Ionization is the process of removing one or more electrons from a neutral atom. This results in the loss of units of negative charge by the affected atom. The atom becomes electrically positive (a positive ion). The products of a single ionizing event are called an electron-ion pair.
Specific Ionization
Specific ionization is that number of ion pairs produced per centimeter of travel through matter. Equation 6-1 expresses this relationship.
Specific ionization is dependent on the mass, charge, energy of the particle, and the electron density of matter. The greater the mass of a particle, the more interactions it produces in a given distance. A larger number of interactions results in the production of more ion pairs and a higher specific ionization.
A particle’s charge has the greatest effect on specific ionization . A higher charge increases the number of interactions which occur in a given distance. Increasing the number of interactions produces more ion pairs, therefore increasing the specific ionization.
As the energy of a particle decreases, it produces more ion pairs for the same amount of distance traveled. Think of the particle as a magnet. As a magnet is passed over a pile of paper clips, the magnet attracts the clips. Maintain the same distance from the pile and vary the speed of the magnet. Notice that the slower the magnet is passed over the pile of paper clips, the more clips become attached to the magnet. The same is true of a particle passing by a group of atoms at a given distance. The slower a particle travels, the more atoms it affects. Stopping Power Stopping power or linear energy transfer (LET) is the energy lost per unit path length. Equation 6-2 expresses this relationship.
where
Specific ionization times the energy per ion pair yields the stopping power (LET), as shown in Equation 6-3.
Stopping power, or LET, is proportional to the specific ionization.
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