Directional gamma ray monitor

To measure the gamma ray flux within a small solid angle about an axis X-X, the flux incident on a detector is measured when a shielding block which is arranged to subtend the said small solid angle with the detector is on the axis and in front of the gamma ray detector and a position A where it is displaced from the axis, and determining the difference between the two measurements. A further fixed shield reduces the effect of the shielding block on the background radiation signal when it is moved to position A. A light shines through apertures when the shielding block is displaced to indicate the location of axis X-X. (author)