Image deconvolution and coded masks in neutron radiography

Nearly all neutron radiography facilities use a setup similar to a single pinhole camera. With such facilities, a compromise between high flux at the detector plane and high spatial resolution has to be made: high resolution can be obtained with a small aperture diameter D and a large distance L between aperture and object. As flux is proportional to (D/L)2, high resolution affords a long exposure time and yields a bad signal to noise ratio. Small L/D ratios provide a low noise level but the resulting image will be blurred. Investigations were carried out whether this situation can be improved by reconstruction of blurred images that were recorded with high fluence. Several image deconvolution algorithms were tested for this purpose. It is shown that the application of those algorithms in neutron radiography is useful only in some special cases: for a given setup the resolution can be increased considerably in some cases. But unless there are restrictions for choosing L and D freely, there is always a combination of L and D that provides a better resolution without algorithmic reconstruction. The quality of reconstructed images strongly depends on the point spread function of the optical system. The point spread function is an issue of the aperture layout. Therefore the performances of different aperture shapes were simulated by Monte Carlo method. It is shown that optimized apertures (like multiple hole masks) improve considerably the quality of reconstructed images compared to a single hole system.