Lab #3 Neutron Radiography

Engineering Physics, McMaster University
4U04-Nuclear Lab
Lab #3
by Barry Diacon
and Josh Vanderstelt
Neutron Radiography
Objective
To study the properties and illustrate some advantages of neutron radiography as a method
of non-destructive testing complementary to X-ray and gamma radiography.
Theory
In any radiographic image, variations in optical density (“blackness“) are caused by
variations in the attenuation properties of different materials. In X-ray radiography, attenuation
is a function of thickness and density, the latter increasing with atomic number. X-rays and
gamma rays are attenuated mainly by Compton scattering or photoelectron absorption. As
such, their attenuation coefficients are a fairly smoothly increasing function of the atomic
number of the sample material.
In neutron radiography (NR), attenuation is due to thickness and neutron cross section,
the latter being independent of atomic number. Neutron attenuation is caused by nuclear
scattering or absorption. The neutron absorption cross section depends on nuclear structure, so
that even isotopes of the same element can have widely differing thermal neutron absorption
properties. Light elements are poor X-ray absorbers, but can be very good for attenuating
neutrons. Hydrogen, for example, is a weak neutron absorber but is an efficient scatterer. A few
examples of strong neutron absorbers are Cadmium, Samarium and Gadolinium. In this way,
neutron images provide a different picture then that presented in an x-ray image.
Neutron radiography can be useful for thick, high-density components, which can be
opaque to X or gamma rays. Due to the high scattering cross section of hydrogen, one can
image plastic or rubber components, such as gaskets and 0-rings, even though they may be
inside heavy metal containment. This is difficult in X or gamma radiography. 0ne can also image
hydrogenous powders or liquids inside metal containers. Examples are concealed drugs,
pyrotechnics, various ordnance (ammunition), explosive bolts and cords, lubricants and
hydraulic fluids.
Neutrons make their presence known in a number of ways, such as activation or ionization.
Unfortunately, the efficiency of these processes is low for film image-recorders, so some form of
“amplifier“ or converter is required. The usual method is a thin layer of some material that, upon
neutron absorption, emits secondary radiation. This “photo-effective“ radiation interacts readily
with the film emulsion to form an image. Variations in the image optical density (“contrast“) are
dictated by the materials and thickness of the object being radiographed.
The conversion material used in this lab is Gadolinium which has the following reactions:
155Gd + n → e + 156Gd
ic
157Gd + n → e + 158Gd
ic
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Engineering Physics, McMaster University
4U04-Nuclear Lab
All of the isotopes of Gd in the above reaction equations are stable — i.e. non-radioactive.
T h e eIC symbolizes an “internal conversion“ electron. The addition of a neutron to the
155
Gd
157
Gd nucleus produces an excited nuclear state. Rather than emitting a gamma ray, deor
excitation is produced (—30% of the time) by direct transfer of the nuclear excitation energy to
an orbital electron, ejecting it from the atom with the nuclear energy minus the electron binding
energy.
Image Comparison
To compare facility to facility and describe the quality of a radiograph, two image quality
indicators (IQI) are included on most radiographs.
The beam purity indicator (BPI), Figure 3.1, contains two disks made from boron nitride,
two disks made from lead, and two cadmium wires in a block of Teflon. By looking at the optical
density, as measured with a densitometer, behind the various disks, information regarding the
causes of film darkening can be gained.
Boron is a strong neutron absorber, with a large cross
section for neutrons. As a result, it is assumed that all of the
thermal neutrons hitting the boron nitride disks will be
absorbed.
Similarly lead is a good absorber of gamma and x-ray
radiation, so it is assumed that all of the gamma and x-ray
radiation hitting these disks will be absorbed.
One of the lead disks and one of the boron nitride disks
are positioned at the front of the Teflon block, and the other
two disks are positioned at the back of the Teflon block.
The optical densities behind the boron nitride disks
provide the contribution resulting from thermal neutrons, as
thermal neutrons should be stopped by a boron disk. Similarly
from the lead disks, we see the proportion of gamma or x-ray
contribution to our image.
Figure 3.1: BPI (above & below)
The difference in optical density
behind the boron nitride disks of the
same shape and size is the result of the
separation between the disks and the
film. Any additional darkening behind
the boron nitride disk that is positioned
farther from the film is presumed to be
the result of scattered neutrons,
neutrons that went around the disk that
was farther from the film, but were
stopped by the disk that was closer to
the film.
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Similarly with the lead disks, the optical density behind the disks tells us what portion of
gamma and x-ray contribution came from angled sources, possibly from inelastic scatter of
neutrons, or pair production in the shielding, and what portion came directly down the beam,
possibly originating in the reactor core.
The following optical density values are to be taken directly from the densitometer values
measured at the corresponding location.
Definition of BPI Equation Terms
max{DB}
Larger film density measured from the 2 images of the boron nitride (BN) disks.
min{DB}
Smaller film density measured from the 2 images of the boron nitride (BN) disks.
max{DL}
Larger film density measured from the 2 images of the lead disks.
min{DL}
Smaller film density measured from the 2 images of the lead disks.
DH
Film density measured in the center of the BPI's hole image.
DT
Film density measured through the BPI Teflon between the lead disks.
∆DB
Difference between the high and low boron nitride disks.
∆DL
Difference between the height and low lead disks.
Table 3.1: Definition of BPI Equation Terms
These measurements are intended to roughly correspond to the film darkening resulting from the
following radiation.
ƒ
Hole = η + s + γ
ƒ
Boron, film side = γ
ƒ
Lead, film side = η + s + p
ƒ
Lead, source side = η + s
ƒ
Teflon = η + s + γ
* Where:
- s is the percentage contribution of scatter
to film darkening
- p is the percentage contribution of pair
production to film darkening
- η is the percentage contribution of
neutrons to film darkening
- γ is the percentage contribution of
gammas to film darkening
Note that in the Teflon image, not all of the radiation exposes the film.
From this the contribution to film darkening from the various sources can be determined as follows.
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DH − max{DB} + ΔDL
DH
DT − min{DL}
γ =
DH
Δ DB
s=
DH
Δ DL
p=
DH
η=
Dx is the film density measured with the
densitometer at the image of material x, where x =
H (hole), B(Boron), L (lead), T (Teflon)
DT for example, is the film density at the point on
the film where Teflon is imaged
Table 3.2: Equations of sources
The sensitivity indication (SI) is another image quality indicator. It is intended to provide
resolution information relating to the imaging system. The indicator is made of steps of varying
thickness of a hydrogenous material. Under these steps are shims that have holes of various
sizes in them. Additionally aluminum shims are placed to form gaps, thin lines of relative neutron
transparency. The ability of a user to see these holes and gaps is used to measure the
resolution of the imaging system.
Value of H
Hole Size,
mm
Absorber
Thickness, mm
1
0.51
0.64
2
0.51
1.27
3
0.51
2.54
4
0.51
5.08
5
0.25
0.64
6
0.25
1.27
7
0.25
2.54
8
0.25
5.08
9
0.13
0.64
10
0.13
1.27
11
0.13
2.54
12
0.13
5.08
*
* The value of H reported is the largest consecutive value that is visible in
the image.
TABLE 3.3: Hole sizes in the SI indicator
Gap
Gap size (mm)
1
2
3
4
5
6
7
0.25
0.13
0.10
0.076
0.051
0.025
0.013
Figure 3.2: SI indicator cross section.
The sensitivity indicator is intended to be used for comparison
of resolution between facilities. Whether or not a 0.013 mm
gap can actually be seen in an object will depend on the
specific materials present, the thickness of the gap, the object
separation from the film, and other parameters. In short,
resolution depends on both the imaging system and the object
being imaged.
Table 3.4: Gap sizes
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Image Category
From the measurements taken with the BPI and the SI indicators, the film category is
determined. Film category is used to compare the quality of radiographs between facilities.
Customers specify required film quality to ensure that a low quality image will not prevent them
from identifying significant defects in parts that may be visible with a better imaging system. The
following table provides the requirements for a film to be classified as a specific category film.
The worst value determines the film category. When determining film category, a bigger thermal
neutron count or a larger number of holes and gaps visible is better, while a smaller percentage
of film darkening from gamma, scatter and pair production is better.
Category
I
II
III
IV
V
NC (%)
65
60
55
50
45
H
6
6
5
4
3
G
6
6
5
5
5
S (%)
5
6
7
8
9
γ (%)
3
4
5
6
7
P (%)
3
4
5
6
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Table 3.5: Film classification categories
No Umbra Device
Figure 3.3: Image of No Umbra device
ASTM standard E803 governs the NU Device construction and use, and describes the
related math. The device is constructed from a series of cadmium wires mounted on a piece of
aluminum with known spacing. The aluminum is angled at 45 degrees from the base. When the
device is imaged, the wires produce images at different distances from the film. The closest wire,
that does not have an area where the entire beam is blocked behind it, (known as an umbra
shadow), is identified, and this distance is then used to determine the L/D ratio.
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Knowing the L/D ratio for an imaging system can be very valuable in comparing the quality
of the imaging systems. But the length of an imaging system is a point that is often debated,
concerning where exactly to measure from; the beginning of the aperture, the middle of the
aperture, the end of the aperture, and where to measure to, the parts, or the conversion screen.
Similarly the aperture size is not as easily defined as might be expected, as leakage around and
through the sides of the aperture can make the affective aperture size different from the physical
dimensions.
The No Umbra device (NU device) seeks to remove the subjectivity of determining L/D
ratios. This devices is imaged, and the image is used to determine what the L/D ratio is for the
imaging system based on its affects at the imaging plane. To understand the operation of this
device, the neutrons can be thought of as being like light. You may have noticed that when you
are between two streetlights at night, the area where the shadows from the two streetlights overlap
is much darker then the areas where a shadow is being cast from only one light. This area where
no light falls can be called the umbra shadow. If the light source is physically large, or the distance
behind the object is large, this affect can also be seen with a single source. The area where all of
the light emitted from the source is blocked is called the umbra shadow, and if only some of the
light from the source is blocked this is a penumbra shadow.
The NU devices looks to determine the distance at
which the cadmium wire needs to be for the device to no
longer have an umbra shadow in the produced radiograph.
The geometry of the device can then be used to calculate the
L/D ratio, based on this shadow formation, giving a precise,
repeatable method to calculate L/D ratios that can be
meaningfully compared between facilities.
Figure 3.4: Schematic of NU device (above & right)
The NU device can either be used to directly find the position where no umbra shadow exists,
through an iterative process, or the ratio can be calculated directly from a single image through
the following equation. ASTM E803 describes this process in more detail.
L / D = [ U1 X1 / U1− U2
X0 ]/ rod diameter
Where,
U1 = umbral width of a rod near the image plane
U2 = umbral image width of a rod near the distance where the umbra disappears
X0 = distance from the film to the rod chosen for U1
X1 = distance between the two rods chosen for analysis
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Procedure
1.
Radiograph the various objects provided, including the Risø fuel pin (from the Risø
National Laboratory in Denmark).
Place the Risø fuel pin directly against the cassette
for the first radiograph. Set the Risø pin some distance away from the cassette for the
second radiograph. For each radiograph vary the exposure time.
2.
In one of the images, place the L/D device near the center of the image at a 45 degree
angle from the fixture.
3.
Include the opaque box containing the “mystery” object. In one case, place the box flat on
the cassette, in the second place the box on its end so that the contents can be viewed in
two different aspects.
4.
Use the Beam Purity Indicator (BPI), shown in Figure 3.1, to determine the effect of the
presence of radiation other than neutrons in the test facility beam. An optical densitometer
is used to measure the logarithm of the relative intensity of the light which can pass
through portions of the BPI radiograph image. (By “relative” is meant the ratio of the
incident over transmitted light.) Calculations with the “D” parameters derived from the PBI
can show various aspects of the purity of the beam.
5.
Use the Sensitivity Indicator, shown in Figure 3.2, to determine the resolution parameters H
and G for this facility. The values for the H parameter are shown in Table 3.3 and those
for the G parameter are shown in Table 3.4.
Figure 3.5: Imaging System. Various shielding and operational changes have been made, but the imaging
system is the same as depicted here. Neutrons are thermalized in the water surrounding the core. A small
aperture allows neutrons into the collimator. A bismuth filter acts to remove gamma radiation from the
imaging system.
A rectangular, divergent collimator shapes the neutron into a slightly divergent
rectangular beam; a shutter acts to stop the beam to allow insertion of objects to image, film, etc.
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Discussion
1.
Determine the dimensions of the “mystery” object inside the opaque box and calculate
the volume of this object. What is the accuracy of this calculation?
2.
Which aspects of the object affect image resolution? Which aspects of the object
affect image contrast? HINT: how do neutrons interact with matter?
3.
What is the smallest feature which can be resolved?
4.
Discuss the resulting images in terms of material properties. Comment on the effects of
exposure time and film-object distance.
5.
Discuss any effects of converter-film separation in terms of the screen radiation(s).
6.
Why is a nuclear reactor probably the best source of neutrons for radiography? What are
the disadvantages?
7.
Do a little research to examine some applications of NR in reactor operations. Discuss
using the radiograph of the RIS0 fuel pin to illustrate your points.
8.
Discuss the effects of gamma radiation on a neutron radiographic image. What are the
possible sources of gamma rays?
9.
Discuss the BPI data in terms of the material properties of the BPI and the screen. If the
pair production figure is high, what does this say about the beam gamma radiation?
10.
According to the standard ASTM E545, what is the Quality Category of the radiographs
produced in this lab? What does the Quality Category mean?
11.
Discuss the pros and cons of gamma versus neutron radiography. Is either method clearly
superior?
12.
Nray Services Inc. Recently switched from aluminum film cassettes to magnesium film
cassettes. Why would they do this?
13.
What is the L/D ratio for this imaging system? What affects to L/D ratios have on an
imaging system.
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References
1.
Berger, Harold, Ed., Practical Applications of Neutron Radiography & Gaging. ASTM,
1976.
2.
“Standard Method for Determining Image Quality in Direct Thermal Neutron Radiographic
Examination”, Annual Book of ASTM Standards, Vol 03.03, E 545.
3.
“Standard Practices for Thermal Neutron Radiography of Materials”, Annual Book of ASTM
Standards, Vol 03.03, E 748
4.
“Standard Test Method for Determining the L/D Ratio of Neutron Radiography Beams”,
Annual Book of ASTM Standards, Vol 03.03, E 803.
5.
Knoll, Glen F., Radiation Detection and Measurement, 2nd Ed., John Wiley & Sons,
Canada Ltd., 1989.
6.
Barton, J.P., Ed., Proceedings of the First World Conference on Neutron Radiography.
Reidel Publishing Co., 1981.
7.
Barton, J.P., Ed., Proceedings of the Second World Conference on Neutron Radiography.
Reidel Publishing Co., 1986.
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