Section 6.2.1.3.1. Collimators and filters

In general, neutron-beam applications are flux-limited, and a major advantage will be realized by adopting advanced techniques in flux utilization. A reactor neutron source is large, and stochastic processes dominate the generation, moderation and general transport mechanisms. Because of radiation shielding, background reduction and space requirements for scattering instruments, reactor beam tubes have a minimum length of 3–4 m and cannot exceed a diameter of about 30 cm. The useful flux at the beam-tube exit is thus between 10⁻⁵ and 10⁻⁶ times the isotropic flux at its entry.

The most important aspect of beam-tube design, other than the size, is the position and orientation of the tube with respect to the reactor core. The widespread utilization of D₂O reflectors enables significant gains to be obtained from tangential tubes with maximal thermal neutron flux, and minimal fast neutron and γ fluxes. A similar result is accomplished in a split-core design by orienting the beam tubes toward the unfuelled region of the reactor core (Prask et al., 1993).

There are two major groups of neutron-scattering instruments: those located close to and those located some distance from the neutron source. The first group is located to minimize the impact of the inverse square law on the neutron flux, and the second group is located to reduce background and provide more instrument space. In both cases, the first beamline component is a collimator to extract a neutron beam of divergence α from the reactor environment. The collimator is usually a beam tube of suitable dimensions for fully illuminating the wavelength-selection device. The angular acceptance of a collimator is determined strictly by the line-of-sight geometry between the source and the monochromator. Some geometric focusing may be appropriate, and a Soller collimator may be used to reduce α without reducing the beam dimensions. For technical reasons, the primary collimator is essentially fixed in dimensions and secondary collimators of adjustable dimensions may be required in more accessible regions outside the reactor shielding.

Neutron-beam filters are required for two main reasons: (i) to reduce beam contamination by fast neutron and γ radiation and (ii) to reduce higher- or lower-order harmonics from a monochromatic beam. Numerous single-crystal, polycrystalline and multilayer materials with suitable characteristics for filter applications are available (e.g. Freund & Dolling, 1995).