Determination of the fine structure constant by measuring the quotient of the Planck constant and the neutron mass

In the silicon crystal the (331) reflection was used. The reflected neutrons had a wavelength of 0.249 nm corresponding to a velocity of 1590 ms/sup -1/. The wavelength follows from the lattice parameter of the silicon crystal. This was measured at the PTB by comparing it with the lattice parameters of the reference silicon crystal. The principle of the velocity measurement is as follows: a roughly monochromatic beam of polarized neutrons is produced by Bragg reflection in a magnetised crystal of Heusler alloy. The beam is modulated by periodically changing the direction of the polarization vector, the frequency being about 750 kHz. For this purpose, a novel type of modulator is used called a meander coil. It produces a magnetic field which is periodic in time and space. After a flight path of several meters the neutrons are incident on the silicon crystal mentioned above. Those with the correct wavelength are back-reflected and returned to the modulator. Here, the beam is again modulated and the two modulations are superposed. The resulting total modulation is analysed by the Bragg reflection in a second magnetised crystal of Heusler alloy.<>