Fabrication of free air well type ionization chamber and calculational assessment & measurement of its operational characteristics

Introduction: Well type ionization chamber is one of the measuring systems for activity determination of Brachytherapy sources. The chamber has a cylindrical volume in which a cylindrical tube is mounted in the middle of the chamber. For activity measurement, the brachytherapy sources are transferred to the middle of the tube. Materials & methods: For designing the well type chamber, the measurement principals of well type chambers were considered and MCNP 4C code as a calculation tool was used. The designed chamber was simulated and the response of the chamber was evaluated. In this investigation, we studied chamber operational parameters such as operating voltage, leakage current, reproducibility, reference measuring point, recombination and polarization factors as well as response stability for 137Cs, 57Co and 241Am sources. Results: The chamber leakage currents at operating voltage in comparison with chamber responses for the measurement of the above mentioned sources were negligible. Responses of the fabricated chamber for these sources are reproducible and its reference measurement position for these sources was obtained as 6 cm from the bottom of the chamber. Recombination factor at the well type chamber was negligible and polarization factor is close to 1, therefore these two factors were not considered in our measurements. The measurements repeatability in different intervals shows the stability of the chamber response for each source. Also results of chamber current measurement in term of source strength were compared to response of simulated chamber for different source positions and energy ranges of the used sources. Discussion: The results show that the measurement of reference positions for each source in simulated and fabricated chambers are quite in a good agreement. Regarding the reliable operational properties of the fabricated chamber, this chamber can be used for activity measurement of brachy therapy sources in which the energy ranges are greater than 50 keV.