Module 5: Basic Concepts in Radiation Physics - Exposure

EXPOSURE

"Exposure" may have at least two meanings:

• In physics, "exposure" relates to the amount of ionization produced by x-rays or gamma rays in air under standard conditions.
• In the industrial situation, "exposure" means being in the presence of ionizing radiation.

Radiation Dose:

The radiation dose refers to the absorption of radiation energy per unit mass of absorber.

• The conventional dose unit is the rad, which is defined as the absorption of 0.01 J per kg of tissue mass.
• The SI unit is the gray (Gy). 1 Gy = 100 rad, or the absorption of 1 J per kg tissue mass.

Dose Equivalent:

• The conventional unit of dose equivalent is the rem. One rem of any kind of radiation produces the same biological effect as 1 rad of 200 keV X-rays.
• The SI unit is the sievert (Sv), where 1 Sv = 100 rem.

Annual Regulatory Limits:

Regulations dictate the limits to which various individuals may be exposed to ionizing radiation. These are:

• Members of the public: 100 mrem
• Occupational limits:
  • Total effective dose equivalent: 5 rem.
  • 5 year limit: 10 rem.
  • Annual administrative limit: 2 rem.

Contamination:

Contamination is simply the presence of radioactive material where it is not wanted. Persons may be contaminated either externally, internally or both. Exposure does not necessarily imply contamination.

ALARA:

ALARA ("as low as reasonably achievable") means making every reasonable effort to maintain exposure to ionizing radiation as far below the regulatory dose limits as practical, taking into account economic, societal and other relevant considerations.

Reduction of External Dose:

This is achieved by:

• minimizing the time spent near the radiation source;
• maximizing the distance away from the source;
• making use of available shielding;
• minimizing the quantity of radioactive materials handled.

Reduction of Internal Dose:

This is achieved by:

• minimizing and controlling contamination;
• using proper protective clothing;
• not eating, drinking, or smoking in contamination areas;
• checking yourself for contamination prior to leaving a potentially contaminated area.