Excitation - Occurs when radiation excites an electron from an occupied orbital into an empty, higher energy orbital.

Ionization - Occurs when radiation carries enough energy to completely remove an electron from an atom or molecule.

Ionizing Radiation - Radiation that carries more than 1216 kJ/mol of energy.

e.g.  UV_B rays (higher end of the UV spectrum), x-rays, gamma rays, cosmic rays.

Nonionizing Radiation - Radiation that carries less than 1216 kJ/mol of energy.

e.g.  radiowaves, microwaves, infrared, visible light, UV_A rays (lower end of the UV spectrum).

• 1216 kJ/mol is the amount of energy required to remove an electron from a water molecule.  
  • The division between ionizing and nonionizing radiation is split above and below 1216 kJ/mol because living tissue is mostly made up of water (70-90% by weight)

How Water Becomes Ionized

• When water is ionized and an electron is lost, H₂O⁺ ions are produced:

• H₂O⁺  is an example of a free radical.
  • Free Radical - Extremely reactive particle that contains an unpaired valence-shell electron.
• H₂O⁺  is so reactive (very strong oxidizing agent) that it can remove electrons or hydrogen atoms from molecules in the cell.

• Much larger amounts of nonionizing radiation would be required to equal the strength of a very small amount of ionizing radiation.

Three ways to measure ionizing radiation

1. Measure the activity of the source in curies (disintegrations/sec) - EASIEST
2. Measure the radiation to which an object is exposed in roentgens by measuring the amount of radiation produced when this radiation passes through a sample of air.
3. Measure the radiation absorbed by the object in radiation absorbed doses (rads) - HARDEST but most useful.
   • rad = absorption of 10^-5 J of energy per gram of body weight
     • Cells become inactivated at 100 rads.
     • Fatal level is 400-450 rads.
   • Radiation biological effectiveness (RBE) - corrects for the rate at which radiation passes through the tissue.
   • Roentgen equivalent man (rem) - The product of the absorbed does (rads) and the biological effectiveness (RBE).

     

Next:  "Natural Versus Induced Radiation"