Based on an original design by F. H. Attix, an improved free-air ionization chamber was constructed at the National Radiation Standard Laboratory of the Institute of Nuclear Energy Research (INER, Taiwan), as the primary standard for x-rays generated at energy levels between 50 keV and 300 keV. Various improvements were made to the Attix design, including modifications of the chamber structure and the measurement technique. The changes include: (i) a three-section design with fixed central cylinder; (ii) thickening of the shielding box; (iii) a circuitous mechanism for the operational handle, to avoid unwanted scattering; (iv) a ruler mechanism to provide fine position control; and (v) increased thickness of the aluminium wall of the chamber. After initial experimental verification, a comparison with the National Institute of Standards and Technology (NIST, USA) using a transfer standard chamber yielded differences in calibration factors of less than 1 %. In addition, the overall uncertainty for the x-ray measurement in terms of air kerma is less than 1 % at the 95 % confidence level. These results indicate that the improved free-air ionization chamber can serve as a primary standard at the NRSL.

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The aim of the present study was to estimate the wall effect of the self-made spherical graphite-walled cavity chamber with the Monte Carlo method for establishing the air-kerma primary standard of high-dose-rate (HDR) 192Ir brachytherapy sources at the Institute of Nuclear Energy Research (INER, Taiwan). The Monte Carlo method established in this paper was also employed to respectively simulate wall correction factors of the 192Ir air-kerma standard chambers used at the National Institute of Standards and Technology (NIST, USA) and the National Physical Laboratory (NPL, UK) for comparisons and verification. The chamber wall correction calculation results will be incorporated into INER's HDR 192Ir primary standard in the future. For the brachytherapy treatment in the esophagus or in the bronchi, the position of the isotope may have displacement in the cavity. Thus the delivered dose would differ from the prescribed dose in the treatment plan. We also tried assessing dose distribution due to the position displacement of HDR 192Ir brachytherapy source in a phantom with a central cavity by the Monte Carlo method. The calculated results could offer a clinical reference for the brachytherapy within the human organs with cavity.

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