A neutron sensor material for fabrication of high sensitivity fission counters

Fission counters (FCs) are gas filled detectors in which charged fission fragments are formed due to the impact of incident neutrons on fissile Uranium and the resulting charges are collected as signal current on the application of high voltage across the electrodes. The generation of charged particles in FCs depends purely on the interaction of fissile material with neutron. The preferred fissile material used is Uranium-235 (²³⁵U), but Uranium-238 (²³⁸U) and Thorium-232 (²³²Th) can also be used for the required applications. The fissile materials are normally introduced within FCs in the form of coating/thin films on suitable substrates, such as SS, Inconel, Al (for low activation) etc. One of the primary considerations to choose a material for the base substrates is the minimization of unwanted secondary radiation interactions in the reactor environment which increase the background, reduce detector efficiency and limits its dynamic range. In addition, presence of impurity elements, such as Co (in SS and Inconel), generates very high energy gamma radiation followed by neutron capture which increases difficulties in handling such detectors. In view of these problems, development of high sensitivity FCs with 1S Al as base material and with UO₂ coating with desired thickness and uniformity has been initiated. However, due to the presence of tenacious Al₂O₃ layer on Al substrate, direct deposition of coatings on it develops issues like cracking and peeling off. In order to address these issues, in the present study, a suitable chemical/electrochemical modified alloy zincating process was adopted to deposit a Zn coating on Al by galvanic displacement and subsequently electrolytically deposit Cu underlayer to engineer the Al surface for UO₂ deposition. Subsequently, the UO₂ coating was applied by DC electrodeposition. The whole tetralayer UO₂/Cu/Zn/Al composite coating structures and their growth behaviour were investigated in detail using atomic force microscope (AFM), field emission scanning electron microscope (FESEM), Rutherford Backscattering Spectroscopy (RBS) and secondary ion mass spectrometry (SIMS). Finally, the entire coatings process has been successfully implemented for the deposition of adherent uniform UO₂ coating on large scale Al tubes as a technology demonstration for the development of high sensitivity FCs in DAE, India. (author)