**3.1 Electron beam energy**

One of the key factors that lead to the non-uniformity of electron beam irradiation is the character of depth dose distribution, which depends on the energy of electrons. **Figure 1a** and **b** shows the dependency of the relative absorbed dose on the penetration depth in water parallelepiped irradiated with 4 MeV, 6 MeV, 8 MeV, and 10 MeV, calculated using the GEANT4 toolkit [44]. **Figure 1a** shows that the higher the electron energy, the deeper the penetration of electrons into the object is and the lower maximum dose is achieved during irradiation.

To illustrate the dependency between the electron energy and depth dose distribution, let us introduce the following parameters of the absorbed depth dose distribution:

*Lmax* is the distance from the surface of the object with the maximum absorbed dose;

**Figure 1.**

*(a) Depth dose distribution in water parallelepiped irradiated with 4 MeV, 6 MeV, 8 MeV, and 10 MeV electrons; (b) Parameters of absorbed dose distribution in water irradiated with 10 MeV electrons.*

*Lopt* is the optimal distance from the surface of the object at which the dose value is equal to the surface dose;

*<sup>K</sup>* <sup>¼</sup> *Dmin Dmax* is the dose uniformity coefficient, which is the ratio of the minimum value of absorbed dose *Dmin* to the maximum absorbed dose *Dmax* in the object.

As can be seen from **Figure 1b** when the water parallelepiped is irradiated with 10 MeV electrons, *Lmax* is 28.0 mm and *Lopt* is 38.75 mm, and the irradiation uniformity coefficient is 0.72. With an increase in electron energy from 4 MeV to 10 MeV, *Lmax* increases from 10.25 mm to 27.5 mm, and *Lopt* increases from 15 mm to 38.75 mm (**Figure 1a**). Therefore, varying the beam energy allows to change the dose uniformity of the irradiated object.
