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

NUCLEAR PHYSICS AND RADIO PROPAGATION 467
helium I, its normal liquid form. Liquid helium II, cooled to 2.19 °K or
lower, acquires properties so unique that it is often described as a fourth
state of matter.106
The studies in superconductivity led to the almost simultaneous dis-
covery at the Bureau and at Rutgers University of a new and wholly un-
expected relationship, called the isotope effect in superconductivity. A con-
nection was observed between loss of electrical resistance at very low tempera.
tures and the mass of the atomic nucleus. The pure mercury isotope Hg'98,
it was found, becomes superconducting at a temperature about 0.02 °K
higher than natural mercury. From the discovery that heavier mercury
isotopes react at lower temperatures than the lighter ones, it was inferred that
the must exert an important effect on the superconducting properties
of the metal.107 The finding appeared to offer a key to a basic understand-
ing of how superconductivity comes about.
Still other lines of research in cryogenics were to be explored after
a new laboratory was erected in Boulder, Colo, in 195L
NUCLEAR PHYSICS AND RADIO PROPAGATION
The postwar research program of the Bureau's atomic physics division
ranged widely, from investigations of intense neutron sources and of arti-
ficial radioactive isotopes to safety precautions at atomic plants and measure-
ment techniques. The wartime studies in radiation hazards, atomic metal-
lurgy, and atomic energy levels continued when their administration was
transferred from the Manhattan District to the Atomic Energy Commission.
The AEC also undertook support of a number of studies at the Bureau in
atomic and molecular spectra, radiometry, electron optics, mass spectrornetry,
X rays, radioactivity, and determination of atomic and nuclear
An early achievement was construction of a national primary neutron
standard, to serve as a reference unit of neutron intensity for the quantitative
measurement of the strength of neutron sources and fluxes. In the absence
of the standard, such measurements had previously been possible with an
accuracy of only about 20 percent. The new standard, consisting of a
beryllium sphere with a radium bromide center, made possible intercom-
parison of neutron measurements in other laboratories and later proved in-
106
On the Kelvin scale of absolute temperatures, zero is equivalent to C or
—459.7° F.
E. Maxwell, "Isotopic effect in the superconductivity of mercury," Phys. Rev. 78,
477 (1950); NBS Annual Report 1950, pp. 7—8, 32—33; Annual Report 1951, pp. 16, 26.
NBS Annual Report 1948, p. 216; Annual Report 1951, p. 36.