Page 17 - Neutrons for Sciences and Society

This was rapidly confirmed experimentally by Hoffman et al8. As a consequence the neutron will interact with the magnetic moment of atoms and can be used to study magnetic structures, a great advantage, where X-rays can
contribute little9. In practice the
flux of neutrons produced by the
radium-beryllium source was much
too feeble to perform these studies.
Chapter 1 - Pre-history
All changed in November
1943 with the start-up of the first
experimental reactor at Oak Ridge
in Tennessee, USA. Still called a
“pile”, the X-10 Graphite Reactor
consisted of blocks of graphite
into which were inserted bars of
uranium, producing 3.5 MW of heat, with a flux of neutrons
in the centre of about 1012 neutrons/cm2/s. The primary role of this reactor was to produce plutonium required to build atomic weapons. During this wartime period a spectrometer was installed adjacent specifically to use a neutron beam to learn of effective cross-sections of the neutron with various atomic nuclei which were to be used in the construction of the atomic bomb. After
the war a two-axis diffractometer10 was built by Ernest Wollan which could record diffraction patterns. In June 1946 Clifford
8 J.G. Hoffman, M. Stanley Livingston and H.A. Bethe, Phys. Rev., (1936) 51, 214-215, DOI 10.1103/Phys- Rev.51.214
9 2018 addition: This remained true until the 1980s. With the advent of synchrotron radiation sources producing extremely powerful X-ray beams, X-ray methods now contrinutes significantly to the study of magnetism.
10 A single-crystal was placed on the first axis and served to select a monochromatic beam of neutrons. These then impinge on the sample mounted on the second axis about which the detector rotates to measure the diffracted intensity.
Fig. 1.3: Clifford SHULL working on his two-axes diffractometer in the fifties.
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