Experimental investigation of angular distributions of exposure rate in natural environments

Angular distributions of exposure rate in natural radiation environment were measured using a 1''diameter x 1'' NaI(Tl) scintillation detector which was placed on the axis of a set of concentric circular lead shieldings. Incident polar angle of γ ray was determined by changing the relative position between the detector and the shieldings. Incident γ rays were integrated over the whole azimuthal angle (0 - 2π) in each polar angle. And then, the angular distribution was obtained by subtracting the responses between each polar angle. As the incident solid angle of γ ray could be set large enough, it was possible not only to set the measuring time short but also to measure with high accuracy. Measurements were made under various environmental conditions; two on open field and one in a pine grove at a height of 1 m, and one at a height of 10 m on a tower. And also, one measurement was made in a one-storied wooden house. Experimental results supported the angular distributions expected from calculations, previously reported by other authors, and it was shown that this method can be applied well even where a calculational evaluation is difficult due to the complexity of its topography. (author)