Neutron Tomography as Tool for Applied Research and Technical Inspection

Abstract

Similar to the imaging with X-ray, neutron imaging systems have been shifted more and more from film exposure towards direct digital detection in the last decade. This approach enables the option for tomography because the image content is measured and understood as dataset now. With the help of mathematical algorithms it becomes possible to derive the necessary information about the full three-dimensional volume of an object from a set of projections from different viewing angles.

Neutron tomography (NT) with best performance requires a strong and well collimated neutron beam. Such beam lines for either thermal or cold neutrons are available at reactor based sources (FRM-2, HMI, ILL) or at spallation neutron sources (PSI). The beam geometry for neutrons is in the best case a parallel one, delivering a sharp 1:1 transmission image of the object under investigation. Therefore, the limited detector resolution is responsible for the tomography resolution generally.

In practical reality, objects with outer dimensions up to 40 cm can be inspected with a nominal resolution of up to 0.1 mm. This compares well with similar X-ray tomography systems used in non-destructive testing (NDT) tasks.

However, the application fields in neutron tomography are different, given by the different interaction probability with matter. Neutrons have the advantage to penetrate most metals (especially heavy ones) very efficiently and to detect hydrogenous compounds very sensitively. Therefore, neutron tomography has application fields in moisture detection, test of adhesive connections, structure analysis for soil and geologic samples and the non-invasive investigation of objects from our cultural heritage.

Based on some practical applications, the paper describes the present state of the art of this technique and gives a prospect for further improvement.