Radiation dose and units Exposure• ability to produce ionization in air under standard temperature and pressure • SI unit Coulombs/kg in air (old unit: Roentgen , 1 R = 2.58 x 10-4 C/kg air) cannot be used to describe dose to tissue Absorbed dose• The amount of energy absorbed per mass -energy absorbed per unit mass of tissue • SI units Gray 1 Gy = 1 J/kg (old unit rad, 100 rad = 1 Gy) ➢ Free radicals of biomolecules can be restituted by hydrogen donating compounds, such as thiols and cysteine = repair ➢ Alternatively, they can be fixed by reaction with oxygen or oxygen mimicking compounds = permanent damage Water radiolysis depends on the pH and LET of the radiation 7 8

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