Experimentally Determined KQ Values at the IAEA Dosimetry Laboratory
Abstract
Purpose
To present IAEA- determined photon beam quality correction factors kQ for the IBA FC65-G and PTW 30013 Farmer-type ionization chambers and compare them with the tabulated values in IAEA TRS 398 (Rev.1).
Methods
In 2022, the IAEA Dosimetry laboratory (DOL) established Calibration capability for ionization chambers in linac photon beam. This has allowed the laboratory to determine the kQ for Farmer chambers individually following TRS 398 (Rev.1) and the methodology for the dissemination of megavoltage photon-beam calibrations from the IAEA Dosimetry Laboratory to SSDLs and end users (see SSDL Newsletter 77). The IAEA maintains measurement traceability to the BIPM, and, upon user request and programmatic justification, it performs calibrations in five high energy photon beam qualities in addition to a Co-60 beam. The TPR20,10 – namely the beam quality specifier according to TRS 398 (Rev.1) - for the five beams is 0.665, 0.710, 0.737, 0.760 and 0.780. The relative standard uncertainty associated with the determination of the calibration coefficients in linac beams is 1.1 %. Chambers studied in this work were the IBA FC65-G and PTW 30013. Measurements were performed without the waterproof sleeve using manufacturer- recommended polarizing voltages. For each chamber and beam quality, measured kQ values were compared with the corresponding tabulated kQ from TRS‑398 (Rev. 1).
Results
Beam quality correction factors kQ were determined for 8 IBA FC65-G and 10 PTW 30013. The highest variation compared to the published data for a chamber model and energy observed was 0.65 %. The data are consistent with the standard uncertainty associated to the tabulated kQ given in IAEA TRS 398 Rev.1 which is 0.6%.
Conclusion
Results show limited differences between calculated and measured kQ values for the IBA FC65-G and PTW 30013 chambers calibrated in photon beam energies. The IAEA will continue collecting chamber‑specific kQ data.