An Overall Stem Effect, including Stem Leakage and Stem Scatter, for a TM30013 Farmer-type Chamber

Abstract

The stem effect is a leakage current generated when the chamber stem is included in the radiation field size. Such an effect can be divided into stem leakage and stem scatter. When a chamber is calibrated in air, the chamber response is likely to be affected by the photons scattered from the chamber stem. These interactions contribute to the apparent measured exposure. We calculated the overall stem effect correction factor that was caused by the metal stem of the ionization chamber. We measured the stem effect of a Farmer-type ionization chamber that had recently been in use for exposure dose measurements. In addition, we calculated and compared the ratios of stem leakage and stem scatter to the overall stem effect. We measured an overall stem effect, including the stem leakage and the scatter of PTW model TM 30013 (vented to air, sensitive volume 0.6 cm(3)) Farmer chamber, in the exposure measurement. We measured the dependences of the stem scatter (k(sem.scatter)) and the stem leakage (k(sem.leak)) on the length of chamber stem exposure when measuring the exposure dose of high-energy X-rays generated by a linear accelerator (LINAC). Electrons ejected from the metal stern were collected by the central electrode, increased to a maximum and then decreased. Most of the overall stem effect was caused by stein scatter and was determined to within 4% according to the length of the stem exposed in repeated measurements of with various radiation fields.