IONIZATION CHAMBERS IONIZATION CURRENT In an electric field
IONIZATION CHAMBERS
IONIZATION CURRENT • In an electric field, the drift of negative and positive charges (electrons and ions), gives a current. • Under steady state irradiation, if the charges are collected efficiently and recombination is negligible, the steady state ionization current is a measure of the rate at which pairs are produced within the volume.
IONIZATION CURRENT • If no net field is applied, obviously there can be no current. • In this case, the ions and electrons created will disappear due to recombination or diffusion. • If the voltage is increased significantly, the E-Field will separate the charges more rapidly, eventually eliminating the chances for recombination.
IONIZATION CURRENT • When recombination becomes negligible, as mentioned, the measured current becomes the rate of formation within the volume. • This voltage is know as the saturation voltage. • Operation in this region is known as ion saturation. • If production of ion pairs is uniform throughout the volume, due to the charges drifting toward the anode and cathode, there will be an imbalance in concentration. • Whenever there’s an imbalance in concentration, diffusion occurs in the direction from high to low. • Thus opposing the electric field.
DERIVATION OF THE PULSE SHAPE • Assuming the saturation voltage is being applied, and negative charges are free electrons only… • Also, assume that all electron/ion pairs are created along the same line located at “x”.
DERIVATION OF THE PULSE SHAPE
DERIVATION OF THE PULSE SHAPE
DERIVATION OF THE PULSE SHAPE
DERIVATION OF THE PULSE SHAPE
DERIVATION OF THE PULSE SHAPE
DERIVATION OF THE PULSE SHAPE
FRISCH GRID • The position dependence of the pulse amplitude can be removed in electron sensitive chambers by use of a “Frisch Grid”. • The radiation must be made to enter only between the grid cathode. • The grid is maintained at an intermediate voltage, and is made to be essentially invisible to the electrons. • Due to the location of the resistor, the ions drifting toward the cathode give no signal pulse, nor do the electrons drifting toward the grid. • Only the electrons which drift from the grid to the anode will give rise to a voltage pulse.
FRISCH GRID
FRISCH GRID
FRISCH GRID
STATISTICAL LIMITS
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