Development of energy-selective fast neutron imaging for nondestructive elemental analysis

This study presents an imaging technique with material decomposition ability using a compact deuterium-deuterium (DD) neutron generator. The method takes benefit from the variation of the neutron energy as a function of the emission angle. At the available deuteron energy of the in-house DD source of 80 keV, the deuteron energy varies from about 2.2 MeV in the backward direction in relation to the incoming deuteron to about 2.8 MeV in the forward direction. This feature is used to measure the neutron beam attenuation through a given sample at various energies. Since neutron cross-sections are characteristic for a given element and strongly depend on the neutron energy, a unique material-dependent contrast can be obtained at each energy. By combining the information obtained from multiple energies, a linear system of equations can be generated. The unknowns of this system of equations are the concentrations of the elements in the sample. The solution of this system of equations yields the composition of the sample. This elemental decomposition method can be combined with a tomographic imaging technique, which allows obtaining a spatial mapping of the elemental composition of an interrogated heterogeneous object. The first part of this report focuses on the theoretical and experimental characterization of the neutron field around the generator. The developed arrangement allows performing measurements remotely and in an automated way. Its layout is presented, along with the angle-dependent neutron spectrum. Tests with selected representative material samples provided a proof of principle of the energy-selective element detection capability of this approach. In the second chapter, a large number of material samples is investigated with regard to their neutron attenuation in the available energy range. The measurements are compared with an existing cross-section database. The agreement between the measurements and the attenuation calculated using the database can serve different purposes. For some elements, the tabulated data is sparse, and the measurements may contribute to an extension of the state of knowledge. Other already well-characterized elements confirm that the measurements are meaningful, although room for further improvement of the measuring setup and its signal acquisition technique was identified. An important factor affecting the accuracy of the overall measurement setup is the electrical perturbation originating from the radio frequency generator of the deuterium plasma source. Some of the observed discrepancies are reduced by careful calibration. The third section deals with applying the elemental decomposition approach to tomographic imaging with two test cases. For this purpose, an extended neutron detection system in the form of an arc-shaped array of individual detectors is used. The individual detectors are made of plastic scintillators coupled to silicon photomultipliers. The results of tomographic 2D element mapping within a cross-sectional view of the selected heterogeneous test samples are presented and discussed. Elemental decomposition was successful for several of the materials interrogated. Finally, the last chapter of the present report contains proposals for future improvement of the developed technique, which could result, for example, in a speeding up of the measurement, and an increase of the element-sensitivity by extending the energy range of the available neutrons