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Neutrons for Science
The beam is pulsed by the first pair of choppers. The pulses are funnelled down to a 5 cm high beam using supermirrors to the final monochromating chopper pair. Only neutrons of a specific speed (hence energy) can pass through these. Those with half or multiples of the speed are eliminated by the intermediate pair of choppers. The monochromatic neutrons are then scattered by the sample, and are sorted by arrival time and position at the detectors, giving the change in energy and momentum in the sample.
Since neutrons slowed by the sample can arrive during the time frame of the next pulse, this overlap can be reduced by rotating the 3rd and 4th discs more slowly slowly (in Fig. 3.3 the later are shown as two pairs of counter-rotating disk choppers), suppressing an integer fraction of pulses and increasing the time between pulses, but with an inevitable loss of intensity.
3.2.4 Polarised Neutrons
In many experiments, elastic and inelastic, it is necessary to use polarised neutrons, that is neutrons which mostly occupy one of the two possible states of spin 1/2 (in the presence of a magnetic field the magnetic moment of the neutron may be either parallel
or anti-parallel to the field). There are several ways of producing such beams where the mean magnetisation points in one direction. All methods depend on the differences occurring between the two spin states of the neutron when it encounters polarised material. This was suggested by Felix Bloch in 1936 (in a publication57 of less than a page length) where he hypothesises the existence of the neutron’s magnetic moment. In 1937 Hoffmann, Livingstone and
57 F. Bloch, Phys. Rev., (1936), 50, p259-260, DOI 10.1103/PhysRev.50.259 63