Image reconstruction and multiple hole apertures in neutron radiography at FRM-II

Most neutron radiography facilities are based on the principle of a pinhole camera. With this setup the best spatial resolution and the maximal neutron flux at the specimen cannot be achieved simultaneously. A high flux is desirable in order to obtain a high signal-to-noise ratio within a short exposure time. The setup of a neutron radiography facility is always a compromise between high flux and high spatial resolution. Investigations were carried out whether the spatial resolution can be increased without decreasing the neutron flux by help of image deconvolution algorithms. The blur due to the neutron beam geometry, as well as the blur from the detector system was corrected. Whenever only a small neutron fluence is achieved in a projection, choosing a setup with lower spatial resolution and application of reconstruction algorithms improve the data quality considerably. As examples projections of a PC main board and specimens from a chestnut tree are shown in this paper. For increasing the neutron flux without loss in spatial resolution multiple hole apertures were tested. The application of this kind of apertures is useful whenever only a very small neutron fluence would be obtained by a single hole aperture. This is demonstrated in this paper by a phase contrast radiograph of a model air plane engine and a projection of monochromator crystals.