Aeqand other factors in a60Co beam for a spherical or cylindrical minimal phantom and for an ionization chamber with a known Awallcorrection factor

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Abstract
Three types of tissue-air ratio (TAR) are summarized. These TARS differ in their definition of the in-air absorbed dose. The first defines it as the absorbed dose at the centre of a spherical or cylindrical minimal water phantom in free space. The second defines it as the maximum primary absorbed dose in a semi-infinite water phantom. The third defines it as the absorbed dose in an imaginary infinitesimal mass of water within the cavity of a chamber in free space, where the absorbed dose is averaged within the cavity. It is concluded that the 60Co TAR data compiled by Godden and the 60Co TAR data of Johns and Cunningham should be reviewed. Aeq and other factors are evaluated for spherical and cylindrical minimal water phantoms in a 60Co gamma-ray beam. A method of obtaining Aeq and other factors for an ionization chamber with a known Awall correction factor is also reported. The work indicates some discrepancies with previously published material.