Simulation of Detector Response : How Does the Electron Multiplication Differ Within Ionization Chambers with Various Geometries?

University essay from KTH/Medicinteknik och hälsosystem

Abstract: This degree project was performed in collaboration with the division of nuclear physics at the department of physics, KTH Royal Institute of Technology. A partial goal of the project was to create a simulation model, where the relationship be- tween the multiplication of electrons that occurs in an ionization chamber and the different pressures of air in the detector could be visualized. The main goal was then to use this model in order to examine the behaviour of electron multiplication for different geometries of the simulated ionization chamber. The simulation was performed in Python 3.7 (Python Soft- ware Foundation, DE, United States), and geometry was modified by increasing and decreasing the simulated inner and anode wire radius of the chamber. Results showed that the peak of the multiplication curve occurred at different pressures for different geometries. When the anode wire radius was fixed, the peak occurred at a lower pressure for an increase of the inner radius, whereas, when the inner radius was fixed, the peak occurred at a higher pressure for an increase of the anode wire radius. The number of created electrons are dependent of Townsend’s coefficent, α, which in turn is dependent of the relationship between pressure and electric field strength. The electric field strength within an ionization chamber varies for different geometries, and therefore is the relationship between pressure and electric field that results in the max- imum value of α, and thus the maximum peak of the multiplication factor, consequently given by different pressures for different chamber geometries. If the results from the simulations in this project are to correspond with actual experimental data, the knowledge of this geometry-dependence may be used to include or exclude the multiplication peak in further measurements depending upon preference.

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