| According to the demands of neutron scattering instruments to be built at China Advanced Research Reactor (CARR), the designs of several new instruments, i.e. two cold neutron guides (CNG), one neutron stress diffractometer and one high resolution neutron powder diffactometer (HRPD), have been simulated and optimized using Monte-Carlo simulation softwares, MCSTAS and VITESS, first. Requirement on the size of the cold neutron source (CNS) by CNGs has been also studied. The results of this thesis have provided essential data as a basis of the neutron instruments designs.At the pre-design size of the CNS 20cm 20cm, the transmission efficiency and neutron current have been campared among various design scheme of the CNGs with characteristic wavelength of 0.2nm and 0.4nm. It is shown that, by adopting supermirror guides with m=2 instead of 58Ni or natural Ni guides, the transmission efficiency could be increased by the factor of 1.3 ~ 1.7, and the length of direct sight of the curved guides could be shorten greatly, which is useful for arranging shielding. The optimal length of straight guides have been determined by calculating their uniform effect on neutron beam. The performance of the CNGs, including transmission efficiency, neutron flux, flux spectrum, divergency, uniform and gravity effect, have been analysed technically. That the scheme of supermirror guide is optimum has been given and adopted.According to the requirements of the CNG on neutron flux, critical wavelength and distance between inpile guides and the CNS, the relation between the source size and the critical angle of the coating material of the inpile guides as well as neutron flux at exit of the CNGs have been studied. It is shown that, the diameter and height of the CNS,18cm - 22cm and 30cm - 36cm respectively, is necessary, if the 58Ni coating or supermirror coating with m=2 is adopted. The reasonable size of CNS has been suggested.The neutron stress diffiractometer, wich is designed for measuring residual stress or applied stress in materials and engineering components, was simulated and optimized. The position and size of the monochromator, focusing curvature radii and tilting angles of the germanium focusing monochromator under different different take-off angle, size and divergency of the second collimator and intensity loss due to the use of it were determined. The neutron flux at different wavelength, under different reflection and take-off angle of the monochromator and divergency of the first collimator, were given. The simulation on the aperture used for limiting beam size in front of the sample showed that, smaller the size of the aperture and further the distance from the aperture to the sample, greater the loss of the intensity and more serious the spread of the beam along vertical direction at the sample position which makes the practical sampling volume greater than expected. (211) reflection of the polycrystal a-iron sample is used in the simulation experiments, and both the resulting full-width-at-half-maximum(FWHM) of the diffraction peaks and peak shifts under tensive and compressive strain are in accord with that expected from analytical methods. The instrumental resolution curves, under various combination of the first collimator and second collimator and take-off angle of the monochromator, are given.The HRPD, wich is designed for the study of crystal structure and magnetic structure of materials, was simulated and optimized. Some parameters of several components in the initial design, such as collimators and monochromator, were modified after optimization. It is shown that, under the high resolution mode and the high intensitymode, respectively, the neutron flux at the sample position can reach above 8.4 10 and 3 107 neutrons cm-2 -s-1, and the minimum FWHM of the diffaction peaks is 0.245 and0.355 and the correspondent d/d is 1.4 10-3 and 2.3 10-3 , respectively. The main performance of the diffractometer were analysed in detail.
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