Measurements of organic scale by neutron capture gamma ray and neutron moderation

The measurement of organic scale, such as the type found in refineries' iron pipes, is usually difficult by gamma-ray and X-ray radiography methods. As the density and atomic number of iron pipes are much higher than that of organic scale, most of the attenuation of gamma rays or X rays takes place in the iron, and only a very small fraction is attenuated by the organic scale. This makes the contrast in the radiographic film very poor. Results of radiographic tests are also not immediately available because of the need for film processing and developing. Accessibility should be maintained on both sides of the pipe, which may not be possible in certain cases. In the ultrasonic technique used for inorganic scale measurements, the pipe surface has to be well prepared, the technique cannot be used on hot pipes, and the signal from organic scale is also poor. In the other nondestructive techniques, such as the electrical resistance and polarization resistance methods, penetration of the pipe wall is required, and the measurement is taken at only one position of the pipe. The neutron capture gamma-ray method has been successfully used for the measurement of iron pipe corrosion and inorganic scale measurements. In this technique, neutrons are allowed to be incident on the pipe whose organic scale thickness is to be measured. Following a neutron capture process in the organic scale, which contains hydrogen and carbon atoms, the hydrogen atoms at 2.223-MeV gamma rays and the carbon atoms at 4.945 MeV are emitted. Because of the lower absorption cross section for carbon, the intensity of its gamma rays is low compared to that of hydrogen atom capture. The neutron moderation technique was recently used for scale and corrosion measurements in desalination plant pipes. In this method, fast neutrons emitted from a neutron source penetrate the iron pipe wall and interact minimally because of the low total cross section for iron for fast neutrons