IMAGING TUTORIALIMAGING TUTORIAL

Dott. Dario TresoldiDott. Dario Tresoldi

CNR IPCF MECNR IPCF ME

Why Neutrons Imaging? Why Neutrons Imaging?

Neutron imaging has wide industrial and scientific Neutron imaging has wide industrial and scientific significance and can provide detailed information significance and can provide detailed information concerning the inner structure and composition of objects. concerning the inner structure and composition of objects. The principle of neutron imaging is based on the The principle of neutron imaging is based on the attenuation, through both scattering and absorption, of a attenuation, through both scattering and absorption, of a directional neutron beam by the matter through which it directional neutron beam by the matter through which it passes.passes.

The technique is also non-destructive in nature, and has The technique is also non-destructive in nature, and has been effectively applied to artefacts of archaeological been effectively applied to artefacts of archaeological significance. significance.

The neutron imaging technique, rather than being in The neutron imaging technique, rather than being in competition with X-ray imaging, is entirely and ideally competition with X-ray imaging, is entirely and ideally complimentary to it.complimentary to it.

Neutron Tomography schematic diagramNeutron Tomography schematic diagram

Neutron Imaging Technique Neutron Imaging Technique Neutron Radiography involves placing an Neutron Radiography involves placing an

object in the path of the neutron beam, object in the path of the neutron beam, and measuring the shadow image of the and measuring the shadow image of the object that is projected onto a neutron object that is projected onto a neutron detector, often consisting of a scintillator detector, often consisting of a scintillator optically coupled to a CCD.optically coupled to a CCD.

Neutron Tomography takes this a step Neutron Tomography takes this a step further and entails rotating the sample in further and entails rotating the sample in the beam and recording multiple 2D the beam and recording multiple 2D images through an angular range of 180°. images through an angular range of 180°. From the data set, a 3D representation From the data set, a 3D representation through the object can be constructed. through the object can be constructed.

Es: Tomography Reconstruction of Es: Tomography Reconstruction of a Lensa Lens

Control software (Es: Control software (Es: Andor Tomography) Elaboration software (Es: Neutomo)Elaboration software (Es: Neutomo) 3D viewer (Es: VGStudio)3D viewer (Es: VGStudio) Final Results ( Final Results ( video1 , video2 )video1 , video2 )

Spatial Resolution Spatial Resolution Digital cameras have finite minimum regions of detection Digital cameras have finite minimum regions of detection (commonly known as Pixels), that set a limit on the Spatial (commonly known as Pixels), that set a limit on the Spatial Resolution of a camera. Resolution of a camera. However the spatial resolution is affected by other factors However the spatial resolution is affected by other factors such as the neutron beam properties, the distance of the such as the neutron beam properties, the distance of the sample from the detector (L/D) and the scintillator sample from the detector (L/D) and the scintillator proprieties.proprieties.The limiting spatial resolution is commonly determined The limiting spatial resolution is commonly determined from the minimum separation required for discrimination from the minimum separation required for discrimination between two high contrast objects, e.g. white points or lines between two high contrast objects, e.g. white points or lines on a black background. Contrast is an important factor in on a black background. Contrast is an important factor in resolution as high contrast objects (e.g. black and white resolution as high contrast objects (e.g. black and white lines) are more readily resolved than low contrast objects lines) are more readily resolved than low contrast objects (e.g. adjacent gray lines). (e.g. adjacent gray lines). The contrast and resolution performance of a camera can The contrast and resolution performance of a camera can be incorporated into a single specification called the be incorporated into a single specification called the Modulation Transfer Function (MTF). Modulation Transfer Function (MTF).

The MTF Method

To measure the spatial resolution you can use the MTF To measure the spatial resolution you can use the MTF (Modulation Transfer Function) Method.(Modulation Transfer Function) Method.

A good neutron absorber (Es: Gd) is put in the beam just to A good neutron absorber (Es: Gd) is put in the beam just to create 2 different regions that will appear black and white create 2 different regions that will appear black and white in the radiography.in the radiography.

The plot of the intensity as function of the axis perpendicular The plot of the intensity as function of the axis perpendicular to the object is the edge response that theoretically is a to the object is the edge response that theoretically is a step function.step function.

In practice the intensity goes from the black to white level In practice the intensity goes from the black to white level with some intermediate grey levels.with some intermediate grey levels.

The derivate function is the LSF (Line Spread Function) that The derivate function is the LSF (Line Spread Function) that corresponds to the system response to the impulse.corresponds to the system response to the impulse.

The FWHM (Full Width at Half Maximum) of this function is The FWHM (Full Width at Half Maximum) of this function is almost the spatial resolution, because represents how large almost the spatial resolution, because represents how large a system see a very small object.a system see a very small object.

The MTF is the Fourier Transform of the LSF The MTF is the Fourier Transform of the LSF

Es: Comparison between different Es: Comparison between different scintillatorsscintillators

In this example, using the software MTF Calculator, a spatial In this example, using the software MTF Calculator, a spatial resolution in the range 200-300 micron for neutron scintillators resolution in the range 200-300 micron for neutron scintillators of thickness between 200-400 micron has been calculatedof thickness between 200-400 micron has been calculated

Acronym

Producer Composition Thickness Emission

Nig1 Applied Scintillation Technologies ND screen 4:1 ratio AST phosphor Half thickness 225 blue

Nig2 Applied Scintillation Technologies NDg screen AST phosphor Half thickness 225 (yellow) green

Nig3 Applied Scintillation Technologies ND screen 4:1 ratio standard thickness 450 Blue

Nig4 Applied Scintillation Technologies ND screen 4:1 ratio mounted tick aluminium 225 Blue

PSI RC TRITEC AG NR Al 1 100 150x150 Mixture ZnS/6LiF 2:1 mounted aluminium

200 green

Nim ZnS (Ag)/6LiF 4:1 400 Blue

ICCD Cameras ICCD Cameras

Intensified CCD cameras Intensified CCD cameras combine an image combine an image intensifier and a CCD intensifier and a CCD camera. The image camera. The image intensifier has useful intensifier has useful properties which allows the properties which allows the camera to have very short camera to have very short exposure times. ICCD are exposure times. ICCD are also cameras which can also cameras which can exploit high gain to exploit high gain to overcome the read noise overcome the read noise limit but also have the limit but also have the added feature of being added feature of being able to achieve very fast able to achieve very fast gate times.gate times.

Energy selective imaging

ICCD gated cameras allows to do energy selective imaging ICCD gated cameras allows to do energy selective imaging on a neutron pulsed source by selecting neutrons in a well on a neutron pulsed source by selecting neutrons in a well determined time relationship with the spallation pulse. In determined time relationship with the spallation pulse. In this way the contrast of objects with different absorption this way the contrast of objects with different absorption proprieties can be improved. proprieties can be improved.

E.S. radiographies done at ISIS (ROTAX ISTRUMENT) of a soldering

TOF=15.7 ms

Exp Time=100 us

Tot exp time=600 s

TOF=15.9 ms

Exp Time=100 us

Tot exp time=600 s

Bragg Edge analysisBragg Edge analysisTo 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:

= 2 d sind = interplanar distance, 2 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 2 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 Es. Bragg Edge Analysis of Cu PowderPowder

In this example several radiographies E.S. has In this example several radiographies E.S. has been collected of a aluminium box containing been collected of a aluminium box containing Cu Powder.Cu Powder.

The software EnergySelective shows as the The software EnergySelective shows as the transmission changes as a function of the transmission changes as a function of the neutrons energy.neutrons energy.

With the Bragg_Fit software a Bragg Edge Analysis With the Bragg_Fit software a Bragg Edge Analysis can be done can be done

Thank youThank you