Application of the neutron radiography for the detection of plastic explosives

Neutron Radiography is a non-destructive testing method that has been applied in special cases of inspection, where it is difficult to take X and gamma radiographs. It employs a collimated beam of thermal neutrons which on transmission through the specimen under test produce an image on a detector such as photographic emulsion, a scintillation screen or a solid state nuclear track detector. X and γ rays interact with the electrons of the atomic shell, and theirs attenuation coefficients increases with the atomic number in a regular way, whereas, absorption or scattering of thermal neutrons is an interaction with the nucleus showing a random behavior of the total cross section on the atomic number. So, in many technical applications, thermal neutrons can be used to great advantage. In the application in study, the use of thermal neutrons is attractive because, unlike X-ray transmission, neutron transmission is more sensitive to the presence of the light elements, H, C, N, and O, which are the constituents of plastic explosives. This work aims to demonstrate the potentiality of the neutron radiography technique, allied to the computerized tomography by transmission, to both detect and visualize plastic explosive samples in several hidden conditions, using a simple scanner as a digitization instrument. Each tomographic essay was obtained in the J-9 channel of the Argonauta Research Reactor of IEN/CNEN,in groups of six neutron radiographic projections, performed with an angular increment of 30 deg. Two groups of tomographic reconstructions were generated, distinguished by the digitization process in the reconstruction plane, utilizing a scanner and a microdensitometer, respectively. The reconstruction of the bi-dimensional image of the transverse section, in relation to this plane, was processed making use of the Image Reconstruction Algorithmic based on the Maximum Entropy principle (ARIEM). From the qualitative analysis of the images, we conclude that the neutron radiographic system was able to detect the explosive sample in a satisfactory way, while the quantitative analysis confirmed the application effectiveness of a scanner to acquire the projection dates when the objective is only a reconnaissance. Acknowledgements: The authors would like to thank to Instituto de Engenharia Nuclear/CNEN, specially to all SUMAR Department staff, the FAPERJ and CNPq for their technical and financial support, respectively. (author)