Dott. Dario Tresoldi CNR IPCF ME IMAGING TUTORIAL Dott. Dario Tresoldi CNR IPCF ME

Why Neutrons Imaging? 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 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 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 Acronym Producer Composition Thickness Emission Nig1 Applied Scintillation Technologies ND screen 4:1 ratio AST phosphor Half thickness 225 m blue Nig2 NDg screen AST phosphor Half thickness 225 m (yellow) green Nig3 ND screen 4:1 ratio standard thickness 450 m Blue Nig4 ND screen 4:1 ratio mounted tick aluminium PSI RC TRITEC AG NR Al 1 100 150x150 Mixture ZnS/6LiF 2:1 mounted aluminium 200 m Nim ZnS (Ag)/6LiF 4:1 400 m

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 Exp Time=100 us Tot exp time=600 s TOF=15.9 ms Exp Time=100 us 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

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