Telescopic measurements of backscattered radiation from secondary collimator jaws to a beam monitor chamber using a pair of slits

摘要: A contribution to field‐size dependent output by backscatteredradiation (BSR) from secondary collimator jaws a beam monitor chamber of linear accelerator was measured with Farmer ionization chamber, positioned 200 cm the source behind low‐melting‐point alloy slab 10‐cm wide slit. Another slit against jaws. Both slits were in form 6.3‐mm‐diam hole middle and aligned source. The use pair intended eliminate any influence on ion readings due charge flattening filter jaw forward scattering. In addition, setup permits observe degree dependence BSR. Charge measurements Therac‐20 18‐MV x rays showed 7.5% BSR whereas 6‐ Varian Clinac‐1800 <2% telescopic method found be easy permitted direct determination contributions.