Experimental Arrangement And Data Acquisition System At LNMRI For Exposure And Air-Kerma Measurement Of Cobal-60 Gamma Rays#

A graphite cavity ionization chamber made by the Osterreichisches Forschunggszentrum Seiberdorf, has been installed as a primary standard for 60Co radiation at the Laboratorio Nacional de Metrologia das Radiacoes lonizantes, (LNMRI). This paper describes several aspects of the experimental arrangement: the radiation source and all modifications made in the beam collimating system in order to improve its flatness; the additional shutter built to avoid output changes due to the cobalt pellets movement inside the capsule; the special insulated room constructed inside of one of the existing facilities to minimize the temperature gradient inside the room and the chamber positioning system using a position transducer. In addition, description of the automated data acquisition system, the mathematical formalism for the calculation of the quantities of interest and the statistical treatment are given. The results of an intercomparison made with the BlPM has shown an agreement better than 0.1%. Furthermore, comparisons made with several secondary standards under the same experimental conditions have also shown an excellent agreement.