Monochromatic Neutron Beam Production at Brazilian Nuclear Research Reactors

摘要

Monochromatic beams of neutrons are obtained from a nuclear reactor polychromatic beam by the diffraction process, using a single crystal energy selector. In Brazil, two nuclear research reactors, the swimming pool model IEA-R1 (5 MW) and the Argonaut type IEN-R1 (170 W) have been used to carry out measurements with this technique. Neutron spectra have been measured using crystal spectrometers installed on the main beam lines of each reactor. The performance of conventional-artificial and natural selected crystals has been verified by the multipurpose neutron diffractometers installed at IEA-R1 and a simple crystal spectrometer in operation at IEN-R1. A practical Figure of Merit formula was introduced to evaluate the performance and relative reflectivity of the selected planes of a single crystal. A total of 16 practical natural crystals were selected for use in the neutron monochromator, including a total of 24 families of planes. Twelve of these natural crystal types and respective best family of planes were measured directly with the multipurpose neutron diffractometer. The neutron spectrometer installed at IEN-R1 was used to confirm test results of the better specimens. The usual conventional-artificial crystal spacing distance range is limited to 3.4 A. The interplanar distance range has now been increased to ～10 A by use of naturally occurring crystals. The neutron diffraction technique with conventional and natural crystals for energy selection and filtering can be utilised to obtain monochromatic sub and thermal neutrons with energies in the range of 0.001 to 10 eV. The thermal neutron is considered a good tool or probe for general applications in various fields, such as condensed matter, chemistry, biology, industrial applications and others.