Measuring Radiation Intensity

Exposure is a quantity describing how much ionization is produced (i.e., how many electrons are liberated) in air by gamma- or x-rays.  The unit of exposure is the Roentgen or Coulombs/kg (in the SI system).

  • 1 Roentgen = 2.58 x 10-4 Coulombs of charge produced per kilogram of air.

The quantity, absorbed dose, describes how much energy is deposited in a material by a beam of radiation and is not restricted to x-rays or gamma rays passing through air.  The unit of absorbed dose is the rad or Gray (in the SI system).

  • 1 rad = 100 ergs of energy deposited in one gram of material
  • 1 Gray = 100 rad

Different types of radiation may deposit the same amount of absorbed dose but produce different effects and different levels of damage.  For instance, charged massive alpha particles will interact more intensely and deposit energy over a shorter distance within a cell than uncharged massless gamma rays and will be more effective at producing biological damage within that cell, even though equivalent amounts of energy are deposited overall.

The quantity, dose equivalent, is derived by multiplying the absorbed dose by a quality factor (QF) which depends on the type of radiation being measured.  The unit of dose equivalent is the rem or Sievert (in the SI system).

  1. Dose equivalent = Absorbed dose x QF
    • QF = 1 for gamma rays, x-rays and most beta particles
    • QF = 2 -11 for neutrons, depending on energy
    • QF = 20 for alpha emitters under conditions of internal exposure
  2. 1 Sievert = 100 rem

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