A position sensitive gas detector for tomography with thermal neutrons

A position sensitive gas detector originally designed for X-ray detection has been modified for the detection of termal neutrons aiming its utilization in tomography. This work describes the modifications done for this purpose, including the incorporation of neutron to-charged particle converter materials, as well as the measurements carried out to evaluate the detector performance for each of them. The first converter studied was a natural gadolinium foil placed close to the detector window, which produces an internal conversion electron arising from the interaction of the neutron with the gadolinium nucleus. This electron causes the aimed ionization in the standard argon-methane filling gas, as used in conventional X-ray detectors. Another converter studied was high-enriched 3 He, replacing thus both former gadolinium foil and the argon-methane. The 3 He fulfills the main requirements for thermal neutron detection due to its excellent features such as high absorption cross section reaching 5330 barns, low efficiency for gamma-ray conversion and good charge amplification factor. The 3He -3 (n,p)3H nuclear reaction releases 764keV, impinging 573keV to the proton and 191 keV to the tritium particles which are emitted practically into opposite directions, since the neutron otself, due to its very low energy (0.025eV) doesn’t contribute significantly to the momentum balance. Termographic systems with thermal neutrons incorporating the developed detector, present some advantages with regard to the earlier 1st generation equipment, sch as a better resolution and a reduction in the image acquiring time.The prototype of the detector used in this work has been manufactured at the Laboratorio de Detetores/CBPF, while all the experimental work has been carried out at the Instituto de Engenharia Nuclear – IEN/CNEN using its Argonauta reactor and facilities, as well as pieces of equipment provide by Laboratorio de Isntrumentação Nuclear-COPPE/UFRJ