Precautions-External Exposure

 

External Radiation -- 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," external radiation exposure (dose) is directly related to the duration of exposure. By reducing the time you are in a radiation field, you will correspondingly reduce your dose.
  • DISTANCE--By maintaining as much distance as possible between you and a source of radiation, you can reduce your dose significantly. The relationship between radiation exposure and distance follows the inverse square law. For example, 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 (1/(2 squared) = 1/4).
  • SHIELDING--By utilizing appropriate shielding, radiation exposure can be reduced significantly. Appropriate shielding depends on the type of radiation:
    • S-35, C-14, P-33 and H-3 have a weak beta, and require no shielding.
    • P-32 has a strong beta, and requires plexiglas shielding. Thin lead sheets would be inappropriate for P-32 because interaction of the high energy beta with high atomic number material (e.g. lead) will result in secondary Bremsstrahlung (x-ray) radiation.
    • I-125 emits x-ray radiation and requires heavy shielding (high atomic number), such as lead or concrete blocks.