
What is The Manhattan Project?
The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons.
When did The Manhattan Project begin?
The Manhattan Project began in 1942.
Did Joseph Rotblat work on The Manhattan Project with other people?
Joseph Rotblat worked in Egon Bretscher's group investigating high-energy gamma rays, and then with Robert R. Wilson's cyclotron group.
What kind of things did Joseph Rotblat do on the Manhattan Project?
Joseph Rotblat investigated whether high-energy gamma rays produced by nuclear fission would interfere with the nuclear chain reaction process.
Are there any other interesting aspects about Joseph Rotbalt on the Manhattan Project?
Joseph Rotblat continued to have strong reservations about the use of science to develop such a devastating weapon.

Question: What did he do after he left?
Self-contained rewrite: What did Joseph Rotblat do after he left the Manhattan Project?