In this paper the characteristics of cavity chambers of different wall material (graphite, bakelite, Al) and different volume are studied. Using this method, the dose rate of γ-rays of Ra is obtained. The experimental result is (corrected for density effect of wall material) 8.74±0.27 γ/mg/hr, at 1 cm off the source (without Pt packing). Using the correction factor of the different thicknesses of Pt packing of Laurence, this value becomes 8.15±0.27 γ/mg/hr (Source in 0.5 mm Pt).This result is in good agreement with previous works within the experimental errors.