1. Radiation Protection and Safety 3/12/2016 1 Interactions with Electrons   Energy-loss Mechanisms Electrons and positrons have similar stopping powers and ranges Positrons create gamma rays by annihilation Can excite or ionize atoms Bremstrahlung xrays

2. Radiation Protection and Safety 3/12/2016 2 Interactions with Electrons   Collisional Stopping Power Betas can lose a large fraction of their energy in a single collision an atomic electron (same mass). Quantum mechanics imposes certain requirements on the equations describing collisions with atomic electrons. Separate stopping power equations exist for electrons and positrons. Elastic scattering is dominant at lowest energies (<100eV) At ~200 eV, ionization is comparable to scattering Many secondary electrons are produced

3. Radiation Protection and Safety 3/12/2016 3 Interactions with Electrons   Collisional Stopping Power

4. Radiation Protection and Safety 3/12/2016 4 Interactions with Electrons   Radiative Stopping Power Passing close to nucleus – Coulomb forces Passing far from nucleus – electron screening + Coulomb X ray losses are much greater in high-Z materials and is the dominant mechanism of energy loss by high energy betas

5. Radiation Protection and Safety 3/12/2016 5 Interactions with Electrons   Radiation Yield The average fraction of an electron’s energy radiated as bremsstrahlung in slowing down A larger fraction as the electron energy increases To limit bremsstrahlung emission, use a low-Z material to reduce beta particles range and then a high-Z material to absorb any xrays

6. Radiation Protection and Safety 3/12/2016 6 Interactions with Electrons

7. Radiation Protection and Safety 3/12/2016 7 Interactions with Electrons