Page 74 - Neutrons for Sciences and Society

Neutrons for Science
inelastic scattering instruments, and is worth a brief description. A velocity selector (a rotating drum with helical slots) roughly monochromatises the beam of neutrons. The neutrons are first polarised parallel to the direction of the beam using a super- mirror. They then pass through a flipper where the polarisation direction is turned through 90 degrees. Then they pass along the axis of a solenoid with the field along the propagation direction of the neutrons. In this field the spin direction of the neutrons rotates about this axis (the phenomenon known as Larmor precession). The angle depends on the speed of the neutron. After the solenoid the neutrons pass through another flipper which turns all spins
by 180 degrees, after which the second part of the apparatus is identical to the first. In the second solenoid the neutrons are re-polarised. After another rotation by 90 degrees the final polarisation is measured.
In the absence of a sample the final polarisation is identical to the initial polarisation. The presence of a sample between the
two solenoids can change the velocity of the scattered neutrons, modifying the precession in the second solenoid; the polarisation will hence be different from the initial polarisation. Mezei showed that measuring this change of polarisation allows a very precise determination of the velocity change. The name of the method spin-echo comes from the much-used nuclear magnetic resonance technique. The resolution attained in the first version of these instruments was about a factor of 10 better than that available with back-scattering techniques. More recent versions using longer wavelength neutrons have improved this by another factor of 10. [2018 addition: Why is this important? The triple axis
and time-of-flight spectrometers are able to observe atomic and 65