Technical Note: Scanning of parallel‐plate ionization chamber and diamond detector for measurements of water‐dose profiles in the vicinity of a narrow x‐ray microbeam

摘要

Purpose Scanning of dosimeters facilitates dose distribution measurements with fine spatial resolutions. This paper presents a method of conversion of the scanning results to water‐dose profiles and provides an experimental verification. Methods An Advanced Markus chamber and a diamond detector were scanned at a resolution of 6 μm near the beam edges during irradiation with a 25‐μm‐wide white narrow x‐ray beam from a synchrotron radiation source. For comparison, GafChromic films HD ‐810 and HD ‐V2 were also irradiated. The conversion procedure for the water dose values was simulated with Monte Carlo photon‐electron transport code as a function of the x‐ray incidence position. This method was deduced from nonstandard beam reference‐dosimetry protocols used for high‐energy x‐rays. Results Among the calculated nonstandard beam correction factors, P wall , which is the ratio of the absorbed dose in the sensitive volume of the chamber with water wall to that with a polymethyl methacrylate wall, was found to be the most influential correction factor in most conditions. The total correction factor ranged from 1.7 to 2.7 for the Advanced Markus chamber and from 1.15 to 1.86 for the diamond detector as a function of the x‐ray incidence position. The water dose values obtained with the Advanced Markus chamber and the HD ‐810 film were in agreement in the vicinity of the beam, within 35% and 18% for the upper and lower sides of the beam respectively. The beam width obtained from the diamond detector was greater, and the doses out of the beam were smaller than the doses of the others. Conclusions The comparison between the Advanced Markus chamber and HD ‐810 revealed that the dose obtained with the scanned chamber could be converted to the water dose around the beam by applying nonstandard beam reference‐dosimetry protocols.