Intensity of gamma-radiation from a radium-covered disk at points along the central normal, with and without absorbing layers, is of interest in connection with the use of radium applicators for gamma-ray treatments. By integrating the effects of narrow rings of the active material, a theoretical formula is derived for the intensity at a distance $p$ from a disk of radius $R$ if the rays go through one or more filters whose absorptions are known and through a thickness of tissue $h_0$. This formula involves the so-called exponential integral for which tables are available. Numerical values have been calculated for certain special cases and these are given in two tables which enable the intensity to be determined at various distances from any uncovered disk in air and also from any disk covered by a layer of brass 2 mm. thick. A given amount of radium spread on a 4 cm. disk gives at distances greater than 4 cm. about the same intensity as when concentrated on the end of a small tube, but the effect near the active layer is over ten times less intense in the former case. This result shows that burns of the skin may be avoided by using sufficiently large disks without decreasing the depth action materially.

New unit of gamma-ray intensity is proposed, to be called a milli-eve and to be defined as the intensity at a distance of 1 cm. from a point-shaped radium preparation of one milligram.

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