High-tech material research

It was shown in the past years that Neutron Imaging have become powerful, competitive and promising methods for material research, many industrial applications and a tool for different branches in university related studies. One main reason for the progress is given by the development and application of dedicated digital neutron imaging devices in combination with scintillator screens as the backbone of the detector system. Using scintillations screens the exposure time has been reduced from hours to seconds and nowadays even down to micro second scale. Therefore it is obvious that the performance of the scintillator screen directly influence the performance of a NI beamline.

In collaboration with the Paul Scherrer Institut (PSI) RC Tritec developed optimised scintillator screens to guarantee the customer best possible light output combined with optimal spatial resolution.

Dependent to the neutron energy different scintillation plates are used.

Thermal and cold neutrons (0.12 – 100 meV): Most important features of the scintillator plates

The scintillation (thermal or cold neutrons) is a twostep mechanism: first a core reaction with ions of high capture cross section (155/157Gd, 6Li or 10B) to create a secondary radiation took place followed by the excitation of a luminous material showing a fluorescence emission in the optimal range of the detection system. You can have a separated system with 6LiF / ZnS as absorber / fluorescence pigment or a single component system like Gd2O2S:Tb with Gd as the absorbing ion integrated in the fluorescent pigment.

A perfect combination of absorber ion and fluorescence pigment applied homogeneous in a reasonable thickness to reach high light output (save measurement time) and high resolution (perfect image) is needed. Actually, the combination 6Li/ZnS is known to have the highest light output, while Gd-based phosphors have the highest resolution due to their high cross capture section. Customer can choose between different thicknesses to adjust the performance to his needs. The scintillation material is applied with a binder on an aluminium substrate. Due to the proper fixing of the scintillation material handling of the screens is very easy (mechanically stressable).

Fast neutrons (> 0.8 MeV): Most important features of the scintillator plates

For imaging with fast neutrons a polypropylen plate filled with a ZnS-phosphor is used. The scintillation is also a twostep process. The neutrons interact with the hydrogen atoms of the polypropylene plate to build up backscattered protons. These excite the ZnS to give the corresponding detectable light (a fluorescence emission (blue to orange-red) in the optimal range of the detection system can be selected). Furthermore, the plate thickness and the ZnS content can be varied for adjustment to customer needs.

Typical applications for the neutron scintillation screens

Tomographic investigations allow acquiring information non-destructively about the material composition and distribution, as well as the inner structure of samples. Further, the tomography (3D) provides a significant advantage over the radiography (2D); namely each voxel (3D-pixel) contains information of the local attenuation coefficient. In addition to the structural data it is therefore possible to locally quantify the sample materials within the limit of the pixel/voxel resolution.

As neutron imaging works with a so-called parallel beam the resolution is directly coupled to the field-of-view. The pixel resolution is calculated by dividing the field-of-view by the number of pixels along the x and y direction. Nowadays the digital cameras used for neutron imaging have about 2k x 2k pixels. With field-of-views ranging from 27mm x 27mm up to 400mm x 400mm the resolution ranges from 13.5 µm/pixel to 200 µm/pixel. Hence, an appropriate scintillator needs to be chosen. For the high-resolution measurements with a Gadolinium based scinitillator is ideal whereas for the measurements with resolutions starting from 50 µm a 6LiF:ZnS screen is recommended.

Example of neutron tomographies of a diesel particulate filter using a standard and high-resolution setup is shown in the figure below.  

scintillation screen
Tomography of a soot filter

For further information please visit: Paul Scherrer Institute