Oct 04 2007

Analysis of: Klein, A. G. and Werner, S. A., Neutron Optics, Rep. Prog. Phys., Vol. 46, 1983

Published by at 7:47 pm under Neutron Experimentation

As stated in the abstract, alluded to in the title, and reiterated throughout Klein and Werner, “A range of phenomena similar or analogous to those of classical optics is exhibited by slow neutrons.”  The abstract goes on to say that this includes “reflection, refraction, diffraction and interference”.  It would not be possible for neutrons to have wave characteristics which behave like massless photons in most ways, no matter their velocity, if they were affected by gravity.
Attempts are made in the paper to insert Newtonian gravity, however calculations make use of the quantum in the main.  Additionally, the effect of gravity, as an experiment that is rotated, is given in terms of phase shift.  With wavelengths on the order of Angstroms, it is easy to see that phase will be effected by the gravitational force acting on parts in the test apparatus, in terms of tension, compression, shear, and bending.
There are two experiments referenced in the paper that attempt to include the effect of the Earth’s gravitational field.  In the first, Koester 1965, 1967, as in Dabbs et al, there is a straight path to the neutron sensor after single edge diffraction, in this case at “K5” [p. 282] [2.7.2 Measurements of scattering lengths based on mirror reflection.]
“In the very first neutron interferometer, built by Maier-Leibnitz and Springer (1962) (see figure 19(a))”, the flight path spans 9.5 meters, dimension D in the figure.  “The mean effective wavelength was 4.4 Ǻ”, which corresponds to a velocity of 899 m/s.  Neglecting travel through the prism, a drop of 551 μm would be expected due to a gravitational acceleration of 9.81 m/s2 .  No such drop is mentioned, nor is an adjustment in location of the “scanning slit” mentioned.  If the neutrons were pulled by gravity and coming in at an angle off horizontal, we would expect an effect on the interference pattern.  The position of the “main slit” [Fig. 19(b)] is varied only ± 60 μm. [3.4.1 Interference by division of the wavefront.]
In the same section, with a different apparatus, [Fig. 20], “Klein and Opat (1976)”, there is a shorter flight path, 2.0 m, and a slower velocity, 20 Ǻ, 198 m/s.  Again, no compensation for gravity is shown, and the Fresnel diffraction pattern is implied as being the same as for light of a similar wavelength.  This implication is supported by the main emphasis of the paper.
In the section [3.4.2 Interference by amplitude division], neutron interference is said to be “topologically similar to the Raleigh interferometer of classical optics… (Zeilinger 1981)”, and also “analogous to the Lummer-Gehrke interferometer of classical optics”.  Other types of experiments pointed out as being similar are “band pass monochromators” and “the so called ‘super-mirrors’ (Mezei 1976, 1978, Mezei and Dagleish 1977) which are highly efficient neutron polarizers.”

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