Improved free-air ionization chamber for the measurement of x-rays

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.