What is Neutron Radiography?

Non-destructive testing (NDT) is comprised of a wide array of techniques utilized to evaluate the properties and structures of materials without causing damage to the specimen. Common techniques of NDT are: ultrasound, liquid penetrant testing, radiography, Eddy current testing and coherence interferometry.  Each technique is unique and adapted to a specialized purpose. 

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There are many applications in the industry that neutron imaging is well suited for.  Neutrons can penetrate high density material in a way that X-rays cannot.  Exploiting this property allows one to take neutron images of critical aerospace components such as turbine blades and various fuses.  Neutrons can also be used to image biological specimens high in hydrogen content and archeological specimens that might be encapsulated in material that X-rays cannot penetrate.  Neutron imaging is truly a unique modality that offers a new method in the NDT industry.


The Physics of Neutron Imaging

Because neutrons are inherently electrical neutral, they do not interact with the atomic electrons of elements in the same way that photons or electrons do.  Instead, neutrons interact with the nuclei of elements, giving neutrons a unique ability to probe materials in a way that gamma rays or X-rays cannot.  While X-rays follow nearly linear attenuation with density, neutrons follow no such trend. 

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Neutron Imaging Metrics

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To qualify and quantify image quality, several metrics must be observed.  These metrics are not necessarily governed by any body such as the American Society for Testing and Materials (ASTM) or the American National Standards Institute (ANSI), but guidelines are given for peer reviewed tests and methods that have become more universal across industries.  In particular, for the neutron radiography method, ASTM E545-14 and E748-16, sets up a fabrication process, test outline for image quality indicators, their setup, alignment, imaging parameters, and analysis, for qualifying an image to be at a certain minimum acceptable standard.