Environmental Health & Safety

Radiation Safety

Time, Distance and Shielding

The concept of time, distance and shielding is integral in maintaining radiation exposures as low as reasonably achievable (ALARA).

Time
Assuming that "all things are equal," radiation exposure (dose) is directly related to the amount of time of exposure. By reducing the amount of exposure time, you will correspondingly reduce your dose.

Distance
By maintaining as much distance (up to a maximum) as possible between you and a source of radiation, you can reduce your dose significantly. The relationship between radiation exposure (dose) and distance follows the inverse square law. If the dose rate at one foot from a radiation source is 1 millirem per hour, the dose rate at two feet is reduced to .25 millirem (i.e., 1/(2 squared) = 1/4).

Shielding
By utilizing appropriate shielding, radiation exposure can be reduced significantly. In general, 1 centimeter of Lucite (an acrylic/plastic material) is adequate to shield P-32 and 1 millimeter of lead is adequate to shield I-125. Due to the low energy and low penetrability of H-3, C-14, and S-35, no shielding is necessary when working with these isotopes.

The Radiation Safety Manual has a more detailed discussion on radiation shielding and the interaction of radiation and matter.