An ionization chamber for therapy-level dosimetry of electron beams

This ionization chamber has been designed at the National Physical Laboratory in collaboration with the Hospital Physicists' Association; it is especially suitable for use with electron beams below 10 MeV. The sensitive volume is discshaped, 20 mm in diameter and 2 mm deep along the beam axis. Polarity effect is less than 0.1% at energies up to 6 MeV, allowing the chamber to be used with most existing dosemeter electrometers. The chamber is intended to be calibrated by the user against a secondary standard X-ray chamber using a Perspex phantom in 60Co gamma-radiation. Factors for converting 'exposure' readings to absorbed dose in water are tabulated for beam energies up to 7 MeV.