The Geel Linear Accelerator (GELINA) neutron time-of-flight facility of the Institute for Reference Materials and Measurements (IRMM) is one of the leading research institutes for neutron cross section measurements in the world. Neutrons with different kinetic energies are produced by high energy electron pulses hitting an uranium target. These neutrons are then used to interact with the samples under investigation in the different experimental stations along the 12 individual flight paths of the GELINA facility.
Up to now, the verification of the beam profile has been performed by the means of x-ray sensitive films, using the existence of a strong -ray burst associated with the neutron production. This method has some disadvantages in both giving the right information about the neutron distribution as well as the need of "wet chemistry" for the developing of the film.
In this masterthesis, a scintillation based neutron camera was used to record the neutron distribution. At flight path 13, the shortest flight path at GELINA, with a flight path length of about 8 m, several experiments have been performed. They were targeted at the correct outlining of the beam, the use of different collimators, the investigation of the neutron distribution as function of camera distance and the neutron energy.
The results from these experiments demonstrate the advantages of the neutron camera over conventional x-ray film for the recording of neutron distributions. The experiments had a direct consequence for the correction of the outlining of the flight path tube as well as for the correct positioning of the neutron target itself. The incorrect positioning of the neutron target has never been detected before.
Different collimator materials were compared in different energy ranges. For this comparison different materials were used as last collimation before the neutron camera. These materials included neutron scatterers like Pb or Cu and neutron absorbing materials like Li. Here two results can be highlighted. (1) The collimator material does not influence the geometric development of the neutron beam profile for short distances and is as expected from theoretical calculations. No significant influence of different energy ranges is seen in the neutron distribution. This is mostly valid for the plateau, the penumbra on the contrary does show a dependence on the collimation material for different energy ranges. (2) On the other hand, scattering of neutrons out of the beam by the last collimator can be a problem for short distances from the collimator and has a clear dependence on the collimator material and neutron energy. The experiments concerning this show that for low neutron energies neutron absorbing materials are ideal and that for medium to high neutron energies neutron scatterers such as Pb are more ideal.
The neutron camera in combination with the time-of-flight method at GELINA has shown clear advantages over the use of conventional x-ray film. This has been demonstrated with respect to linearity, ease of use, real sensitivity to neutrons and the possibility to have energy dependent measurements. It is a valuable tool for the positioning and design of flight path elements which has a direct influence to the scientific output of the neutron cross section experiments.
If you want to cite this thesis in your own thesis, paper, or report, use this format (APA):
PEETERS, S. (2012). Determination of neutron beam profiles at Gelina.
Unpublished thesis, Xios, TIW.