Page 73 - Neutrons for Sciences and Society

Chapter 3 - Why invest so much money in a source of neutrons?
Bethe58 produced a small number of the first polarised neutrons using the transmission of neutrons through iron magnetised with a magnetic field, a method proposed by Bloch in his article.
We now know how to polarise neutron beams efficiently. I presented above the use of helium-3, where the nuclear spins
can be polarised by optical pumping. More often a crystalline ferromagnetic monochromator or a polarised mirror (or super- mirror) is used. To detect the polarisation after scattering a similar device is used. Using a specific magnetic field configuration (a spin flipper) it is possible to invert the polarisation direction of the neutron.
3.2.5 Neutron Spin-echo
This technique was invented in 1972 by Ferenc Mezei59 and allows very small changes of neutron energy to be measured. The principle is shown in Fig. 3.4. It is totally different from other
Fig. 3.4: Layout of the neutron spin echo spectrometer IN15 (updated layout)
58 J.G. Hoffman, M. Stanley Livingston and H.A. Bethe, Phys. Rev., (1937), 51, p214-215, DOI 10.1103/Phys- Rev.51.214
59 F. Mezei (1972) Z. Physik 255, 146, DOI 10.1007/BF01394523 64