·Table of Contents
·Computer Processing and Simulation
New Advances in the Development of MODERATOA. Bonin, B. Lavayssière,
EDF / Division R & D / REME, 6 quai Watier, 78401 Chatou Cédex (France)
ENS / CMLA, 61 avenue du Président Wilson, 94235 Cachan Cédex (France)
In this paper, we present the physical model of MODERATO, and results that show the influence of radiographic configuration on images.
The peculiarity of MODERATO is to simulate detectors consisting in metallic screens and argentic films with a Monte-Carlo model. The advantage of such a model for the detector over a refined analytical model is to enable to simulate any radiographic configuration. This model takes into account the specificity of front and back screens, and can simulate different kind of films according to their parameters. The model of detector is very flexible, as any combination of filters, screens and films can be simulated.
The simulation of particle interactions is more complex in the detector than in the objects. In the objects and in the flaws, inelastic and elastic collisions (Compton and Rayleigh interactions) and photoelectric absorption of photons are taken into account, whereas the electrons are neglected. However electrons cannot be neglected in the detector because they are responsible for the formation of the image. In the detector, MODERATO simulates the generation of electrons by the photons (during photoelectric absorption and Compton scattering) and their own interactions.
Electrons can encounter many different interactions, and we have designed models for filters, screens and photographic films. We have developed a simplified model for ionization by neglecting ion formation. Coulomb diffusion, that is mainly responsible for backscattering of electrons, is only simulated in back screens.
Films are described by their physical properties (number of grains, distribution of grain sizes, etc.) so that different films can be simulated. They are considered to be composed of silver-halide grains in gelatin.
The detector has just been completed and is now thoroughly assessed. The simulation is rather time-consuming if we simulate actual exposure times. However the dynamic of the simulated images is much wider (255 gray levels at least for simulated images, around 50 in photographic images), so that less photons (i.e. shorter exposure duration) are necessary to obtain comparable images.
Fig 1: Simulated image and real image
Fig 2: Influence of the radius of the source
The image profiles of Figure 2 show an increase of geometric unsharpness with the source radius. Due to film unsharpness, the profiles are not smooth so it makes interpretation harder. However the difference of slopes of flaw profiles indicates that the image of the flaw is sharper with a smaller source.
An offset of the position of the source leads to a less contrasted image, and a wider flaw image. We compare on Figure 3 the profiles obtained with a centered source, and two offset sources.
|Fig 3: Influence of the position of the source|
In Figure 4 we notice that magnification varies with the distance between the source and the object.
|Fig 4: Influence of the distance of the source|
Position of the flaws
In Figure 5 the position of the flaw varies perpendicularily to the film (40 mm off the film and 70 mm off the film). Profiles are quite similar. We know that the depth of the profile depends on the ratio flaw/object, that does not vary with the position of the flaw in the object. We present radiant image profiles to suppress the effect of film unsharpness because differences are difficult to detect. On these profiles, we notice that the definition of the flaw image is better when the flaw is close to the film (Zdéf=40 mm), which is natural.
|Fig 5: Influence of the position of the flaw in the object|
In Figure 6, we show three simulated images with the same flaw and similar object of different thickness. The thinner is the object, the sharper is the profile of the flaw because the ratio flaw/object increases from (a) to (c).
Fig 6: Simulated images with the same flaw in an object of 60 mm (a), 50 mm(b) and 40 mm(c)
We present the corresponding profiles in Figure 7 for image (a) and (c). The profile of the sharpest image is deeper.
|Fig 7: Influence of object thickness|
We see on the profiles of Figure 8 that the image is slightly smoother with a thicker front screen. We checked also by simulation that for important thickness the screen behaves like a filter. The threshold value between reinforcement and filtering that was determined by simulation is similar to the one experimentally determined at EDF.
The film thickness has influence on the smoothness of the image which is shown on Figure 9. We also observed that, as one electron can only darken a few grains, there is a threshold above which the film thickness does not have any influence.
|Fig 8: Influence of front screen thickness||Fig 9: Influence of film thickness|
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