Measurement of the collection efficiency of a large volume spherical ionization chamber in megavoltage therapy beams.

The collection efficiency of a 5.7 cm diameter spherical ionization chamber has been measured in 4 MV and 10 MV x-ray beams at various distances from the source. This chamber was found to have a substantial inefficiency due to its large volume and the high dose rate and pulsed nature of the therapy beams. It was also found that the efficiency depended on the dose rate of the machine because the inter-pulse separation time of the linac is significantly less than the ion transit-time for this chamber. Thus, ionization from more than one beam pulse is collected by the chamber at the same time. The efficiency was determined using three techniques (i) the two-voltage technique, (ii) the voltage extrapolation technique and (iii) a method originally devised for determining the collection efficiency of large volume ionization chambers in diagnostic radiology. The results show that methods (ii) and (iii) agree well, but that the two-voltage technique predicts a much lower efficiency. At about 4 m from the source, the collection efficiency for this chamber varied between 98% and 97% for dose rates between 50 and 250 cGy/min for 4 MV and between 97% and 90% for dose rates between 100 and 600 cGy/min for 10 MV. At isocenter, the comparable figures were 78% and 56% respectively for 4 MV and 65% and 38% respectively for 10 MV.