Measures for Progress: A History of the National Bureau of Standards

THE BUREAU AND THE ATOMIC BOMB 387
over, Snow, and Gordon, brought out in January 1944 to take charge of
purification of U235 scrap so it could be used again, and to prepare especially
purified reagents for use in analyses of the uranium and plutonium.°6
The newcomers were briefed in a series of five lectures given by
Robert Serber, Oppenheimer's colleague at the Radiation Laboratory at
California. The lectures were set down shortly after their delivery by
Dr. Edward U. Condon (to succeed Dr. Briggs as Director of the Bureau 2
years later) in a 26-page pamphlet entitled "The Los Alamos Primer." 67
"The object of the [Los Alamos] project," the primer began, "is to
produce a practical military weapon in the form of a bomb in which the
energy is released by a fast neutron chain reaction in one or more of the
materials known to show nuclear fission." The materials were designated
as 25 [U235], 28 [U238], and 49 [plutonium 239]. "Material 49," the primer
went on, "is prepared from neutron capture reaction in 28. only microgram
quantities have so far been produced. There is another project going on
presently to produce 49 for us in kilogram quantities."
On the basis of current calculations, the primer continued, "the
simplest estimate of the minimum size of the bomb is a critical mass of
200 kilograms, in a sphere twice that size." Upon that assumption, "the
immediate experimental problem is largely concerned with measuring the
*
neutron properties of various materials and with the ordnance problem * *
to determine the critical size and time scale, working with large but subcritical
amounts of active material."
The hazard of radiation that preoccupied every laboratory experiment
and industrial process involving live material and that called on so
much engineering effort in the construction of the plants also haunted
Los Alamos. But the consuming concern at Los Alamos as the bomb
approached realization was predetonation. The primer attempted to estimate
the possibility of a premature or incomplete explosion, particularly one
that might give the enemy a chance to inspect or recover the materials of
the bomb.
Three sources of neutrons were recognized that might provide back.
ground giving rise to the danger of predetonation: (1) cosmic rays,
(2) spontaneous fission, or (3) nuclear reactions which produce neutrons.
Thus, while "there will always be some chance of predetonation," every
Twelve other members of the Bureau, including physicists, chemists, glassblowers,
instrumentmakers, and metallurgists, were at other installations of the Manhattan Dis.
trict. Letter, LJB to War Manpower Commission, May 10, 1945 (NBS Box 502, AP).
Thirty-six copies of the primer, classified "Secret—Limited Circulation," were mimeo-
graphed for the use of key members of the project. Quotations here are from Dr. Condon's
personal copy. The primer was declassified on Feb. 25, 1963. Certain of its information
is alluded to in the Smyth Report, pp. 213 if.