Zip Out

By U.S. Army chronology, the Atomic Age was born on Dec. 2, 1942, a good 32 months before Hiroshima. Last week the Army, beaming proudly, released a detailed description of the birth.

In a squash court under the stands of the University of Chicago's football stadium, a curious structure had grown, watched by the hopeful, nervous eyes of some of the world's best physicists. It was built of dead-black graphite bricks with small cubes of uranium or uranium oxide imbedded in some of their corners.

This was the world's first uranium pile. Within it, if all went well, would rage the first nuclear chain reaction. Physicist Enrico Fermi, Italian-born Nobel Prizewinner, was sure that all would go well. He had figured every smallest detail, advancing through theory and mathematics far into the unknown.

On Dec. 2, a small group of physicists gathered in the squash court for the final test. Partly shrouded in balloon cloth,* the pile squatted black and menacing. Within it, all knew or hoped, a monstrous giant sat chained. Control rods plated with cadmium (which readily absorbs neutrons) had been thrust into holes in the graphite. When the control rods were removed, Fermi had calculated, the chain reaction would start spontaneously, and the giant would be free.

One of the rods was automatic, controlled by a motor which could shoot it back into the pile when instruments warned that neutrons were getting too thick. Another (called "Zip") was attached to a heavy weight by a rope running over a pulley. When in the "withdrawn" position, it was tethered by another rope; a man with an ax stood ready to cut it free, send it zipping into the pile if anything went wrong. The last rod, marked in feet and inches, was to be worked by hand.

But all the physicists knew that they were in dangerous, unknown territory. So above the pile was stationed a "liquid-control squad" to douse mutinous neutrons with cadmium-salt solution.

Into the Unknown. Fermi ran the test. At 9:54 a.m. he gave an order. A whining motor withdrew the automatic control rod. The Geiger counters on the instrument panel clicked a little faster; a pen drew a slightly higher curve on a strip of paper.

"Zip out!" ordered Fermi a few minutes later. Physicist Walter H. Zinn pulled out the Zip rod and tied it carefully. The counters clicked still faster. The graph pen moved up again.

"Pull it to 13 feet, George," commanded Fermi. Physicist George Weil drew the final control rod part way out of the pile. Faster clicked the counters. He drew it out another foot; then another six inches.

At 11:35, the counters were clicking furiously. The physicists watched fascinated as the curve climbed steadily upward. Then, Wham! With a clang, the automatic control rod (which had been set for too low a neutron count) slammed back into the pile. "I'm hungry," said Fermi calmly. "Let's go to lunch." The other rods were inserted, the pile quieted down.

Neutrons Away. At 2 o'clock the physicists gathered again in the squash court. One by one, on Fermi's orders, the control rods were withdrawn, the counters clicked faster. The pile was alive with neutrons now; the giant was straining his bonds. But it was not quite a chain reaction. The neutron curve moved up, leveled off.

At 3:25 Fermi ordered the control rod out another foot. "This is going to do it," he said. "The curve will... not level off." Now the counters were roaring, not clicking, the graph curve was climbing upward. Fermi studied the instruments, grinned broadly: "The reaction is self-sustaining."

For 28 minutes the physicists watched as the curve climbed sharply upward. The giant was flexing his muscles.

"O.K." said Fermi. "Zip in." The Zip rodshot into the pile. The counters slowed their clicking. The graph curve sagged. But the world outside the squash court would not be the same again.

-To exclude neutron-absorbing air from the pile, the Army ordered a balloon with square sides. The puzzled contractor (Goodyear Co.), not privy to the atomic bomb project, protested the balloon would never fly right, was told to go ahead and make it anyhow.

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