The test pieces used initially for evaluation included practical honeycomb assemblies and also quantitative indicators for spatial resolution and contrast sensitivity. As the program proceeds, the intention is to obtain more detailed data about the images, MTF, and dynamic range as examples. Storage phosphor systems are now available for x-ray imaging. These phosphors utilize trapping mechanisms to store the x-ray image information, which can be released in the form of visible light when the phosphor is exposed to a laser scanning beam. The system includes a scanner that correlates the laser spot with the emitted light, as detected by a photomultiplier tube. The flat panel typically makes use of amorphous silicon in a transistor and photodiode combination. A phosphor screen provides the light input for this x-ray detector. Both of these methods provide resolution values in the order of four line pairs/mm (lp/mm) and excellent dynamic range, typically greater than 11 bits.

There are many camera systems that can be considered for the Air Force inspection application. Simple lens-coupled systems offer reasonable large area coverage (8 to 12 inch field of view) with moderate resolution (1 lp\mm) and dynamic range (8 bit) at an affordable cost. These simple systems can be used successfully for many of the inspections, such as honeycomb assemblies, where crushed core, water, corrosion, foreign objects, and similar details are the object of the inspection. For crack detection or other more demanding applications, small area, high resolution cameras can provide much improved image quality, typical values being 20 lp/mm and a dynamic range in the order of 12 bits.

In keeping with the next century theme of the CNDE meeting, one can also predict that these electronic x-ray image systems will be more widely used for nondestructive testing applications and that the inspections will take advantage of the many features offered by the digital technology that accompanies these imaging methods.

The accelerator that is planned for use with the electronic neutron system offers attractive neutron beam capabilities, 106n/cm2-s for thermal neutrons and more than 107n/cm2-s for fast neutrons. The lens-coupled, single mirror camera system developed under the Navy program was shown to provide real-time imaging (30 frames/s) capability. Camera features included remote controls for both focus and lens selection. A small field-of-view (about 3 in.) provided much improved spatial resolution. The detection screen used for slow neutrons was Li-6 enriched LiFZnS(Ag). Although this screen did not provide as much signal through the intensified-CCD camera as copper activated ZnS, the long decay time (many seconds) was a disadvantage. The screen selected for fast neutron imaging was ZnS(Ag) dispersed in the hydrogeneous binder polypropylene. This common approach to fast neutron imaging makes use of knock-on protons from fast neutron interactions with the hydrogeneous binder to stimulate light emission from the phosphor. Fast neutron data on these screens showed that light output saturated for a screen thickness above about 3mm. Fast neutron tests with the radioactive neutron source Cf252 proved the sensitivity, but the low neutron flux and poor geometry made the production of interesting images difficult. Reactor beam tests with thermal neutrons, however, permitted excellent images.

The conclusion was that the developed camera would provide a useful imaging system with the Navy neutron source, both for slow and fast neutrons. The large weld area field of view, 12 in. x 12 in., provides good throughput and a spatial resolution in the order of I lp/mm for thermal neutrons, and about 0.5 lp/mm for fast neutrons, where scatter limits the image sharpness. A strong feature of the camera was the elimination of the "hit" problem (high amplitude image spots resulting from scattered radiation) usually encountered with CCD high energy x-ray and neutron systems.

The flexibility of the camera in terms of screen changes makes the camera broadly useful for x-ray, gamma ray, and neutron imaging. The impact on neutron inspection can be substantial in that the new camera will contribute to increased industrial use of neutrons for inspection applications that can take advantage of new non-reactor neutron sources.