MODIFICATIONS IN NEUTRON BRAGG DIFFRACTION TECHNIQUES FOR STRAIN/STRESS MEASUREMENTS IN POLYCRYSTALLINE MATERIALS

P.Mikula, P.Lukas, J.Saroun, M.Vrana

Nuclear Physics Institute, 25068 Rez near Prague, Czech Republic

V.Wagner

Phys. Technische Bundesanstalt, 38116 Braunschweig, Germany

Non-destructive neutron diffractometry plays an increasing role in the investigation of residual stresses in polycrystalline materials. Depending on the neutron source, in principle, investigations by the scattering of thermal neutrons can be performed by two different types of diffractometers (spectrometers) using

the diffraction of a constant neutron wavelength produced by a crystal monochromator at a steady state source [1,2] or
the entire neutron spectrum from a pulsed source in combination with the TOF technique [3,4]

In the former case, conventional high resolution powder diffractometers are usually used, where the $\delta$d/d resolution is given by the quality of the mosaic monochromator, and the beam collimation is determined by a system of Soller collimators and the scattering geometry.

In this contribution, several improved modifications of the conventional diffractometer for strain measurements are reported. They consist of the use of Bragg diffraction optics - focusing in real and momentum space - based on employing bent-perfect crystals as monochromators (and analyzers in a triple axis arrangement). New performances can easily be implemented on conventional diffractometers and two of them are used in NPI and PTB [5,6]. They provide $\delta$d/d resolution (represented by FWHM of the diffraction profile) of 10$^{-3}$ rad and sufficient luminosity (without any Soller collimator) which permits their effective employment even at medium power reactors for an investigated sample volume of roughly 20 mm$^{3}$. Besides the elastic macrostrain investigations, high instrumental resolution permits us to obtain information on substructure factors determining the diffraction peak profile [7]. The feasibility of our instruments and examples of recent measurements will be demonstrated.

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