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Telescopic measurements of backscattered radiation from secondary collimator jaws to a beam monitor chamber using a pair of slits
Author(s) -
Kubo Hideo
Publication year - 1989
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.596381
Subject(s) - collimator , ionization chamber , optics , linear particle accelerator , beam (structure) , dosimetry , physics , field size , scattering , radiation , point source , materials science , ionization , nuclear medicine , ion , medicine , quantum mechanics
A contribution to field‐size dependent output by backscattered radiation (BSR) from secondary collimator jaws to a beam monitor chamber of a linear accelerator was measured with a Farmer ionization chamber, positioned 200 cm from the source behind a low‐melting‐point alloy slab with a 10‐cm wide slit. Another slit was positioned against the collimator jaws. Both slits were in the form of a 6.3‐mm‐diam hole in the middle and were aligned to the source. The use of a pair of slits was intended to eliminate any influence on the ion chamber readings due to field‐size dependent charge contribution from the flattening filter and collimator jaw forward scattering. In addition, the setup permits to observe the degree of field‐size dependence on BSR. Charge measurements from the Therac‐20 18‐MV x rays showed a 7.5% field‐size dependence on BSR whereas 6‐ and 18‐MV x rays from Varian Clinac‐1800 showed <2% dependence on BSR. The telescopic method was found to be easy to use and permitted direct determination of BSR contributions.