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Either an X-ray machine or a radioactive source (Ir-192, Co-60, or in rarer cases Cs-137)
can be used as a source of photons. Neutron radiographic testing (NR) is a variant of
radiographic testing which uses neutrons instead of photons to penetrate materials. This can
see very different things from X-rays, because neutrons can pass with ease through lead and
steel but are stopped by plastics, water and oils.
Since the amount of radiation emerging from the opposite side of the material can be
detected and measured, variations in this amount (or intensity) of radiation are used to
determine thickness or composition of material. Penetrating radiations are those restricted to
that part of the electromagnetic spectrum of wavelength less than about 10 nanometres.

Weld radiograph with a burn-through defect

The beam of radiation must be directed to the middle of the section under examination and
must be normal to the material surface at that point, except in special techniques where known
defects are best revealed by a different alignment of the beam. The length of weld under
examination for each exposure shall be such that the thickness of the material at the
diagnostic extremities, measured in the direction of the incident beam, does not exceed the
actual thickness at that point by more than 6%. The specimen to be inspected is placed
between the source of radiation and the detecting device, usually the film in a light tight
holder or cassette, and the radiation is allowed to penetrate the part for the required length of
time to be adequately recorded.
The result is a two-dimensional projection of the part onto the film, producing a latent
image of varying densities according to the amount of radiation reaching each area. It is
known as a radiograph, as distinct from a photograph produced by light. Because film is
cumulative in its response (the exposure increasing as it absorbs more radiation), relatively
weak radiation can be detected by prolonging the exposure until the film can record an image
that will be visible after development. The radiograph is examined as a negative, without
printing as a positive as in photography. This is because, in printing, some of the detail is
always lost and no useful purpose is served. Before commencing a radiographic examination,
it is always advisable to examine the component with one's own eyes, to eliminate any
possible external defects. If the surface of a weld is too irregular, it may be desirable to grind
it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface
irregularities (which will be visible on the radiograph) may make detecting internal defects
difficult.
After this visual examination, the operator will have a clear idea of the possibilities of
access to the two faces of the weld, which is important both for the setting up of the
equipment and for the choice of the most appropriate technique.
Defects such as delaminations and planar cracks are difficult to detect using radiography,
which is why ultrasonics is the preferred method for detecting this type of discontinuity.

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