Experimental determination of pCo perturbation factors for plane-parallel chambers

For plane- parallel chambers used in electron dosimetry, modern dosimetry protocols recommend a cross- calibration against a calibrated cylindrical chamber. The rationale for this is the unacceptably large ( up to 3 - 4%) chamber- to- chamber variations of the perturbation factors ( p(wall))(Co), which have been reported for plane- parallel chambers of a given type. In some recent publications, it was shown that this is no longer the case for modern plane- parallel chambers. The aims of the present study are to obtain reliable information about the variation of the perturbation factors for modern types of plane- parallel chambers, and - if this variation is found to be acceptably small - to determine type- specific mean values for these perturbation factors which can be used for absorbed dose measurements in electron beams using plane- parallel chambers. In an extensive multi- center study, the individual perturbation factors p(Co) ( which are usually assumed to be equal to (pwall)(Co)) for a total of 35 plane- parallel chambers of the Roos type, 15 chambers of the Markus type and 12 chambers of the Advanced Markus type were determined. From a total of 188 cross- calibration measurements, variations of the pCo values for different chambers of the same type of at most 1.0%, 0.9% and 0.6% were found for the chambers of the Roos, Markus and Advanced Markus types, respectively. The mean pCo values obtained from all measurements are (p) over bar (Roos)(Co) = 1.0198, (p) over bar (Markus)(Co) = 1.0175 and (p) over bar (Advanced)(Co) = 1.0155; the relative experimental standard deviation of the individual pCo values is less than 0.24% for all chamber types; the relative standard uncertainty of the mean p(Co) values is 1.1%.