Absolute measurement of the exposure rate from X and gamma rays by free air and cavity ionization chamber method

In this paper the characteristics of the two types of ionization chamber - free air chamber and cavity ion chamber - are given. These two ion chambers constitute standards in the nuclear radiation metrology and dosimetry national laboratories, taking into account radio-physical parameters like accuracy, stability and physical method of measurement. The measuring methods are based on electronic equilibrium and Bragg-Gray relation, respectively. Depending on the desired application they can measure exposure and/or exposure rate as well as absorbed dose and/or absorbed dose rate. For the cavity chamber we also derived some relations that make us able to determine the sources activity. The two ion chamber prototypes were made in the Institute of Nuclear Physics and Engineering, Bucharest-Romania. (author)