Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 44

It is intuitively evident that radiation dose decreases with increasing distance from the source. In the case of a “point source” (For most practical purposes, a source appears like a point at a distance equal to about 10 times the longest physical dimension of the source.), the dose rate decreases rapidly according to the inverse square law:

(9)

where I₁ and I₂ are the dose rates at distances d₁ and d₂, respectively. Thus, a dose rate of 100 mrad h⁻¹ at a distance of 0.5 m from a point source decreases to 25 mrad h⁻¹ at 1 m, and to 6.25 mrad h⁻¹ at 2 m.

For the case of a line source, such as a brine‐carrying pipe that has a buildup of scale along the pipe wall containing radium, the dose rate decreases more slowly with increasing distance than from a point source. The same is true for an area source, such as a spill of a radioactive liquid. However, at distances approaching 10 times the maximum linear dimension of the spilled area, the dose rate begins to fall off as though the spilled area were a point.