Microdosimetry of pulsed radiation fields employing the variance method

The relationship between dose mean lineal energy and relative variance has been exploited previously to derive these quantities from current measurements in steady and uniform radiation fields. Recently others made the observation that utilization of two detectors can make the variance technique practicable in time-varying fields. The authors report here the first measurements of y-bar/sub D/ for 10 MeV x rays and 9 and 18 MeV electrons from a pulsed linear accelerator using the variance method. Two independent analog-to-digital converters were used to obtain data from two spherical proportional counters in synchrony with the beam pulse. The method is described in detail and results reported for site diameters of 1/2, 1, and 2 μm. Data for an accurate determination of y/sub D/ can be obtained with this technique in less than a minute, making possible an essentially ''on line'' determination of y-bar/sub D/ or z-bar/sub D/ in a clinical situation