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

Abstract

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.

Assessment of fissionable material behaviour in fission chambers

“…Although the geometrical characteristics of the CFUR43 are not totally known, we have taken its geometrical and physical features from [9,12]: Ar as internal gas, pressure at 1 bar, interelectrode gap of 0.25 mm (the suggested valué is between 0.2 and 0.3 mm [12]) and length of 17.55cm (z max : 0.71 cm; z min : -1.045cm [9]). These valúes lead, considering a cylindrical volume, to an Ar mass of about 4.2 x 10~8 kg; i.e.…”

“…This valué is obtained collapsing the uncertainty data for a particular neutrón spectrum according with this formula [5,7] : Z^AI,EXP\ jf Cj (12) where a¡ is the effective cross-section in one of the energy groups and a the one-group cross-section. For inventory calculations, this procedure to derive one-group uncertainty data guaranties that the uncertainty using different group structure remains constant [17].…”

Saturation current of miniaturized fission chambers

“…We will establish first a general law, and we will apply it then to the 4 most common chamber geometries: detectors with circular plane electrodes, rectangular plane electrodes, cylindrical electrodes and spherical electrodes. Two examples of previous evaluations of N for cylindrical chambers, and especially an approximation by Poujade and Lebrun [4] for fission products emitted perpendicularly to the anode axis, are presented and commented in Ref. [1].…”

Research activities in fission chamber modeling in support of the nuclear energy industry

“…First, its sensitivity can be optimized to monitor a given neutron flux spectrum by selecting the most relevant fissile deposit in terms of isotopic composition and mass. Second, its three operating modes, which are namely the pulse [1], Campbelling (or fluctuation) [1] , [2], [3] and current modes [1], [4], [5], make it possible to follow the flux amplitude over a large dynamic range from low-flux level at starting-up to high-flux level at power [6], [7], which corresponds to about 15 decades. Third, a fission chamber operated in fluctuation mode is capable of discriminating the [3].…”