Measurement of the directional electron contributions to dose in a phantom irradiated by 8 MV X-rays

A method of measuring the directional components of electron dose in a material exposed to 8 MV X-rays is described. A hollow wooden spherical shell which is divisible into ten sectors is used. The detector is a conventional thin-walled ionisation chamber. The internal diameter of the shell is large compared with the dimensions of the ionisation chamber. The dose contribution from each sector is separately measured and expressed as a fraction of the full equilibrium dose. The principles of the method are discussed and some correction factors are described. The build-up of dose with thickness of material in each sector is measured and tabulated. The measured data are used to calculate the loss of electron equilibrium at a point in an air channel in an otherwise uniform body.