In June of 1941, just before my seventeenth birthday, Harry Goldberg, my physics instructor at Brooklyn Technical High School, asked me if I would be interested in a summer job at Columbia University. He could not tell me what the job was, but he indicated that I would be helping to build something. Being young and eager to learn (I was seventeenth in a class of five hundred graduating the following year), I accepted. Soon after school let out, I reported to a Mr. Herbert Anderson at Pupin Hall, Columbia’s physics building.

When I arrived that morning with two other teenage boys, we signed a mimeographed form promising to keep our work secret. Then we were introduced to Dr. Walter Zinn and Dr. Enrico Fermi, who was in charge.

We started our first day by choosing different tasks according to our interests and skills. The jobs offered to us were soldering and sealing eight-inch cubical cans made of tin plate, machining graphite blocks to a four-by-four-by-twelve-inch configuration, and carrying or moving the various materials with which we all were working. Another important effort, supervised by Herb Anderson and Walter Zinn, was drying the uranium oxide powder that went into the eight-inch tin-plate containers. This was done in an adjacent building in a room with a concrete floor. Red bricks were placed close together on the floor to make an insulated surface. Uranium oxide powder was then spread over the bricks and dried with electric heaters.

I knew that machining and handling graphite was a dirty job, one I would rather avoid, so I chose to solder and seal the tin-plate containers, which were filled with hot uranium oxide when they arrived at my workstation.

I have no idea how many containers I sealed, but the flow of them seemed unending during the hot July days of that summer. The seal had to be perfect so that no moisture entered when the container cooled. On my workbench were three gas-fired soldering-iron-heating furnaces. We wore leather gloves to avoid burning our hands. The work I was assigned was not unpleasant, but as I learned years later, the danger in it was unseen. The containers I was sealing—this was openly discussed—were filled with U 235. There was absolutely no mention of risk from radioactivity.

We must have handled an enormous number of graphite blocks and containers of uranium oxide over the weeks, gradually assembling them into a twelve-foot cube. Finally one day in August I found myself in the large ground-floor room of Pupin sweeping the residue from our work into a dustpan. Dr. Fermi, a fine, sensitive, softspoken person, came into the room with a serious countenance. We were alone. He was carrying a metal device that looked like a long-handled spatula. He approached the large black cubical structure and inserted the spat- ula into an opening about waist high that had been provided in the center