Imaging DD fusion neutrons using a coded aperture of sub-mean-free-path thickness

Neutron imaging is a useful tool with applications ranging from NIF implosion geometry to material radiography for industrial and security applications. Pinhole arrays typically used for such imaging require thick substrates to obtain good contrast along with a small pinhole diameter to obtain good resolution, resulting in pinholes that have large aspect ratios. This leads to expensive pinhole arrays that have small solid angles and are difficult to align. We have previously proposed a coded aperture with scatter and partial attenuation (CASPA), that relaxes the requirement of thick substrates for good image contrast.

Here, we show results using a 3.5 mm resolution 6.8 mm thick tungsten CASPA to image the neutron source of a dense plasma focus (DPF). The LLNL MJOLNIR DPF produces roughly 10¹¹ DD (2.5 MeV) fusion neutrons that have a mean-free-path of 23.8 mm through tungsten, which are typically imaged with pinholes through approximately 110 mm thick substrates to generate absolute imaging contrast. Using conventional cross-correlation decoding techniques, our reconstruction was successful in imaging a source diameter of 5.2 ± 1.8 mm, limited by the aperture cell size. We discuss how to optimise the CASPA imaging system to reduce noise in the reconstructed image, how to improve the imaging resolution, and the limitations on achievable resolution.