Scanning the radiation field of a 25-MeV medical betatron using a new uniformity check system
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Radiation fields of high-energy machines can be readily scanned for beam flatness by means of a new mechanism. It is very useful in alignment of compensating filter and patient-dose uniformity checks.
The scanner is carefully positioned in front of the machine in the treatment room and a remote-control console placed outside drives an upright probe and displays the relative position of the probe. The sensor in the probe consists of a semiconductor p-n junction detector made of 1-mm diameter silicon discs doped with phosphorus and boron with 0.1-mm “depletion layer” between the discs. The detector current is measured through, an external circuit with very low impedance. The short-circuit current is equal to the radiation-induced current at zero bias, i.e., proportional to exposure rate.
The dosimeter has an output of 33.6 pA/rad/min. Amplifiers in the console provide an output voltage which is linear with field strength up to 10 V for 500 rads/min. Console output goes into anX-Y plotter with position signal in theX axis and dose signal in theY axis. The scanner method yields the tracings of the radiation field in about 10 min whereas alternate methods may take several hours.
In addition to measuring field geometry the scanner is useful in the studies of wedge isodose curves; dosimetry of irregular fields and radiation treatment planning for cancer patients.
KeywordsBoron Radiation Field Depletion Layer Exposure Rate External Circuit
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