**3.2 Density of the irradiated object**

Considering that biological objects vary in terms of density and density distribution, it is important to investigate the impact of the object density on the parameters of the absorbed depth dose distribution, such as the optimal distance *Lopt* and dose uniformity *K*. While the irradiation of biological objects is performed with 4–10 MeV electrons, it is feasible to estimate the depth dose distribution coefficients for the objects irradiated within this energy range having the density from 0.3 g/cm<sup>3</sup> to 1.6 g/cm<sup>3</sup> , which is similar to that of biological objects at industrial irradiation facilities. **Figure 2a** shows that dose uniformity *K* varies from 0.62 to 0.72 and practically does not depend on the density of the irradiated object for the water parallelepipeds with densities ranging from 0.3 g/cm<sup>3</sup> to 1.6 g/cm<sup>3</sup> .

**Figure 2b** shows the dependency of optimal distance *Lopt* on the electron energy for irradiation of parallelepipeds with densities of 0.3 g/cm<sup>3</sup> , 0.6 g/cm<sup>3</sup> , 1.0 g/cm<sup>3</sup> , and 1.6 g/cm<sup>3</sup> . As it can be seen, the higher the energy of electrons, the greater the value of *Lopt*, which means that at higher energies a greater dose uniformity can be achieved for the objects of greater thickness. At the same time, the lower the density of the irradiated object, the greater the growth rate of *Lopt* value with an increase in electron energy.

According to the computer simulation using the GEANT4 toolkit and MATLAB, the analytical interdependencies of electron energy, dose uniformity *K*, optimal distance *Lopt,* and the object density can be expressed as follows [45]:

*Electron Beam Processing of Biological Objects and Materials DOI: http://dx.doi.org/10.5772/intechopen.112699*

$$L\_{opt}[\text{cm}] = 4\left[\frac{\text{cm}^4}{\text{MeV} \ast \text{g}}\right] \times \rho^{-0.96} \times E[\text{MeV}] - \text{1.59}\left[\frac{\text{cm}^4}{\text{g}}\right] \times \rho^{-0.46},\tag{1}$$

$$K = 0.01 \left[ \text{MeV}^{-1} \right] \times E \left[ \text{MeV} \right] + \text{0.57},\tag{2}$$

where ρ is the object density <sup>g</sup> cm3 , *E* is electron energy. These dependencies were obtained with a maximum interpolation error of no more than 2%.
