In the second method, the linear attenuation coefficients (m) of these materials were measured using a small volume ionization chamber (0.1 cc, Type 23323, PTW Freiburg, Germany) in a telecobalt unit (Theratron Phoenix, Best Medical, Canada). Schematic diagram of the experimental set-up used for the measurement of m is shown in Figure 1. For this purpose, the ionization chamber with its Co-60 build-up cap was placed at 110 cm distance from the telecobalt source and 5 × 5 cm2 field size was opened by the collimator of the telecobalt machine. Lead blocks were used to define a field size of about 2 × 2 cm2 at the plane of the ionization chamber. For the measurement of the linear attenuation coefficient of water, a locally made 25 × 25 cm2 water phantom with open top and a thin polythene sheet at the bottom was used. The readings of the ionization chamber without the phantom material and with various thicknesses of the phantom materials for 5 minutes irradiation were recorded and the linear attenuation coefficient (μ) was calculated using the relation