127 research outputs found
Improvements in the Modelling of Micro Fission Chambers Operated in Current Mode
No full textPublished in a revised form in IEEE Trans. Nucl. Science. Reference: S. Chabod, A. Letourneau, P. Gourdon, C. Laye, Improvements in the modelling of sub-miniature fission chambers operated in current mode, IEEE Trans. Nucl. Science 57 (2010) 2702International audienceWe recall the theoretical bases necessary to model micro fission chambers operated in current mode. For these detectors, we propose formulae for the lower and upper limits of the saturation zone, obtained through analytical and numerical resolutions of the charge transport equations. Using these results, we calculate the voltage extension of the saturation plateau, as function of parameters such as the electrode geometries or the characteristics of the filling gas
Energy-time correlation of slowing-down neutrons
No full textWe formulate the energy-time correlation of neutrons slowing down through multiple elastic scatterings in a homogeneous infinite medium. From the correlation result, we obtain the neutron thermalisation time, expressed through special functions appearing in the calculation steps. We validate our theoretical approach by comparing the ensuing formulae with results of Monte Carlo calculations
Saturation current of miniaturized fission chambers
No full textWe present the detailed formulae of the saturation currents for the four main categories of fission chambers operating in current mode. The results obtained are function of simple parameters: number of fission reactions within the chamber deposits, geometric characteristics of the electrodes and filling gas properties. A direct relation between the saturation current values and the ambient neutron flux is thus established. These results should reduce the number, the duration and the cost of the calibration procedures required to operate the fission chambers
Charge collection efficiency in ionization chambers operating in the recombination and saturation regimes
No full textWe solve the electric charge transport equations in the recombination and saturation regimes using an iterative perturbation method. We then calculate the charge collection efficiencies of ionization chambers. The formulae obtained are presented in the form of series for which we calculate the first coefficients. Our approach allows to account for the spatial as well as the temporal variations of the primary charge density N(r,t) in the calculations. Finally, we apply our method to study different density patterns, N, including the textbook case N=N0δ(t) and the charge clusters and columns
A perturbation method to examine the steady-state charge transport in the recombination and saturation regimes of ionization chambers
No full textWe developed a perturbation method to predict the behaviour of ionization chambers functioning in current mode when the space charges are neglected. This method allowed us to go for the first time beyond the Thomson problem by taking into account, in the output current calculation, the spatial variations of the primary charge density induced by the incident ionizing particles. The results obtained are presented under the form of series for which we studied the convergence radius and quality in a simple case. Additionally, we solved the Thomson problem when the space charges are neglected and established for the first time the explicit relationship linking the input voltage to the output current. Finally, we calculated the voltage that delimits the transition between the recombination regime and the saturation plateau
Impact of space charges on the saturation curves of ionization chambers
No full textWe apply a perturbation method to quantify the impact of space charge accumulation on the output current of ionization chambers. We then obtain the formulae of the saturation curves delivered by these detectors under the form of series for which we calculate the first coefficients. The spatial variations of the primary charge density are taken into account in the calculations. For uniform ionization rates, we demonstrate that the amplitude of the space charge perturbations depends only on the values of the electronic and ionic Langevin factors of the filling fluid. We show that the influence of space charges can be neglected when the Langevin factor, λ, is small. These results were confirmed using numerical resolutions of the charge propagation equations. Finally, we evaluate the voltage that delimits the transition between the recombination regime and the saturation plateau when the space charges are taken into account
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