A system for discriminating the direction of a high-energy ray source, wherein a collimator (2) is formed of double rings, and a space of Da is secured between the inner ring (2a) and the outer ring (2b) and also a space Db is secured between the inner ring (2a) and a sensor (1). A distribution function for signals can be adjusted by adjusting these spaces. When the collimation of the collimator is at such a degree that allows an incidence to sensors on both sides of unit sensors positioned apart 180° from each other, an insensible incidence angle by a space between the unit sensors can be eliminated. Also, when a plurality of ring-like two-dimensional sensors are stacked and formed to be extended in the closed state of the opening part (3) of the collimator formed in the outer periphery thereof, the flying direction of energy ray from a vertical direction to the flat surfaces of the rings can also be discriminated.
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