Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
The nice thing about distance is that particulate radiation has a very short range, even in
air; as for photons, the number reaching a particular point in space per unit time decreases
according to an inverse square law, as the photon beam spreads father from the source.
This concept of an inverse square law requires further explanation. Picture a number of
spheres of different sizes all centered on one spot, e.g., a golf ball, a tennis ball, a bowling
ball, and a basketball, all having a common center and each inside the other (Fig. (2.14)).
Now picture lines drawn from the center and intercepting the surface of all balls. Since
Fig. (2.14) is drawn in two dimensions, the lines intercept the circumference or boundary
of various circles. If a definite size block were then chosen and placed on the surface of
each of the balls (or boundary of each of the circles), the number of lines intercepted by
the block when placed on the smallest ball (or circle) would be greater than the number of
lines intercepted by the block when placed on a bigger ball (or circle). As a matter of fact,
the lines intercepted decrease in inverse proportion to the distance of the surface (or
boundary) from the common center. Think of the lines as being representations of the
path of photons.
Fig. (2.14) Diagram of concept of the inverse square law: The lines A radiation from the center C
spread out as they go from inner circle to outer circle. If a constant length B is placed
across the circumference (edge) of each of the circles, it is seen that fewer lines are
intercepted by the length as it moves from the inner to the outer circles.
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