Neutron radiographic detection limits of fluids in metal pipes

Thermal neutron radiography has been used to image thin films of various fluids in thick-walled high-pressure process pipes. A 3.5 x 106 n/(cm2-sec) thermal neutron beam and standard transfer imaging techniques were used to provide radiographs of the fluid-bearing pipes. In addition, a computer graphics procedure was developed which simulated the neutron radiographs; this method permitted examination of some fluid-pipe configurations not studied experimentally. Actual pipe outer dia ranged from 1/4 to 9/16 in. (OD/ID = 3), but the graphics method was used for some cases outside this range. Fluids in the thermal cross section range 0.29 cm-1 less than or equal to Σ/sub th//sup f/ less than or equal to 3.47 cm-1 were examined for pipes with 0.099 cm-1 less than or equal to Σ/sub th//sup p/ less than or equal to 1.15 cm-1. For Σ/sub th//sup f/ greater than Σ/sub th//sup p/, the smallest measured film thickness t/sub s/ varied 0.006 to 0.040 in. with decreasing Σ/sub th//sup f/; a semiempirical model described t/sub s/ = t/sub r/ + t/sub a/, where t/sub r/ corresponded to resolution and t/sub a/ depended on the bulk attenuation strength of the fluid. For Σ/sub th//sup f/ less than Σ/sub th//sup p/, part of the fluid film t/sub m/ was masked by the pipe. The calculated physical film thickness t/sub p/ = t/sub a/ + t/sub m/ ranged from 0.003 to 0.035 in. for smallest detected fluid films. Detection of low Σ/sub th/ fluids was improved as much as 60 percent when the pipes were tilted 600 toward the neutron beam. Other options for improved detection were investigated using simulated radiographs. (auth)