Calibration of NRSF2 Instrument at HFIR

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The Neutron Residual Stress Mapping Facility (NRSF2) at HB-2B is a new generation-diffraction instrument, adding many new Second Generation features, such as larger beam tube, large sample XYZ goniometer, and KAPPA orienter for a broad range of materials behavior studies. One key feature is the NRSF2 monochromator, which is a double focusing, double crystal monochromator system consisting of two sets of stacked Si crystal wafers. One set of wafers has Si[400] plane normal to the surface while the other set of wafers has the Si[500] normal to the surface. The monochromator crystal diffracts at a fixed diffraction angle of 88{sup ... continued below

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Tang, Fei & Hubbard, Camden R August 1, 2006.

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Description

The Neutron Residual Stress Mapping Facility (NRSF2) at HB-2B is a new generation-diffraction instrument, adding many new Second Generation features, such as larger beam tube, large sample XYZ goniometer, and KAPPA orienter for a broad range of materials behavior studies. One key feature is the NRSF2 monochromator, which is a double focusing, double crystal monochromator system consisting of two sets of stacked Si crystal wafers. One set of wafers has Si[400] plane normal to the surface while the other set of wafers has the Si[500] normal to the surface. The monochromator crystal diffracts at a fixed diffraction angle of 88{sup o} selecting a neutron wavelength determined by the monochromator d{sub hkl}-spacing. This 'Missouri' monochromator system has two independent monochromators, which enable diffraction from the following set of six diffraction planes: Si(511), Si(422), Si(331)AF (Anti-Fankuchen geometry), Si(400), Si(311), and Si(220). These diffraction planes can provide 6 different neutron wavelengths: approximately 1.45, 1.54, 1.73, 1.89 {angstrom}, 2.27, and 2.66 also incorporate seven position sensitive detectors located in a detector shield box. To use this advanced instrument for scientific and engineering measurements, careful calibration needs to be performed to accurately calibrate the seven position sensitive detectors, neutron wavelength, and 2{theta}{sub 0}. Just as in the X-ray diffraction technique, neutron diffraction directly measures the diffraction angle (2{theta}) or diffraction peak position, then based on Bragg's law and a strain free lattice spacing, the strain can be calculated. Therefore anything that can affect the diffracting angle measurement can influence the accuracy of the strain measurements. The sources of difficulties in achieving accurate neutron diffraction peak positions can be classified into three categories. (1) Instrument - These difficulties come from alignment of the monochromator, alignment of the incident and detector slits, leveling of the sample table, 2{theta}{sub 0} offset, and response of the position sensitive detector; (2) Counting statistics - if the peak profile count is too low, then the peak position derived from fitting a profile and background will have larger error. Therefore, adequate counting statistics and well-defined peaks are always good for precise peak position determination; and (3) Sample - Large grain size materials make it difficult to get enough diffracting grains, contributing to the different profile. With a low number the peak becomes 'spot' and results in inaccuracy in peak position. Texture in the sample can change the effective elastic constants and also affect the peak intensity. Phase and composition inhomogeneity can make it difficult to determine an accurate stress-free d{sub 0} for strain calculation. A partially buried gauge volume due to proximity to the sample surface or buried interface can also shift the peak position. The calibration method presented in this report will address the first two categories of difficulties listed above. The FWHM can be minimized for each sample d-spacing by adjusting the horizontal bending of the monochromator crystal. For the monochromator, the optimum FWHM lies between 70 and 110 degree. This range is selected in order to maintain an approximately equiaxed gauge volume and avoid significant increases in peak breadth for the detectors above and below the horizontal plane. To adequately calibrate the position sensitive detectors, 2{theta}{sub 0}, and wavelength, a set of high purity reference powders were selected. Since the selected reference powders have define grain size is, the measurement errors from sample grain size and texture can be excluded, although there may still be micro-strain in the powders, which can broaden the reference peak. In this report, the calibration procedure for the NRSF2 instrument will be presented and calibration results for five monochromator settings from HFIR cycle 403 will be presented. The monochromator settings calibrated include Si(331)AF (Anti-Fankuche n geometry), Si(220), Si(511), Si(422), Si(400), and Si(311). The report presents calibration results for the single PSD that is in the horizontal plane defined by the center of the monochromator, sample, and PSD. Calibration for the out of plane detectors will require additional corrections related to the out of plane angle and finite height of each PSD detector.

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  • Report No.: ORNL/TM-2006/541
  • Grant Number: DE-AC05-00OR22725
  • DOI: 10.2172/969651 | External Link
  • Office of Scientific & Technical Information Report Number: 969651
  • Archival Resource Key: ark:/67531/metadc934477

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  • August 1, 2006

Added to The UNT Digital Library

  • Nov. 13, 2016, 7:26 p.m.

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  • Nov. 21, 2016, 6:34 p.m.

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Tang, Fei & Hubbard, Camden R. Calibration of NRSF2 Instrument at HFIR, report, August 1, 2006; [Tennessee]. (digital.library.unt.edu/ark:/67531/metadc934477/: accessed November 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.