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Chapter 22:
Neutron Optics, Neutron Waveguides, and Applications
Historically, it appears that the first suggestion that neutrons evolve according to wave mechanics was made by W. M. Elsasser in 1936. It was followed, also in 1936, by two experiments (by H. Halban and P. Preiswork,2 and by D. P. Mitchell and P. N. Powers) demonstrating that neutrons suffered diffraction by crystalline materials. The experiments evidencing mirror reflection of thermal neutrons (carried out in 1944 by E. Fermi and W. H. Zinn) are regarded as the beginning of systematic research on neutron optics. After those pioneering works, several optical-like effects for slow neutrons (with associated de Broglie wavelength near and larger than 1 Å) have been established experimentally. We shall not attempt to give here a broad presentation of the various subjects included in neutron optics; comprehensive treatments for such a purpose can be found in Refs. 4–7. Rather, we shall discuss a few general aspects and offer an overview of just one subtopic; namely, the confined propagation of slow neutrons along waveguides of small cross section, based upon Refs. 8–10. Hollow guides of suitably large cross section (with transverse dimension of approximately several centimeters) have been developed, and they are technologically interesting, because they allow for the transport of slow neutron beams along relatively long distances. They are essentially based on a geometrical-optics-like phenomenon, namely, the multiple total reflections of the neutrons on the walls of the guide. On the other hand, we are reminded that light can be transmitted along optical fibers, namely, thin solid (say, glasslike or plastic) dielectric waveguides, the transverse dimensions of which may be tens of wavelengths. Here, some interesting physics lies in the fact that the electromagnetic-wave aspects of light have to be taken into account (that is, a geometrical-optics description may not suffice); in particular, light is transmitted along the fiber only in the form of some specific distributions of the electromagnetic field (solving Maxwell’s equations), named propagation modes. A similar phenomenon occurs with the propagation of light along the retinal photoreceptors as biological waveguides (the transverse dimensions of which are only a few wavelengths) in the eye.
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