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
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
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
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).
Dose equivalent = Absorbed dose x QF
1 Sievert = 100 rem
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
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