Page 63 - Neutrons for Sciences and Society

Chapter 3 - Why invest so much money in a source of neutrons?
It is the only simple substance which absorbs neutrons and is
a gas, hence usable to create a detector (see paragraphs later
on detectors). This is of such importance that the ILL purifies
this rare isotope routinely. In addition it was shown by Larry Passell49 in 1966 that the absorption of neutrons was dependent on the relative orientations of the spin of the neutron and that of the helium-3 nucleus. Only neutrons with spins anti-parallel to that of the helium-3 are absorbed. Hence if we fill a container of polarised helium-3 the container will be transparent to neutrons
in one of the spin states, but would strongly absorb the other. It would therefore be a device for producing polarised neutrons. The advantage in this method, as distinct from others, is that it acts
on neutrons with a wide range of energies. To achieve a working device requires preparing polarised helium-3 then concentrating it. This challenge has met with success by Francis Tasset in collaboration with several laboratories, principally the University of Mainz (E.W. Otten et al). This is not the place to describe the very complex technique required for a functioning system. The project to implement this on instruments at the ILL was financed by the SERC.
Having described absorption of neutrons and practical applications, I will now consider the second type of interaction, neutron scattering. In this case a description in wave terms50 is most appropriate. When the incident neutron wave encounters a nucleus, the wave is emitted where the amplitude varies from one nucleus to another, and can even be negative. If there are several adjacent nuclei these emerging waves can interfere. If the nuclei
49 Passell L. and Schermer R.I., Phys. Rev., 1966, 150, p. 146-151, DOI 10.1103/PhysRev.150.146
50 All elementary particles as mentioned earlier are associated with a wave-like property.
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