The reason for testing it at that date was that only then was [there] enough material available to serve for one test and have another equal amount of material available to actually use in the war against Japan. The test worked beyond all expectations. I had calculated, or rather Serber had calculated, it would give 8 kilotons of energy. But one of Serber's young men had calculated it might give 20. And, of course, I believed the authority, I believed Serber, so I said it would give 8 kilotons, and it gave 20. So it was successful beyond all belief. And thereby established the Plutonium Nuclear Weapon. It was then... a duplicate of that, that was then dropped on Nagasaki a little later.

[Q] The... so you were at Los Alamos essentially till the end of '45.

I was there to the end of '45, the end of the war was about one September, '45, so this was after the war. We were, however, now working on further improvements of the nuclear weapon. It was obvious that it would be more efficient to go back to our original design, namely have a shell of plutonium inside natural uranium and then implode that. And it would, at the same time, it became obvious that plutonium was more expensive to produce than uranium-235, so we were interested in a combined weapon with plutonium in the center and 235 around it, and then natural uranium around that. So this is what we worked on in the later months of 1945. I had got another group leader by then, namely Placzek, who... who had been my friend before. He had been in a project in Canada and brought with him a Canadian physicist, Carson Mark, who, after a while, became Placzek's successor as group leader and then my successor as leader of the Theoretical Division.