Use of the variance-covariance technique for determination of the radiation quality in a pulsed linear accelerator

Abstract

Recent recommendations of a joint task group of the ICRU and ICRP have established the microdosimetric quantity “lineal energy” as the parameter of choice for determination of the radiation quality for purposes of radiation protection. Unlike linear energy transfer (LET or stopping power), lineal energy can be measured directly for tissue-equivalent volumes with dimensions of relevance to radiobiology (of the order of 1 μm). Conventional proportional-counter spectroscopy, the method most frequently utilized for determination of linear energy spectra, is impractical for short-duty cycle pulsed machines. A technique for determination of the dose mean lineal energy, utilizing the relative variance and covariance of two proportional counters operated in synchrony with the beam pulse, is described. Design and construction of a new ultraminiature proportional counter for use in high-dose-rate fields is described. This extremely small cross section (0.25 mm 2) counter should make multi-event measurements practical in the beam of a pulsed linear electron accelerator at full dose rate and single-event measurements at slightly reduced intensities; this in turn should make possible the microdosimetric determination of photoneutron contamination in megavoltage photon beams.