D. W. O. Rogers

Many laboratories with cavity chambers as primary standards for air kerma are considering using additional Monte Carlo calculated correction factors, in particular the correction for attenuation and scatter in the walls,and possibly the correction for point of measurement,Standards labs also use Monte Carlo calculated Spencer‐Attix stopping‐power ratios for graphite to air. The purpose of this article is to investigate the sensitivity of these calculations to their details and to assign uncertainties to the calculated values. We also investigate the correction needed for the Canadian primary standard to account for a polystyrene insulator,and find that it is quite largeThe article shows that the values of correction factors are very robust and insensitive to most details of the calculations except the values of the underlying electron stopping powers which have a significant effect on the stopping‐power ratio and onThe 1% uncertainties on the photon cross‐sections have a negligible effect on these correction factors except forAs a result of these investigations, with no change in the stopping power data used, the Canadian primary standard of air kerma in abeam needs to be increased by 0.54%.

Data are presented to allow the photon beam quality specifier being used in the new AAPM TG-51 protocol, %dd(10)x, to be extracted from depth-dose data measured with a 1 mm lead foil either 50 cm or 30 cm from the phantom surface. %dd(10)x is the photon component of the percentage depth dose at 10 cm depth for a 10x10 cm2 field on the surface of a phantom at an SSD of 100 cm. The purpose of the foil is to remove the unknown electron contamination from the accelerator head. Monte Carlo calculations are done: (a) to show these electrons are reduced to a negligible level; (b) to calculate the amount of electron contamination from the lead foil at the depth of dose maximum; and (c) to calculate the effect of beam hardening on %dd(10). The analysis extends the earlier work of Li and Rogers [Med. Phys. 21, 791-798 (1994)] which only provided data for the foil at 50 cm. An error in the earlier Monte Carlo simulations is reported and a more convenient method of analyzing and using the data is presented. It is shown that 20% variations in the foil thickness have a negligible effect on the calculated corrections.

A new approach is proposed for electron‐beam dosimetry under reference conditions and data necessary to use this approach are presented. The approach has the following features: it uses ion chambers and starts from an absorbed‐dose calibration factor for to be consistent with the present proposal for the new AAPM photon‐beam protocol; it uses to specify the beam quality and the reference depth, (all quantities in cm), recommended by Burns et al. [Med. Phys. 23, 383–388 (1996)]; it has a formalism which is parallel to the formalism for photon‐beam dosimetry; it fully accounts for the impact on stopping‐power ratios of realistic electron beams; it allows an easy transition to using primary standards for absorbed dose to water in electron beams when these are available. The equation for dose to water under reference conditions is: The term is not needed with plane‐parallel chambers but corrects for gradient effects with cylindrical chambers and is measured in the user's beam. The parameter is associated with converting the absorbed‐dose calibration factor into one for an electron beam of quality and contains most of the chamber to chamber variation. Calculated values of are presented as well as Monte Carlo calculated values for plane‐parallel chambers in a water phantom irradiated by a beam since these are needed to calculate The factor is a function of and converts the absorbed‐dose calibration factor to that for the electron‐beam quality of interest. Two analytical expressions are presented which are close to universal expressions for all cylindrical Farmer‐like chambers and for well‐guarded plane‐parallel chambers respectively. Calculated values are presented graphically for electron beams with energies between 5 and 50 MeV.