TIME-OF-FLIGHT FAST-NEUTRON RADIOGRAPHY

Neutron radiography with fast or thermal neutrons is a standard techni que for non-destructive testing (NDT). Here we report results for fast neutron radiography both as an adjunct to pulsed fast neutron analysi s (PFNA) and as a stand-alone method for NDT. PFNA is a new technique for utilizing a collimated pulsed neutron beam to interrogate items an d determine their elemental composition. By determining the time of fl ight for gamma-rays produced by (n,n' gamma X) reactions, a three dime nsional image can be produced. Neutron radiography data taken with the same beam provides an important constraint for image reconstruction, and in particular is important in inferring the amount of hydrogen wit hin the interrogated item. As a stand-alone device, the radiography me asurement can be used to image items as large as cargo containers as l ong as their density is not too high. The use of a pulsed beam gives t he further advantage of a time of flight measurement on the transmitte d neutrons. By gating the radiography signal on the time of flight app ropriate to the energy of the primary neutrons, most build-up from sca ttered neutrons can be eliminated. The pulsed beam also greatly improv es the signal to background and extends the range of the neutron radio graphy. Simulation results will be presented which display the advanta ge of this constraint in particular for statistically limited data. Ex perimental results will be presented which show some of the limitation s likely in a PFNA system utilizing neutron radiography data. Experime ntal and simulation results will demonstrate possible uses for this ty pe of radiographic data in identifying contraband substances such as d rugs.