Answer to Question #225 Submitted to "Ask the Experts"

The answer depends on the size of the "plane" source. For an infinite plane—e.g., a source that approximates infinite from the viewer's perspective—the exposure rate will decrease very little with radial distance and may actually increase due to electronic equilibrium being established in the air near the source. The only significant parameter which will cause a decrease in exposure rate for an infinite plane will be energy absorption in the air itself.

For a source that is larger than a small point but that cannot be approximated as "infinite," a handy equation that can be used is one for a disc source. The exposure rate at a radial distance from a disc source can be estimated as:

I = (Io ln((r² + d²) / d²)) / r²

where:

I	=	exposure rate at distance d;
Io	=	exposure rate in air at 1-meter from a point source;
d	=	distance (m); and
r	=	disc radius (m).

Of course, any units of measure may be used in place of "meters." Hope this helps.

Robert L. Coleman
Oak Ridge National Laboratory