Characterization of an extrapolation chamber as a primary standard dosimeter for beta radiation

Abstract

Inbeta or electron radiation fields, operational dose equivalent quantities, based on absorbed dose intissue, are used for radiation protection of the workers. Low penetration and high scattering in matter arecharacteristics of beta radiation that make its dosimetry more complex. Reliable measurements ofabsorbed dose in tissue must be traceable to an extrapolation chamber, which is the only primary standarddosimeter available for beta radiation. This ionization chamber uses the extrapolation to zero volume asthe method of measurement of absorbed doses in 0.07 mm of tissue, DT(0.07);its requires thedetermination of correction factorsrelated to influences from the detector itself and from the radiationfield geometry. In this work, a 23392 model PTW extrapolation chamber was characterized bydetermining its collector electrode area, its true-null depth and the influence in its response oftemperature, humidity and chamber depth. Values of the true-null depth were 0.46±0.04 mm and0.40±0.04 mm for 90Sr/90Y and 85Kr beta field sources, respectively. Corrections of the chamberionization current, according to the ideal gas law significantly reduced the high influence of temperature;air humidity influence showed to be negligible within the 50% to 70% range. The high dependence of thechamber response with its volume addressed the need to determine accurately its true-null depth. Theseresults are to be used as appropriate corrections for accurate absorbed dose in tissue determination.