Positron annihilation as an additional source of information about plastic deformation in structural materials

Neutron absorption is always a competing process to neutron scattering. Therefore, during strain measurements by neutron diffraction additional information is available through simultaneous gamma-ray spectroscopy. Neutron capture leads to the emission of isotope-specific high-energy prompt gamma radiation, which in turn produces positrons within the bulk of the specimen. The subsequent decay of these so-called antiparticles and the resulting annihilation radiation are influenced by material properties like crystalline defects, such as dislocations created by plastic deformation. Experimentally, this leads to an increase in the mosaic spread of the Bragg reflections for elastically scattered neutrons as well as changes of the shape of the 511 keV line of the positron annihilation radiation. Suitable collimation can assure that the information from both neutron diffraction and positron annihilation comes from the same gauge region. Correlation between the width of the Bragg reflections and the S-parameter (describing the shape of the annihilation line) as a function of the degree of plastic deformation has been established. Experimental work of simultaneous on-line investigation of the plastic deformation behaviour of copper and steel as model substances is performed at the FRG-I research reactor at GKSS and will be reviewed.