It is proposed to use the (γ, n) reaction for the measurement of the integrated void volume fraction in two phase flow of D2O inside a duct. This method is applicable to different channel geometries, and it is shown to be insensitive to the pattern of void distribution over the cross-sectional area of the channel. The method has been tested on mock-ups of voids in a round duct of 6 mm inside diameter. About 40 mC Na24 was used as a source of gamma rays. The test results show that the maximum measured error in this arrangement is less than 2.5% (net void) for a range of 2.7% to 44.44% actual void volume fractions.