The Study Technology Of Measuring Mosaic Angle Of Neutron Monochromator By γ-Ray Diffraction Method

Neutron diffraction technology is an important tool for analyzing themicrostructure of the materials, and has unique advantages as compared with X-raydiffraction analysis method. It can be used to analyze the positions of the lightelements and the adjacent elements in crystal and the magnetic structure of thematerials, which are not analyzed by X-ray diffraction method. Achieving neutrondiffraction analysis is inseparable from the establishment of neutron diffractionspectrometer. The monochromator crystal, which can convert white neutron fromreactors into monochromatic neutron source neutron that is necessary for diffractionexperiments, is one of the key components of neutron diffraction spectrometer.However, this conversion performance of monochromator is closely related to itsmosaic angular distribution. Therefore the development of mosaic angle measuringtechnology of neutron monochromator for the evaluation and selection ofhigh-performance monochromator components is of great significance.It can be achieved for the measurement of mosaic angular distribution ofneutron monochromator by neutron diffraction method. But it requires the use ofhigh flux reactor of thermal neutron, which is not only expensive but also notquickly and easily. For theγ-ray, which is accessible and more convenient to use,owns the characteristic of single energy and a similar penetration compared with thermal neutron, therefore theγ-ray diffraction method for measuringmonochromator mosaic angle can overcome the inadequacies of neutron diffraction.To sum up, this work adoptsγ-ray diffraction method for measuring the mosaicangular distribution of the neutron monochromator.Theγ-ray diffraction method requiresγ-ray source with the characteristics ofhigh monochromaticity and high intensity and suitable wavelength. In view of thisrequest, by literature research and comparative analysis this paper selects 198Au asγsource for the diffraction device, which is obtained from 197Au activated by thermalneutron in reactor, in view of the big capture cross-section of nuclear reaction 197Au(n,γ) up to 98.8 barn and the short half-life of 2.7 days (but it can meet the testrequirements) and the 100% isotopic abundance, which is easy to obtain by thermalneutron activation in reactor. It is important that 198Au can emit theγ-ray of0.003nm wavelength (412KeV energy) that can also meet the diffractionrequirements for the wavelength.The option of the gold size for the diffraction asγsource is an important issuethat needs to consider. Too small to meet the requirement for theγdiffractionexperimental intensity, while too big not to produce the uniform activation since theself-shielding effect of thermal neutron, and because of the self-absorption effect ofγ-ray, the larger gold foil will not increase the intensity ofγ-ray beam after thecollimator obviously, but also the multiple Compton Scattering ofγ-ray can worsenthe monochromaticity ofγ-ray beam. Moreover, too big size of gold would increasethe total intensity of theγ-ray beam, which leads to fail to shield effectively anddifficulty. In this paper, the Monte Carlo method is applied to simulate the differentdimensions of the gold that is activated uniformly, the variation ofγ-raytransmission rate with the thickness of gold foil and change relation ofmonochromaticity with the gold thickness on the Compton scattering. From theseresults, considering monochromator mosaic block size and its structure, we designthe gold size of 0.1×1×0.3cm~3 as the size ofγsource.On the basis of the design of the gold size, we calculated the radioactiveactivity of 198Au after it is irradiated for certain time under certain thermal neutron fluence rate, and we analyzed the influence of the process on the intensity andmonochromaticity ofγsource considering the characteristic, which 198Au has largethermal neutron capture cross section to generate 199Au. On the basis of theestimation on theγsource strength, we designed the lead-shielding device forγsource diffraction based on standards of national radiation protection.It is necessary for single-energy to measure the mosaic angular distribution ofmonochromator byγ-ray diffraction, as well as a single direction of the incidentγ-ray, which is achieved through the collimator. The intensity ofγ-ray beam will begreatly reduced after collimating. In order to obtain clear and sharp Rocking curveson behalf of mosaic angular distribution, we adopt theγspectrometer with highefficiency detection and certain energy resolution. To the above problems, this paperdesigns the collimator and selects the detector and establishes the detection systems.Finally, we discuss preliminarily the geometric layout of the various components indiffraction device to measure the mosaic angle of neutron monochromator.