Measurement of radiation-induced embrittlement on steel samples by neutron small angle scattering

Small angle scattering of slow neutrons can be used for analysing radiation-induced defect clusters in samples of pressure vessel steels. This is shown with samples irradiated by a fast neutron fluence of 2 x 1019 cm-2 for E > 1 MeV at a temperature of 2900C. The measurements were performed using a well collimated beam of slow neutrons from a neutron guide of the FRG-1. the beam was monochromatized by a helical slot velocity selector. Only neutrons with wave lengths above 4 A were used in order to avoid double Bragg scattering. An external magnetic field was provided at the sample position for reducing the effect of magnetic scattering. The neutrons scattered by the steel samples with volumes of about 1 cm3 were measured by a position-sensitive BF3-counter. In most cases the scattering intensity of an irradiated sample was compared to that of an unirradiated specimen of the same material, using an automatic sample changer. In the case of normal pressure vessel steel no effect was found at the above indicated level of irradiation, but samples taken from welds showed rather large radiation-induced cross sections in the region around scattering vectors of 0.1 A-1. It is hardly possible to explain this by assuming dislocation loops, even if magnetic effects are considered; rather the existance of voids must be assumed. In this case mainly small voids with diameters about 20 A contribute to the cross section. It seems rather difficult to see defects of this size in magnetic materials by means of transmission electron microscopy. During annealing at temperatures up to 4500C the void diameters increase, but at the same time the number of voids per cm3 decrease leading to a smaller total void volume. (orig.)