Capillary neutron optics for boron neutron capture therapy

Glass capillaries with narrow channels (diameters of a few microns) can guide and bend thermal and cold neutrons by means of multiple total external reflection. Capillary neutron optics, which consist of arrays of capillary fibers, have many applications including: neutron focusing, neutron imaging, and filtering of fast neutrons and gamma rays to produce a pure beam of slow neutrons for various applications. The transmission of neutrons through straight and bent capillary fibers has been measured using cold neutron beams from the research reactor at the National Institute of Standards and Technology. Experimental results have been compared with ray-tracing simulations. The good agreement indicates excellent neutron reflectivity from the reflecting surfaces of the capillary channels. A neutron optic assembled from 1763 glass capillary fibers has been used to focus a neutron beam from a Ni-58-coated neutron guide with exit dimensions of 50 x 45 mm(2) and focus it to a 0.53 mm diameter spot (FWHM) with an average neutron current density gain of 80. We also report studies of a simple prototype neutron bender/focuser which bends a thermal neutron beam and focuses it to a submillimeter spot. The applications of using capillary neutron optics for boron neutron capture therapy for cancer are discussed.