Section 6.2.2.2. Moderators

Moderators for pulsed spallation neutron sources are nearly always composed of hydrogenous material of about 1 l in volume. Either a thermal or fast reflector surrounds the moderator. Reflectors composed of materials with strong neutron slowing-down properties, such as Be or D₂O, are called thermal reflectors; fast reflectors are composed of materials with weaker slowing-down powers, such as Pb or Ni. In order to retain a narrow pulse width in time, thermal neutrons produced in the reflector region are prevented from reaching the moderator module by judicious use of liners and poisons (typically Cd or Gd) that allow transmission of fast (high and intermediate energy) neutrons, but are opaque to thermal neutrons. Such a moderator arrangement is said to be decoupled, and all thermal neutrons extracted by the beam pipe originate in the moderator itself. For a 0.25 µs-long proton pulse, the target (W or U) produces a fast neutron burst of about 0.5 µs in duration. These very high energy neutrons are slowed down in the reflector and are reflected back into the moderator to produce a thermal neutron pulse of about 1 µs duration. Since thermal neutrons produced in the reflector are prevented from reaching the moderator by the use of liners and poisons, the experiment sees only thermal neutrons originating in the moderator.

By moving the decoupling and poison layers away from the moderator and into the reflector, one can redefine how actively a reflector communicates via neutrons with a moderator and how some of the thermal neutrons produced in the reflector are extracted (Schoenborn et al., 1999). The result is an increase in flux, both peak- and time-integrated, but at the expense of the sharply defined time distribution. With the elimination of all liners and poisons, a fully coupled system is obtained with a flux gain of about 6 × but with poorer wavelength resolution. The wavelength distribution at a given distance from the moderator is shown in Fig. 6.2.2.3(a) for the fully decoupled case and in Fig. 6.2.2.3(b) for the fully coupled case. For a decoupled moderator, the slowing-down power of the reflector is not as critical as it is for the coupled one. In the coupled moderator, it is beneficial to use a thermal reflector in the volume immediately surrounding the moderator because this enhances the peak thermal neutron flux. The decay constant of the neutron pulse can be tailored to match the diffractometer resolution by using a composite reflector composed of an inner thermal reflector and an outer fast reflector. The outer reflector can have a moderate thermal neutron absorption cross section, or the inner reflector can be decoupled from the outer reflector in the same manner that a moderator is decoupled from a reflector. The decay constant can then be varied by simply adjusting the size of the inner reflector (Russell et al., 1996). The wavelength or energy distribution of thermal neutrons produced in the moderator is dependent on the temperature of the moderating medium, as described in Section 6.2.1.2.

Figure 6.2.2.3 | top | pdf | Neutron flux given as a time–wavelength spectrum for (a) a fully decoupled system and (b) for a fully coupled system. Both spectra are based on Monte Carlo codes (LAHET and HMCNP) and are calculated for a target-to-sample distance of 10 m. Comparison of such Monte Carlo results calculated using the geometry of an existing beamline shows agreement with measured values to within 10%.

For neutron protein crystallography, a moderator with an intermediate temperature between a cold and thermal moderator would be most appropriate. This can be achieved with a composite moderator composed of a thermal and a cold moderator in a symbiotic configuration, or a cold methane system.