Application of small-angle neutron scattering (SANS) to irradiated pressure vessel steels and iron based model alloys

It is known that fast neutron irradiation induces nanometre-sized defects in reactor pressure vessel (RPV) steels. SANS can detect changes of microstructure on such a small size scale and operates as volume integrating measuring method, i.e. statistically reliable mean values can be determined.

Purpose of the talk is to describe preparation, measuring procedure and raw-data treatment of a SANS experiment as well as data interpretation considering as example the analysis of irradiation-induced defect/solute clusters in RPV steels and iron alloys. Preparation covers not only preparation and transport of radioactive specimen but also to submit a scientific proposal to get access to a SANS facility. Part of the measuring procedure and data treatment are absolute calibration, background correction etc.

A particular example for the application of SANS in the field of RPV steels is sketched below. More examples will be outlined in the presentation. Fig. 1 shows a SANS image recorded by an two-dimensional position-sensitive detector consisting of 64x64 cells with a size of 1x1 cm^2. It was obtained for a Fe-based alloy placed in a saturation magnetic field. The scattering vector, Q, is proportional to the scattering angle. The anisotropic scattering pattern is caused by the magnetic field and can be used to separate magnetic and nuclear scattering contributions.

The measured dependence of the scattered intensity on scattering vector (also called scattering curve) is presented in Fig. 2 for an RPV steel in two different irradiation conditions and the unirradiated reference state. An increase of the scattered intensity is observed in the range Q > 0.5 nm^-1. This increase is caused by irradiation-induced defect/solute clusters. The size distribution of these clusters is obtained by Fourier transformation of the scattering curve. The underlying assumptions will be discussed in the talk. The calculated size distribution of the irradiation-induced clusters is shown in Fig. 3.

The basic findings of this experiment are a volume fraction of clusters, which increases with neutron fluence and a mean cluster radius of about 1 nm independent of neutron fluence.