Extra-cameral volume effects in ionisation chambers for electron beam dosimetry

The authors record a sudden change in sensitivity of a low-energy electron therapy ionisation chamber after five years' use, attributed to an extraneous ion collection volume in the region of the coaxial cable termination. This is a serious problem as the change in sensitivity is different for the low-energy electron beams for which the chamber is designed, and for the cobalt-60 or megavoltage photon in which it is recommended that the chamber should be calibrated. A series of measurements is suggested to counteract this eventuality. (U.K.)