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Neutron radiography Neutron radiography is suitable for a number of tasks impossible for conventional x-ray radiography Neutron radiography, also known as n-ray radiography,is a very efficient tool to enhance investigations in the field of non-destructive testing as well as in many fundamental research applications As the name implies, a neutron beam penetrates the specimen to be studied. This beam is attenuated by the sample material depending on the material's neutron cross-section. The beam is then detected by a two-dimensional imaging device that outputs an image representing the macroscopic structure of the samples interior. The advantage of neutron radiography is its ability to image very light elements (i.e. with low atomic numbers) such as hydrogen, water, carbon etc. In addition, neutrons penetrate very heavy elements (i.e. with high atomic numbers) such as lead, titanium etc... as well as distinguish between different isotopes of the same element. This makes neutron radiography an important tool tool for the studies of both radioactive materials using the transfer method and for conventional x-ray radiography. MNRC's high neutron intensity beams permit short exposure times, high spatial resolution and high sample throughput.
Methods of neutron radiography Two general classes of neutron radiography are used at
MNRC. The first method is called the "direct method"
and is used for non-radioactive samples. As the name implies, the
film is exposed to the neutron beam directly. A piece of x-ray film
is placed on top of a conversion plate (i.e. a 12 m vapor deposited
sapphire coated layer) and the film and converter plate are placed
in a vacuum cassette. This cassette is directly exposed to the neutron
beam. The film is then removed and processed in the dark room. The second method is called the "transfer method" and is primarily used for radioactive samples. The direct method will not work for radioactive samples as the samples produce particles that would fog the x-ray film. Instead of the film, a converter foil is placed in the neutron beam. Once the converter foil has been exposed, it is removed from the beam and taken to the dark room. The foil is then placed on the x-ray film and left to decay. After an appropriate decay time the image is "transferred" from the foil to the film.
Applications Neutron Radiography has a wide range of uses, including:
Differences between neutron and x-ray radiography Neutron radiography is based on the principal that neutrons interact
with the nucleus of the atom, rather than the electrons, and that
each atomic nucleus has a different probability for absorbing or
scattering a neutron. Neutrons, in particular those traveling at
very low velocities (thermal neutrons), are absorbed in matter according
to laws that are very different from those that govern the absorption
of electromagnetic rays, such as x-rays and gamma rays. The absorption
of x-rays and gamma rays increases as the atomic number of the absorber
increases, but this is not the case with thermal neutrons. Elements
having adjacent atomic numbers can have widely different absorptions
of neutrons and it varies from element to element, even from
isotope to isotope. Also, some low atomic number elements attenuate
a beam of thermal neutrons more strongly than some high atomic number
elements. This means that, contrary to x-rays, neutrons are attenuated
by some light materials, such as hydrogen, boron and lithium, but
penetrate many heavy materials such as titanium and lead. This allows
for some unique applications of neutron radiography. For example,
since hydrogen has a much higher neutron attenuation than lead,
it is possible to determine the height of water in a lead standpipe
by neutron radiography. This is impossible with x-ray or gamma-ray
radiography. These figures show the quite random attenuation behavior of thermal neutrons for different elements, whereas the attenuation of x-rays is clearly dependent on the elements atomic number. The below figures also impressively demonstrate that neutron radiography can yield different yet complementary information than what is obtained using x-rays.
Other Information Resources Nondestructive Evaluation of Aging Aircraft, Airports, Aerospace Hardware, and Materials
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