THE EFFECT OF FREE-ELECTRON COLLECTION ON THE RECOMBINATION CORRECTION TO IONIZATION MEASUREMENTS OF PULSED RADIATION

Three models of the charge collection process in small dosimetric ioni zation chambers exposed to pulsed radiation are discussed. All three m odels allow for the presence of a free-electron component in the charg e transfer, incorporating this into the model in slightly different wa ys, and the resulting collection efficiency formulae are compared over the range of variables normally met within clinical dosimetry. Measur ements of the free-electron fraction for plane-parallel ionization cha mbers and for a Baldwin-Farmer 0.6 cm(3) chamber are presented. The pr oportion of free electrons at the normal operating voltage is often hi gh in small chambers but it is obvious that this can only lead to an i ncrease in collection efficiency if the f-value calculated for purely ionic conduction allows for some improvement. Thus, a 50% free-electro n 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 fr ee-electron fraction, if exposed to larger pulse doses, yielding an ef ficiency of 0.9531 as calculated for ions alone, would have a true eff iciency of 0.9830-a large change.