  Fission chambers were first made fifty years ago for neutron detection. At
the moment, the French Atomic Energy Commission \textsf{(CEA-Cadarache)} is
developing a sub-miniature fission chamber technology with a diameter of 1.5 mm
working in the current mode (Bign). 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.
