Accelerator dosimetry tends toward either specifying the field by way of its components or using a tissue equivalent chamber with a failsafe Quality Factor (QF). This investigation attempts a third approach, the determination of QF directly. A Lucite, three-parallel plate ionization chamber was constructed with a common center electrode and two independent outer electrodes. This configuration allows the position of the center electrode to be manually adjusted to effect first order balance; vernier balance is obtained electronically through adjustment of one of the paired electrometers. The chamber can be pressurized to at least 17 atm with a tissue equivalent gas mixture and, therefore, approximates tissue rad response. This ionization chamber is capable of detecting differences in LET of any radiation by operating on the principle of columnar recombination. The previous investigators use either a single ion chamber or operate their respective systems in the general recombination mode. This technique requires low polarizing voltages and, therefore, is useful only for low dose rates. Both the single chamber and the general recombination system are limited in accelerator dosimetry applications. Very high instantaneous dose rates and extreme variability of sources dictate a balanced ionization chamber system that utilizes columnar recombination. In the present instrument, one of the chambers is operated at low polarizing potential while the other is variable. The difference current between the two chambers is adjusted to zero for background levels and when sensing radiation this difference current is a measure of QF. In operation, the slope, with voltage, of the difference current is proportional to the QF of the radiation. This proportionality is apparently linear at 10 atm over a range of QF from one to at least 7.5. This response has been shown to be dose-rate independent. The directional dependence and pressure dependence of this chamber have been studied and will be reported.
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