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Saturation curves of parallel‐plate ionization chambers
Author(s) -
Fallone B. G.,
Podgorsak E. B.
Publication year - 1983
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.595292
Subject(s) - saturation (graph theory) , ionization chamber , ionization , electric field , atomic physics , saturation current , materials science , optics , computational physics , physics , analytical chemistry (journal) , ion , chemistry , voltage , mathematics , chromatography , combinatorics , quantum mechanics
A new analytical expression is presented to describe the full saturation curve of parallel‐plate ionization chambers. In contrast to the presently known expressions, which hold only for the near saturation region, this empirically determined expression is in excellent agreement with measurements in the whole collection efficiency range from 0 to 1 for x‐ray sources with effective energies from 20 to 150 keV and cobalt‐60 gamma rays. The dependence of the ion collection efficiency and the extrapolated electric field, which is a parameter in the new saturation curve expression, on electric field, dose, dose rate, beam quality, and chamber volume, is discussed. The effect of photoemission from the chamber polarizing electrode for low‐energy x‐ray beams on the saturation current is demonstrated. A universal ionization chamber constant is derived experimentally. It is shown that all parameters of the saturation curve equation and thus the saturation curve itself, can be calculated from one single measurement of ionization current at a given electric field and air gap thickness.