Modeling of the saturation current of a fission chamber taking into account the distorsion of electric field due to space charge effects

Fission chambers were first made fifty years ago for neutron detection. At the moment, the French Atomic Energy Commission (CEA-Cadarache) is developing a sub-miniature fission chamber technology with a diameter of 1.5 mm working in the current mode [1]. To be able to measure intense fluxes, it is necessary to adjust the chamber geometry and the gas pressure before testing it under real neutron flux. In the present paper, we describe a theoretical method to foresee the current-voltage characteristics (sensitivity and saturation plateau) of a fission chamber whose geometrical features are given, taking into account the neutron flux to be measured (spectrum and intensity). The proposed theoretical model describes electric field distortion resulting from charge collection effect. A computer code has been developed on this model basis. Its application to 3 kinds of fission chambers indicates excellent agreement between theoretical model and measured characteristics.