| 摘要 |
A radiation beam scanner system employs a peak detection methodology to measure the intensity and distribution of radiation produced by a medical linear accelerator. The scanner system combines the capability to perform scanning measurements with the capability to perform high accuracy calibrations of the linear accelerator. The system employs two ion chamber detectors, signal and reference, with the signal detector positioned within a tank of water (phantom tank). As the water is irradiated by the linear accelerator, the signal detector is continuously moved within the water by means of electrical stepper motors as the reference detector remains stationary at some point within the radiation beam. The reference detector output is compared to a predetermined threshold and, when the threshold is reached, a peak detector circuit monitors the signal detector output for a radiation pulse peak. Accelerator anomalies are eliminated by calculating a ratio of the signal detector output to the reference detector output. Each ratio is time tagged and correlated to signal detector position information, thereby providing a map of the distribution and intensity of the radiation within the water. The resultant data is evaluated by a qualified medical physicist to ascertain the accelerator's suitability for use in dosimetry. In one embodiment, the pulse frequency of the accelerator is determined and the rate of signal detector movement (scan rate) is set as a function of the pulse frequency of the accelerator. The system includes a unique, integrating, auto-zeroing, calibration electrometer that can be employed to calibrate the accelerometer.
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