Detector Performance for Fast Neutron Radiography and Computed Tomography

Radiography and computed tomography using fast neutrons can provide imaging contrast of large, dense, or heavily shielded materials that are unobtainable by x-rays or thermal neutrons. However, fast neutron imaging is difficult i n practice due to the scarcity of fast neutron sources and the difficulty of producing fast neutron sensitive detectors, thus requiring specialized equipment for both the source and detector aspects of radiography. A world class neutron radiography and computed tomography facility has been developed at the Los Alamos Neutron Science Center to study the use of fast neutrons for non-destructive testing. Two fast neutron detector configurations were recently investigated for use in this facility: an a-Si flat panel with mounted ZnS:Cu scintillator, and a CCD camera optically coupled to various scintillator screens. Results of detector performance metrics, such as the modulation transfer function and the noise power spectrum, are presented. These metrics are reported as a function of shielding thicknesses, and as a function of the scintillator material chosen for use with the camera system. A comparison of the two systems found that the camera had better spatial resolution, while the flat panel had less noise. Of the three scintillators investigated with the camera system, the custom iridium-doped plastic scintillator had highest resolution, and the ZnS:Ag scintillator had the best noise performance.