Telescopic measurements of backscattered radiation from secondary collimator jaws to a beam monitor chamber using a pair of slits.
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 less than 2% dependence on BSR. The telescopic method was found to be easy to use and permitted direct determination of BSR contributions.
Kubo
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Department of Radiation Oncology, University of Rochester Cancer Center, New York 14642.
D006801:Humans; D010315:Particle Accelerators; D011834:Radiation Monitoring / Q000295:instrumentation; D011874:Radiometry / Q000295:instrumentation; D011882:Radiotherapy, High-Energy; D012542:Scattering, Radiation