Three Easy Steps to a Perfect Neutron
Radiograph
Vered Anzenberg
NE 22.54
May 10, 2004
Introduction
Chadwick discovered the neutron in 1932 and by 1935 individuals in Germany,
France, and America had patents for neutron radiography. Although the early neutron
radiographs were hindered by poor quality, nonetheless they showed that neutrons had
properties that could be used to image. By 1948, Kallmann published a paper on neutron
radiography and his techniques are still being used today.
X-rays have long dominated imaging. X-rays interact with the electron shell of
the atom, and so for objects of a relative small thickness, they can penetrate. Also, X-ray
attenuation increases with higher density materials, and hence increases with higher
atomic number. Neutrons on the other hand interact with the nucleus via cross sections,
and so this makes them more sensitive to lighter elements such as hydrogen, oxygen,
boron, etc. Metals however are less probable at interacting with the neutron and so the
neutron can penetrate farther in metallic objects. For neutrons, unlike the X-rays, the
contrast mechanism is established by the darker areas representing the lower attenuation
(i.e. metals) and the lighter areas being that of the greatest attenuation (i.e. light elements
such as hydrogen). The following figure1 shows the attenuation coefficients of X-rays
(line) and neutrons (points). As stated above, the X-rays have increase attenuation with
higher atomic numbers, but the neutrons are pretty scattered:
(Image of graph deleted due to copyright considerations. See footnote for source)
1
http://www.osp.cornell.edu/vpr/ward/NRay.html
The possibility of distinguishing between light and heavy metals allows the applications
of neutron radiography to span across many disciplines. In material science, neutron
radiography is used to look at corrosion in airplane parts and water uptake in concrete.
For nuclear engineers, this imaging technique is used for example to see hydrided
zirconium cladding tubes, plutonium pins, inspect radioactive materials and nuclear
warheads. In biology, water uptake in plants and wood can be seen. Specifically, neutron
radiography is excellent at finding pores, cracks, or slight imperfections in materials.
Most importantly, neutron radiography allows for a nondestructive method of imaging.
(Please see appendix for the above-mentioned neutron radiographs)
So how does one put together a neutron radiograph? There are three main
components to the system: neutron source, a detector, and computed tomography. There
are many variations to the three components, depending on the application that is being
sought.
Neutron Source
Through out the years, optimization of the neutron source has been performed. In
general, there are three types of sources: accelerators, reactors, and radionuclides. The
important things that are considered when choosing the source is that of neutron energy,
intensity, shielding problems, and gamma energy radiation background. In general,
thermal neutron beams have been used for neutron radiography because of their good
attenuation contrast and also they are more easily available. Although the attenuation
contrast for fast neutrons isn’t as great as thermal, the benefit of the fast beam is the
higher energy and therefore greater penetration potential, such as for thick steel samples.
This also applies to accelerators since they also often produce high-energy neutrons.
Therefore the problem of moderation is involved. For radioactive sources as neutron
sources, the important things to consider is half lives of the reactions so that intensity and
yield of the beam is high and again the energy of the neutron.
In summary, the criteria for a neutron beam is:
•
•
•
•
the flux intensity
spatial resolution of the beam
energy spectrum of the beam
the homogeneity profile of the incident beam on the object
•
•
the area of the beam
the background gamma radiation ( noise on the image)
Generally speaking, most facilities use a reactor source. Since these neutrons are in the
several MeV range, for most applications, moderation is a necessity.
Detectors
Detectors are used to record the neutron beam after penetrating a sample. These
are two dimensional integrating position sensitive detectors. Common detectors that are
used are:
•
•
•
•
•
•
X-ray film plate assemblies
Track/Etch films
Imaging plates
Scintillator/CCD cameras
Amorphous silicon (a-Si)
Flat panel detectors
Neutrons are detected in the above by either producing an ionizing particle via a nuclear
reaction, by blackening a film, microtraces in special foils (track etch), charge seperation
in semiconductor materials, or by excitations of electronic states. The neutron beam is
attenuated according to the following law of radiation attenuation:
φ ( x, y , z ) = φ o ( x , y , E ) * e − ∑ ( x , y , E ) * d
(1)
where Фo is the flux density entering the object, d is the thickness of the sample, Ф is the
flux density out of the object, and ∑ is the total macroscopic neutron cross section of the
material. For materials consisting of several elements, the total macroscopic cross section
is:
∑ = ∑ pa ∑ a = ∑ pa ( N aσ a )
a
(2)
a
where the overall compound macroscopic cross section is the sum of the weighted mass
fraction pa times the elemental macroscopic cross section. Or, the total macroscopic cross
section can be seen as the sum of elemental microscopic cross sections, σ, times the
number of nuclei/ cm3 of the absorbing material. The cross sections are related to
attenuation coefficients by taking equation 1 and equivalently rewriting it as:
φ ( x, y , z ) = φ o ( x, y , E ) * e
⎛µ ⎞
− ⎜⎜ ρ ⎟⎟ ( x , y , E )* d
⎝ρ ⎠
(3)
where the attenuation coefficient, µ/ρ, is given for a particular material independent of
density. Equation 1 and 3 are equivalent.
To get a 2D image, the object and/or detector is rotated through 180ْ and the
outgoing flux, Ф, is recorded with the detector. This is done by either placing the object
or detector on a rotating table. A method known as a radon transform and computed
tomography is used to decipher the input of the detector and to form the image.
Computed Tomography
To summarize, the neutrons in the incident beam interact with the object to be
imaged. Based on the attenuation coefficient and the density of the material, the
remaining non-absorbed beam strikes a detector. This procedure is repeated and rotated
in precise increments over 180ْ creating slices. The data received by the detector is
normalized and processed through a median filter to remove the gamma background
counts from the neutron counts. A radon transform is performed over all angles and a
sinogram is formed. Now a filtered backprojection algorithm and an inverse radon
transform can reconstruct each slice. Colors are used to indicate the different densities of
the materials, and directly show the areas of different attenuation coefficients. Finally all
slices are stacked to form a 3D data image, which again is the direct expression of the
neutron attenuation characteristic of the object.
Neutron Radiograph Application: “Detection and Characterization of the
Heterogeneous Distribution of Hydrogen in Titanium Compressor Blades by
Neutron Computed Tomography.”2
High concentrations of hydrogen in metals such as steel or titanium can cause
corrosion, embrittlement, and loss of mechanical properties. For the most part, detection
methods of hydrogen concentrations in metals are destructive. Some techniques include
mass spectrometry, x-ray diffractometry and transmission electron microscopy. Once the
material is tested for hydrogen concentrations, it is no longer usable.
2
McFarland, E., Leigh, J., and Lanza, R. Journal of Advanced Materials. April 1995. pg. 3-10
Ideally, one would like a technique that could continuously over time test
mechanical parts for hydrogen content without destroying the component. An example of
such application is that of airplane wings. To keep the plane operative and in service, it is
imperative that the wings are relatively free of corrosion and embrittlement. However,
one simply cannot remove the wings and destructively test them for corrosion. Another
method that can be employed to determine hydrogen content in airplane wings is neutron
radiography. Since neutron radiography is non-destructive, it can be used to test airplane
wings over their time of usage. The primary goal of this research paper was to detect
trace hydrogen concentrations in titanium airplane compressor blades and compare the
results with both current destructive methods and neutron radiography.
The neutron tomography was performed at McClellan Air Force Base in
Sacramento, CA. The neutron source was a 250 KW reactor that produced a fluence of
5x104 ± 5% neutrons/mm2 –sec. The detector of choice was a LiF-ZnS(Ag) scintillator
screen with a detection efficiency of 15% for incident thermal neutrons incident
perpendicular to the screen. The blades were mounted on a sample rotator and were
rotated at 3˚ increments through 180˚. Tomography was done using the NIH-Image
software package. Neutron radiography was able to calculate the weight fraction of
hydrogen in the sample by:
⎞
⎛ µ ⎞⎛ ρ ⎞⎛ I − I
ω H = ⎜⎜ Ti ⎟⎟⎜⎜ H ⎟⎟⎜⎜ ROI B − 1⎟⎟
⎠
⎝ µ H ⎠⎝ ρTi ⎠⎝ I M − I B
(4)
where ρi and µi are the bulk densities and linear attenuation coefficients, IB is the
background intensity, IROI is intensity of interest and IM is the intensity measured. The
obtained concentration of hydrogen calculated from the neutron radiographs were
[H]=1,000 ppm to [H]=7,000 ppm along the convex region of the blade and 200-1000
ppm along the concave region.
Comparing with the destructive methods, there were no characteristic hydride
features visible with the scanning electron microscope or the x-ray diffraction. However
with the mass spectrometry, concentrations of 554 ppm, 511 ppm and 497 ppm were
visible for the convex, middle and concave regions. The above results demonstrate how
neutron radiography can be used as a more sensitive non-destructive method to image
low concentrations of light elements in high atomic materials, such as metals.
Appendix
Neutron radiographs of different applications:
<The following images have been removed due to copyright considerations.>
Figure 1. Neutron Radiography of Hydrided Zirconium Cladding Tubes. Notice the
hydrogen deposited on the outside of the tubes.
Figure 2: Neutron radiography images of a concrete specimen with water/cement ratio
0.5 placed in silane for (a) 1h, (b) 4h and (c) 24 h
Figure 3: Measurements of water in the spruce sample were made after 3, 13, 28, 118,
180 and 300 minutes.
Figure 4: Neutron Radiographs of drying of a spruce sample. Notice the crack forming
over time due to the drying of the spruce sample.
Figure 5: Water movement in soil. Images taken in intervals of 1 min
References
Berger, H. Neutron Radiography: Methods, Capabilities and Applications. Elsevier
Publishing, New York, 1965.
McFarland, E. Leigh, J. and Lanza, D. Detection and Characterization of the
Heterogeneous Distribution of Hydrogen in Titanium Compressor Blades by Neutron
Computed Tomography. Journal of Advanced Materials. April 1995, pg. 3-10.
http://neutra.web.psi.ch/What/index.html
http://www.ati.ac.at/~neutropt/experiments/Radiography/radiography.html