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03 - Ionization Chambers 03 - Ionization chambers Jaroslav Adam Czech Technical University in Prague Version 2 Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 1 / 130 Principle of operation Charged particle getting through a volume of a gas or noble liquid Interaction proceed through ionization and end excitation of the molecules, electron-ion pairs are created Ion can be created directly by the incident particle, or by the δ-electrons, when the energy from the primary particle if first transferred to the electron which acquires enough energy to make further ionization Electric field is applied by the electrodes, electrons and ions drift to them If the field is high enough, drifting electrons can also ionize the gas (proportional counters) After further increase of the field strength, electrons emit UV light on the anode (Geiger-Muller counter) Electronic signal at the output, pulsed or current regime Position sensitivity by segmenting one of the electrodes (xy) and by timing measurement (z) Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 2 / 130 Ionization detectors without amplification Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 3 / 130 Number of ion pairs Minimum energy W to be transferred from the incident particle to create at least one ion pair Quantified as average energy loss to create the pair Given by the least tightly bound shell, W = 10 - 25 eV Non-ionizing energy loss (excitation) makes number of pairs lower Fully stopped 1 MeV particle produces 30 000 ion pairs Number of pairs is important for resolution Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 4 / 130 Energy dissipation per ion pair (the W-Value) Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 5 / 130 Fano factor Fluctuation in number of pairs affect the energy resolution p The simplest approach postulate Poisson statistics for number of ion pairs (σ = N) Fano factor makes correction to predicted variance to get observed variance Fano factor = 0 if all incident energy converted into pairs, no statistical fluctuation p In gases Fano factor < 1, Poisson distribution valid but fluctuations are smaller than N Significant in pulse mode Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 6 / 130 Principle of ionization chamber Figure : Planar ionization chamber Suppose two metallic electrodes at distance D covering volume of a gas or noble liquid Voltage V applied to anode (thousands of kV) Number of electrons n− given by the number of minimum-ionizing-particles (mip) n dE n− = Dρ (1) W dx ρ is the density and dE/dx is energy loss per g cm−2 Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 7 / 130 Charge carriers in gas/liquid volume Several processes applies to the ion pairs Drift movement by external electric field Diffusion due to random thermal movement Charge transfer Recombination Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 8 / 130 Drift movement Electrostatic force moves the charges, positive ions opposite to electrons and negative ions Drift of electrons characterized by drift velocity in electric field E = V =D dx µ V µ v (e) = = e E = e (2) d dt p pD 2 −1 −1 Electron mobility µe given in unit of bar cm V s , p is the gas pressure Mobility of ions is about 1000 less than of electrons Description with mobility provides calculation of readout times ms for ions, µs for electrons Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 9 / 130 Saturation of the drift velocity Figure : Electron velocity vs. filed No increase in drift velocity after reaching it’s maximum, only in some gases Hydrocarbons, argon-hydrocarbon mixtures In non-saturation gases, E=p proportionality holds up to high fields Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 10 / 130 Diffusion Thermal movement with mean free path of about 10−6 - 10−8 m More important for electrons since their thermal velocity is bigger Point-like collection of electrons form Gaussian spatial distribution widening with time Widening in one direction x; y or z given by diffusion coefficient D p σ = 2Dt (3) D is given by kinetic gas theory or the process is described by a transport model Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 11 / 130 Charge transfer collisions Electron transfer from neutral gas molecule to positive ion in mutual collision Significant in mixtures, net positive charge transfered to species with lowest ionization energy Negative ion can be formed by capturing of free electron (oxygen) Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 12 / 130 Recombination Free electron captured by positive ion making ordinary neutral atom Positive and negative ions recombine, most probable compared to electron and ion case Original charge is lost, no contribution to final signal Recombination rate given by density of positive and negative species n+ and n− and recombination coefficient α dn+ dn− = = −αn+n− (4) dt dt Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 13 / 130 Columnar (initial) recombination Electron-ion pairs created in column along particle trajectory Recombination of electron with it’s parent ion Mainly for heavy ionizing particles (low energy α), minimal for mip Independent of interaction rate Recombination may occur with neighboring ion when electrons are drifting in electric field Depends on the angle between incident particle and electric field Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 14 / 130 Volume recombination Recombination during drift towards electrodes Unlike initial/columnar recombination, this depends on irradiation rate Suppressed by fast charge separation and collection -> high electric field Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 15 / 130 Ionization current, DC ion chamber Constant irradiation rate creates constant formation of ion pairs Steady-state current is measure of the rate Supposing negligible recombination and efficient charge collection Figure : Planar ionization chamber Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 16 / 130 Current-voltage characteristics Electric field by external voltage Current in the circuit equal to ionization current at equilibrium Increasing voltage begins to separate the charges that would recombine High electric field makes recombination negligible After ion saturation, all charges are collected, on increase in current when increasing the voltage Standard operation of ion chambers, current in the circuit is an indication of the rate of ion pairs formation Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 17 / 130 Saturation current Recombination, especially columnar recombination require high voltage Volume recombination important at high irradiation intensity Higher voltage required to get true saturation current More important in neutron measurement, where heavy fragments are detected With chambers filled by ambient air, recombination depends on humidity Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 18 / 130 Perturbations in current due to diffusion Imbalance in steady-state situation supposing uniform production of ions within the chamber Larger concentration of positive ions close to cathode, opposite for electrons close to anode Gradient in concentration formed, diffusion opposite to drift Perturbation in measured current in planar chamber given by ∆I kT − = (5) I eV is ratio of average energy of charge carrier, kT =e ≈ 2:5 × 10−2 V at room temperature close to one for ions, but several hundreds for electrons Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 19 / 130 Losses of saturation current due to diffusion close to one for ions, but much larger, several hundreds for electrons Minimized by high voltage Columnar recombination not fully eliminated Separate measurements of ionization current as a function of voltage 1=I as a function of 1=V to determine true saturation current Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 20 / 130 Operation of DC ion chamber No special requirement on gas since negative charge can be collected as free electrons as well as negative ions Only recombination could affect the amount of charge, suppressed by high enough voltage Few centimeters and tens of hundreds of volts sufficient to reach saturation Air for gamma-ray exposure, denser gases like Argon to increase ionization density Pressure about 1 atmosphere, higher to increase sensitivity Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 21 / 130 Geometry of ionization chamber Electric field given by geometry of electrodes Uniform electric field with planar geometry Cylindrical geometry with electrical field as E(r) = U0 r ln(ra=ri ) Figure : Cylindrical ionization chamber Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 22 / 130 Insulators Small values of ionization current, < 10−12 A Resistance of insulator at least 1016 Ω to keep leakage current below 1% for U = 100 V Leakage by moisture absorbed on surface suppressed by the guard rings and smooth surface of insulators Plastics or ceramic for higher irradiation Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 23 / 130 Measurement of gamma-ray exposure with ion chambers Amount of charge in air-filled ionization chamber Charge is measure of exposure, ionization current gives exposure rate Requires to measure ionization of all secondary electrons, mean free pairs several meters compensation for secondary electrons needed
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