One useful method is to compare the amount of energy a particle yields when it collides with absorbing matter. Some particles yield most of their kinetic energy over short distances; other deplete their energy gradually over longer distances.

For each type of particle one can calculate a quantity called linear energy transfer or LET which is defined as the total energy dissipated divided by the path length required for the particle to come to rest.

1. Using the table Data Sheet of Particle Radiation, calculate the LET for a typical radioactive alpha particle in air.

2. Calculate the LET for a typical radioactive decay electron (beta particle) in air.

3. A primary solar cosmic ray proton travels 1km in the atmosphere. If its kinetic energy is 2GeV, what is its LET value in air?

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   Of course the linear energy transfer depends upon the chemical composition of the absorbing material and the density of the material. In the examples above, the density of air at sea level differs significantly from its density at high altitudes.

   For comparative purposes water is often chosen as the absorbing medium by which the LET of various types or radiation can be compared.

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4. A 600eV electron gives up its energy in water within a distance of 1.09x10^-7m. Calculate its LET rate.
5. A 800keV proton gives up its energy in water within a distance of 1.77x10^-5m. Calculate its LET rate.
6. A 5.3MeV alpha particle gives up its energy in water within a distance of 4.82x10^-4m. Calculate its LET rate.
7. A 100 MeV fission fragment gives up its energy in water within a distance of 1.11x10^-5m. Calculate its LET rate.

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   The nature of the radiation makes a considerable difference in the biological effects it creates even when the same amount of energy is absorbed. In general, the larger the value of the LET, the more biologically reactive the radiation.

   Of course the LET is only the average kinetic energy given up to the absorbing material over the particle's path and it tells little about the effects the added energy has on the absorbing material.

   The absorbed energy will usually cause ionizations to occur through particle-particle collisions when the energy of the incident particle exceeds the ionizing potential of the atoms it encounters.

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8. If the ionizing potential of a hydrogen atom is 10.6eV, how may ionizations will be created by an absorbed alpha particle if its incident kinetic energy of 100MeV is totally absorbed?

9. A general rule of thumb used by physicists is that about 30eV is needed for each ionization in liquid water whereby H₂O is broken into H⁺ and OH^-. How many ionizations are created in water by a single 800keV proton?

10. How many ionizations are created in water by a single 2GeV proton?