Effective point of measurement of thimble ion chambers in megavoltage photon beams

Abstract

The authors confirm that an upstream EPOM shift of 0.6r is too large for thimble ion chambers in high energy photon beams. Proper values for the EPOM shift could be tabulated per beam and per chamber, but they envisage that a single shift for all practical beams may prove sufficient. Moreover, the systematic dependence on chamber characteristics provides evidence that a universal parametrization in terms of a few design parameters is conceivable and has implication for the calculation of chamber correction factors.