Three models of the charge collection process in small dosimetric ionization chambers exposed to pulsed radiation are discussed. All three models allow for the presence of a free-electron component in the charge transfer, incorporating this into the model in slightly different ways, and the resulting collection efficiency formulae are compared over the range of variables normally met within clinical dosimetry. Measurements of the free-electron fraction for plane-parallel ionization chambers and for a Baldwin-Farmer 0.6 cm3 chamber are presented. The proportion of free electrons at the normal operating voltage is often high in small chambers but it is obvious that this can only lead to an increase in collection efficiency if the f-value calculated for purely ionic conduction allows for some improvement. Thus, a 50% free-electron fraction in a chamber which collects ions with efficiency f = 0.9950 at low pulse doses will increase this efficiency to only 0.9982. The same chamber, at the same operating voltage, and therefore the same free-electron fraction, if exposed to larger pulse doses, yielding an efficiency of 0.9531 as calculated for ions alone, would have a true efficiency of 0.9830-a large change.
The recombination correction for a small cylindrical ionization chamber exposed to pulsed radiation is reexamined in the light of new experimental evidence. A simple two-voltage testing technique is described and demonstrated. In general the existing theory for an air-filled chamber (Boag 1950) is found to be accurate over the range of charge densities examined (0.013 p/pulse to 3.8 p/pulse) where p=3.33 X 10(-4)C. m-3 (1 esu cm-3).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.