Neutron Monochromator Crystal Model, Performance, And Production Testing Process

In this thesis, the diffraction theory and fabrication procedure of the crystals used as neutron monochromators are studied systematically. A model is built for the description of neutrons reflected by imperfect crystals ,which can be used to calculate the reflectivity of these crystals by using the Monte Carlo method. The hot-pressing deformation technique is adopted to investigate the process of the mosaic spread of crystals. A practical fabrication procedure of imperfect crystals is obtained. These work make it possible to build a vertical focusing neutron monochromator and will give a technical support for the development of new type neutron monochromators in the future.The main contents and results obtained are as follows:1) A physical model is established to describe the properties of neutrons reflected by imperfect crystals, such as flat mosaic crystals, bent crystals and d-spacing gradient crystal. This model can obtain the solution of the Darwin equation by means of full Monte Carlo method. Some important physical parameters, such as the secondary extinction depth and the average collision number of multi-scattering events, can be calculated and evaluated, because both the effects of extinction and absorbtion in the crystal are considered carefully. This model is also able to deal with the inhomogeneity of crystals.2) A Monte Carlo program code has been developed in FORTRAN language and has been integrated in MCSTAS program as a standard neutron monochromator component. The component can be utilized to simulate and evaluate the performance of the neutron scattering instruments in detail. The performance of monochromator crystals in common use, including PG, copper, germanium and silicon, is analyzed by using the component. A serial of data of integral reflected intensity has been collected and will be used as reference.3) In order to increase the intensity of the incoming neutrons from monochromator, the mosaicity of monochromator crystal should be increased. By hot-pressing deformation technique , so the mosaic spread process is investigated. Some equipments related with the technique are designed and built. A new type vacuum hot-pressing system has been built with the feature of reversal furnace. It can runs continuously among the total four positions of deformation operation. Compared with similar equipments abroad, it markedly improves the efficiency.4) The hot-pressing technique suiting to germanium crystal has been investigated and the hot-pressing procedure is significantly improved. Finally, a set of technical parameters based on the features of our equipments are obtained, which are also valuable to deal with other new type of monochromator crystals.5) In order to measure the increase in mosaic width of deformed crystal quickly, the current neutron powder diffractometer is adapted and the new function of measuring rocking curve is available. The experimental set-up used to check the quality of the deformed germanium wafers increases the value of the powder diffractometer.6) The measurements of rocking curves for deformed germanium crystals are performed. The results show that anisotropic mosaic spread is successfully introduced into the germanium wafers. After the Ge(115) crystals suffered a cycle of deformation and reflattening, the horizontal mosaicity increases to 5' approximately, the vertical mosaicity increase to 10' approximately within the experimental accuracy, Two cycles will increase the mosaicity to approximately 7' and 18' respectively. The peak shape is symmetrical and the anisotropic ratio of mosaicity at different directions is about 3. All these parameters show that the qualitie has reached the advanced international level.