Characteristic functions of point monodirectional electron beams.

Narrow electron beams with approximately Gaussian radial fluence distributions were extracted from a clinical 42 MeV betatron by means of a special multi-diaphragm lead collimator with either 3 or 7.5 mm diameter of the effective diaphragm. From each narrow beam the dose distribution in water, was measured with an automated scanning system, and the corresponding dose distribution of the point monodirectional source was obtained by deconvolution. As a reference, the fluence distribution in air at the same distance z was used, corrected for multiple scattering in air where necessary. At 10, 20 and 40 MeV, non-linear pear-shaped curves for the root mean square radial excursion, sigma(z), as functions of the depth, with maxima of 12, 19.7 and 27.8 mm at about 70 per cent of the practical range were obtained. The second characteristic function, I(z), the integral of the dose over the transverse plane at depth z, was obtained from the measured broad beam depth dose curve and the associated transverse dose distributions.