Replacement correction factors for plane-parallel ion chambers in electron beams

Plane-parallel chambers are recommended by dosimetry protocols for measurements in (especially low-energy) electron beams. In dosimetry protocols, the replacement correction factor P(repl) is assumed unity for "well-guarded" plane-parallel chambers in electron beams when the front face of the cavity is the effective point of measurement. There is experimental evidence that ion chambers which are not well-guarded (e.g., Markus) have nonunity P(repl) values. Monte Carlo simulations are employed in this study to investigate the replacement correction factors for plane-parallel chambers in electron beams.

Using previously established Monte Carlo calculation methods, the values of P(repl) are calculated with high statistical precision for the cavities of a variety of plane-parallel chambers in a water phantom irradiated by various electron beams. The dependences of the values of P(repl) on the beam quality, phantom depth, as well as the guard ring width are studied.

In the dose fall-off region for low-energy beams, the P(repl) values are very sensitive to depth. It is found that this is mainly due to the gradient effect, which originates from the fact that the effective point of measurement for many plane-parallel chambers should not be at the front face of the cavity but rather shifted toward the center of the cavity by a fraction of a millimeter. Using the front face of the cavity as the effective point of measurement, the calculated values of P(repl) at d(ref) are not unity for some well-guarded plane-parallel chambers. The calculated P(repl) values for the Roos chamber are close to 1 for all electron beams. The calculation results for the Markus chamber are in good agreement with the measured values.

The appropriate selection of the effective point of measurement for plane-parallel chambers in electron beams is an important issue. If the effective point of measurement is correctly accounted for, the P(repl) values would be almost independent of depth. Both the guard ring width and the ratio of the collecting volume diameter to the cavity thickness can influence the values of P(repl) For a diameter to thickness ratio of 5 (e.g., NACP02 chamber), the guard width has to be 6 mm for the chamber to be considered as well-guarded, i.e., have a P(repl) value of 1.00.