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

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

STATISTICAL LIMITS