The mysterious wall around exposure

The quantity, exposure, and its associated unit, the roentgen, have been employed for approximately 50 years for the practical measurement of x- and gamma-ray source "output." Practical exposure-measuring devices must be provided with a wall of critical thickness and composition. Confusion arises since the wall is not mentioned in the definition of "exposure." This mystery can be dispelled by the fact that exposure never is measured in the manner in which it is defined.