Energy dependence of electron beam penetration, area throughput rates and electron energy utilization in the low-energy region

Industrial electron accelerators used in radiation processing facilities are rated by their maximum electron energies, which are usually classified in three energy intervals. Low energies are from 75 keV to 300 keV, medium energies are from 300 keV to 5 MeV, and high energies are above 5 MeV. The incident electron energy is determined by the depth and density of the irradiated material, while the electron beam current is determined by the area throughput rate and the absorbed dose. The ITS3 Monte Carlo code has been used to calculate the energy deposition per electron versus depth in polyethylene plates with incident electron energies from 75 keV to 600 keV. This quantity is proportional to the absorbed dose delivered during a continuous irradiation process. The energy depositions in the thin titanium beam window and the air space between this window and the irradiated material are included in these calculations. The beam window and air space have significant effects on the depth–dose distribution within the processed material in the low-energy region. The first group of calculations assumes a window thickness of 40 μm (micro-meter) and an air space of 15 cm with energies extending from 200 keV to 600 keV. The second group assumes a window thickness of 15 μm and an air space of 5.0 cm with energies extending from 100 keV to 300 keV. The third group assumes a window thickness of 6.0 μm and an air space of 2.5 cm with energies extending from 75 keV to 250 keV. The data presented include the energy deposition at the surface of the polyethylene plate, the depth where the exit dose equals half of the entrance dose, and the total energy absorbed in the polyethylene plate as a percentage of the incident electron beam energy. Additional calculations show the effects of changing the thickness of the air space in the limited low-energy region from 75 keV to 125 keV.