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

468 THE NEW WORLD OF SCIENCE (1946-51)
The NBS standard of neutron intensity, a solid beryllium sphere, 4 centimeters in diam-
eter, enclosing a capsule of platinum-iridium containing a grain of radium bromide
compressed to maximum density.
Neutrons are emitted in the sphere by the action of the gamma rays from the radium
at a constant rate of 1.1 million per second.
The establishment of a national neutron standard was made imperative by the
increasing importance of neutrons as bombarding particles in physical and biological
research.
valuable both in the operation of nuclear reactors and in neutron irradiation
research.'°9
Accurate determination was first made at the Bureau of a fundamental
nuclear constant, the gyromagnetic ratio of the proton, in absolute units,
which previously had been made in terms of relative values of other physical
constants. An important contribution to the knowledge of nuclear phenom-
ena, it provided a simple and convenient standard for measuring the absolute
value of magnetic fields, knowledge necessary in the use of such scientific
apparatus as cyclotrons, mass spectrographs, and beta-ray spectrometers, and
in such industrial equipment as servomechanisms and electromagnets.11°
Refinement of application of this constant led to a more precise knowledge
of such other important atomic constants as the magnetic moment of the
L. F. Curtiss and A. Carson, "Reproducibility of photoneutron standards," Phys.
Rev. 76, 1412 (1949); NBS Annual Report 1948, pp. ix, 213. A refinement of the
standard, based on a new determination of its emission rate, was reported in NBS
Annual Report 1956, p. 36.
"' NBS Annual Report 1948, pp. ix, 214.