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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 for the studies of radioactive
materials using the transfer method. This makes neutron radiography
suitable for a number of tasks impossible for conventional x-ray
radiography and is actually complementary to X-ray radiography.
The UCD 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 the UCD
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:
- Imaging casting to ensure that the mold materials don't carry
into the castings as impurities.
- Quality control inspection for micro-crack and deformities in
metal alloy castings
- Validating the proper fill of explosives in actuators
- Studying the flow of oil in automobile transmissions
- Detect leaks in complex piping systems using Isotope tracers
- Facilitate Fluid flow analysis
- Analyze O-ring placements
- Determine Pyrotechnic product quality
- Study radioactive samples
- Image carbon, gun powder grain structure, plastics, lead, and
other heavy metals.
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
figures below impressively demonstrate that neutron radiography
can yield different yet complementary information than what is obtained using x-rays.
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COMPARATIVE RADIOGRAPHS
OF A CIGARETTE CASE
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PHOTOGRAPH
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Other Information Resources
Neutrons
provide unique penetrating radiation
Nondestructive
Evaluation of Aging Aircraft, Airports, Aerospace Hardware, and
Materials
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