IMAGING TUTORIAL
Dott. Dario Tresoldi
CNR IPCF ME
Why Neutrons Imaging?
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Neutron imaging has wide industrial and scientific
significance and can provide detailed information
concerning the inner structure and composition of objects.
The principle of neutron imaging is based on the
attenuation, through both scattering and absorption, of a
directional neutron beam by the matter through which it
passes.
The technique is also non-destructive in nature, and has
been effectively applied to artefacts of archaeological
significance.
The neutron imaging technique, rather than being in
competition with X-ray imaging, is entirely and ideally
complimentary to it.
Neutron Tomography schematic diagram
Neutron Imaging Technique
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Neutron Radiography involves placing an
object in the path of the neutron beam,
and measuring the shadow image of the
object that is projected onto a neutron
detector, often consisting of a scintillator
optically coupled to a CCD.
Neutron Tomography takes this a step
further and entails rotating the sample in
the beam and recording multiple 2D
images through an angular range of 180°.
From the data set, a 3D representation
through the object can be constructed.
Es: Tomography Reconstruction of
a Lens
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Control software (Es: Andor Tomography)
Elaboration software (Es: Neutomo)
3D viewer (Es: VGStudio)
Final Results ( video1 , video2 )
Spatial Resolution
Digital cameras have finite minimum regions of detection
(commonly known as Pixels), that set a limit on the Spatial
Resolution of a camera.
However the spatial resolution is affected by other factors
such as the neutron beam properties, the distance of the
sample from the detector (L/D) and the scintillator
proprieties.
The limiting spatial resolution is commonly determined from
the minimum separation required for discrimination
between two high contrast objects, e.g. white points or
lines on a black background. Contrast is an important factor
in resolution as high contrast objects (e.g. black and white
lines) are more readily resolved than low contrast objects
(e.g. adjacent gray lines).
The contrast and resolution performance of a camera can
be incorporated into a single specification called the
Modulation Transfer Function (MTF).
The MTF Method
To measure the spatial resolution you can use the MTF
(Modulation Transfer Function) Method.
A good neutron absorber (Es: Gd) is put in the beam just to
create 2 different regions that will appear black and white
in the radiography.
The plot of the intensity as function of the axis perpendicular
to the object is the edge response that theoretically is a
step function.
In practice the intensity goes from the black to white level
with some intermediate grey levels.
The derivate function is the LSF (Line Spread Function) that
corresponds to the system response to the impulse.
The FWHM (Full Width at Half Maximum) of this function is
almost the spatial resolution, because represents how large
a system see a very small object.
The MTF is the Fourier Transform of the LSF
Es: Comparison between different
scintillators
In this example, using the software MTF Calculator, a spatial
resolution in the range 200-300 micron for neutron
scintillators of thickness between 200-400 micron has been
calculated
Producer
Composition
Thickness
Emission
Nig1
Applied Scintillation Technologies
ND screen 4:1 ratio AST phosphor Half thickness
225 m
blue
Nig2
Applied Scintillation Technologies
NDg screen AST phosphor Half thickness
225 m (yellow)
green
Nig3
Applied Scintillation Technologies
ND screen 4:1 ratio standard thickness
450 m
Blue
Nig4
Applied Scintillation Technologies
ND screen 4:1 ratio mounted tick aluminium
225 m
Blue
PSI
RC TRITEC AG
NR Al 1 100 150x150 Mixture ZnS/6LiF 2:1
mounted aluminium
200 m
green
ZnS (Ag)/6LiF 4:1
400 m
Blue
Acronym
Nim
ICCD Cameras
Intensified CCD cameras combine an image
intensifier and a CCD camera. The image
intensifier has useful properties which allows the
camera to have very short exposure times. ICCD
are also cameras which can exploit high gain to
overcome the read noise limit but also have the
added feature of being able to achieve very fast
gate times.
Energy selective imaging
ICCD gated cameras allows to do energy selective imaging
on a neutron pulsed source by selecting neutrons in a well
determined time relationship with the spallation pulse. In
this way the contrast of objects with different absorption
proprieties can be improved.
E.S. radiographies done at ISIS (ROTAX ISTRUMENT) of a soldering
TOF=15.7 ms
TOF=15.9 ms
Exp Time=100 us
Exp Time=100 us
Tot exp time=600 s
Tot exp time=600 s
Bragg Edge analysis
To determine the crystal structure of a polycrystalline sample
an intensity diffraction spectrum is recorded.
At certain wavelengths strong intensity maxima are detected
called Bragg peaks following the expression:
l = 2 d sinq (d = interplanar distance, 2q = scattering angle)
In transmission, the total neutron cross section of
polycrystalline materials shows sharp discontinuities called
Bragg Edges.
These Bragg edges occur because, for a given hkl reflection
plane, the Bragg angle increases as the wavelength until 2q
is equal to 180°. At wavelengths greater than this critical
value, no scattering by this particular {hkl} family can
occur and there is an increase in transmitted intensity.
ICCD camera allows to measure the transmission spectrum
Es. Bragg Edge Analysis of Cu
Powder
In this example several radiographies E.S. has
been collected of a aluminium box containing
Cu Powder.
The software EnergySelective shows as the
transmission changes as a function of the
neutrons energy.
With the Bragg_Fit software a Bragg Edge
Analysis can be done
Thank you