Establishing of primary standard for Cs-137 air-kerma of OAP, Thailand.

Graphite wall cavity chambers are the most common detector for precise gamma-ray measurement and are accepted as primary standards for radiation protection and radiotherapy level beams. This paper describes the physical constants and the evaluation of the correction factors that are necessary for the graphite cavity chamber to be established as a primary standard of the Office of Atoms for Peace (OAP). The correction factors to be applied to the primary standards were determined by experimental and Monte Carlo methods. The evaluation of the standard according to the recommendations of ICRU 37 and ICRU 90 report for graphite cavity chamber is presented including the uncertainty budget. Finally, the comparison results using ICRU 37 and ICRU 90 data are presented in this paper to give confidence in the establishment of this primary standard.