is when an alpha or beta-particle passes close to atom, disrupting the electrostatic forces causing an electron to be freed.
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Directly ionizing radiation
are alpha- and beta- particles. They will ionize atoms until they run out of energy. However they have limited energy and do not penetrate far.
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Indirectly ionizing radiation
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are gamma- and X- rays.
They do not interact with the body until they happen to run into particles. This allows them to penetrate far into the body. They can free an energic electron or other particles, therefore creating directly ionizing particles inside the absorber.
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Excitation
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is when radiation particles do not have enough energy to ionize, instead the electrons of the absorber will just gain energy. The atom will release electromagnetic radiation to go back to ground state.