An estimate of the 60Co gamma attenuation in the small phantom enployed in the definition of tissue air ratio.

Tissue air ratio definition employs a concept of dose at the centre of a small mass of tissues that is suspended in air and is just large enough to give electronic equilibrium at its own centre. When the dose at the centre of the said small mass of tissue is to be derived from a knowledge of exposure in free air at the same point, the attenuation in the small mass of tissue should be taken into account. This attenuation for 1250 keV photons has been evaluated through a theroretical model and the attenuation factor is estimated to be 0.975.