Why was Los Alamos needed?

John Manley speaks at a conference in February 1946. Manley supervised the early fast neutron studies that preceded the founding of Los Alamos. He later served as head of the Physics (P) Division, executive secretary of the general advisory committee of the Atomic Energy commission and technical associate director of the Laboratory. He taught physics at the Universtity of Washington from 1951 to 1957 and then returned to the Laboratory as a research adviser. He retired in 1974 and remained in Los Alamos until his death.

The United States was fully engaged in war in the Pacific and Atlantic oceans in the summer of 1942. In the Atlantic, German U-boats sank an average of 100 ships a month in 1942, losing only 21 submarines in the process. In the Pacific, U.S. forces first engaged the Japanese in the Solomon Islands at Guadalcanal in August, setting off a bitter six-month campaign. The outcome of both conflicts was uncertain.

With the prospect of a long war, a group of theorists under the direction of J. Robert Oppenheimer met at Berkeley during the summer of 1942 to develop preliminary plans for designing and building a nuclear weapon.

Crucial questions remained, however, about the properties of fast neutrons. John Manley, a physicist at the University of Chicago's Metallurgical Laboratory, was assigned to help Oppenheimer find answers to these questions by coordinating several experimental physics groups scattered across the country.

The measurements of the interactions of fast neutrons with the materials in a bomb are essential because the number of neutrons produced in the fission of uranium and plutonium must be known, and because the substance surrounding the nuclear material must have the ability to reflect, or scatter, neutrons back into the chain reaction before it is blown apart in order to increase the energy produced. Therefore, the neutron scattering properties of materials had to be measured to find the best reflectors.

Estimating the explosive power required knowledge of many other nuclear properties, including the cross-section (a measure of the probability of an encounter between particles that result in a specified effect) for nuclear processes of neutrons in uranium and other elements. Fast neutrons could only be produced in particle accelerators, which were still relatively uncommon instruments in physics departments in 1942.

"I had to chase around the country because there were nine separate contracts with universities that had accelerators which could be used as neutron sources," Manley recalled. "Measurements of fast neutron properties were going on everywhere from Washington, D.C., to Rice (University in Houston), to Minnesota, Wisconsin, Purdue, and so on."

Manley encountered many problems: The amounts of materials available for these experiments were infinitesimal, the different neutron sources were not calibrated with each other and uranium was available only in microgram quantities from the mass spectrograph devised by physicist Al Nier at Minnesota. "It was really a very discouraging sort of physics -it was all very new," Manley said.

Manley's frustration was compounded by the fact that normal scientific communication between the projects was impossible. Telephone calls were prohibited by government security regulations that governed the uranium project, classified teletypes were hopeless, and it took time to write up the details of experiments.

Manley was forced to travel from project to project trying to resolve the technical questions arising from the use of different neutron sources, detectors and purities of uranium.

"It didn't take very long to realize that you just couldn't run a railroad in this fashion," Manley said. It also was impossible to tell how much uranium-235 or plutonium would be needed for the bomb without estimates based on experiments, and the production plants were already being designed.

The need for better coordination was clear. But how could the scattered experimental efforts involving different accelerators in many locations be brought together? This was the problem that faced Oppenheimer and Manley in August of 1942.

- Robert Seidel

Next time: .Col. Leslie Groves completes the Pentagon building and learns that he has to oversee a "hare-brained" project to build a nuclear weapon.