## 3 Results and Discussion
### Adipose Tissue  
* Photon energy effect: Figure 2 (left side) shows the variation of BUF with incident photon energy for adipose tissue at penetration depths of 0.5, 5, 15, 25, and 40 mfp. The values of the BUF increase up to a maximum value at intermediate energies, then begin to decrease, as seen in Figure 2. The photoelectric effect is the major photon interaction process in the low energy zone, with a cross section that varies inversely with energy as \(E^{3}\). As a result of this process' dominance, the largest number of photons will be absorbed, lowering the BUF value in the lower energy zone. Moreover, the atomic cross section of Compton scattering decreases with increasing \(E\). Compton scattering is a dominant photon interaction activity in the intermediate energy band, however it merely participate in the reduction of photon energy owing to scattering and does not totally remove the photon. As a result, the life period of a photon is longer in this energy zone, and the probability of a photon escaping is likewise higher. The value of BUF rises as a result of this process. Due to multiple scattering  
Figure 1: Comparison of calculated and standard buildup factors (**(left)** EABF and **(right)** EBF) of water medium as a function of photon energy at different depths of: 0.5, 5, 15, 25, and 40 mfp. effect for significant penetration depths, EBF values became quite high for the highest penetration depth of 40 mfp. BUF values reached a maximum of roughly \(4\times 10^{4}\) at deep penetration values. * Penetration depth effect: The variation of BUF has been studied with penetration depth at nine photon energies, 15, 20, 30, 40, 50, 60, 80, 100, and 150 keV, and is shown in Figure 2 (right side). In general, it has been observed that the BUF increases with increasing the penetration depth. In the low energy region, the increasing rate in the BUF is very slow and becomes more fast with increasing energy. Therefore, we confirm the previous interpretation of reaching a maximum at about 100 keV followed by slow decrease for greater energies. * Chemical composition effect: The considered adipose tissue and its equivalent material candidates chemical compositions differs; this also play a crucial role in the magnitude of their buildup factors. Figure 3 and Table 2 show the variation of buildup factors at different energies (30, 80, and 150 keV) and selected penetration depths (0.5, 5, 15, 25, and 40 mfp) for the studied materials. The buildup factors magnitude and its dependency on \(Z_{eq}\) vary according to the photon energy region. For the current studied energy range, we confirm the theoretical hypothesis of the inverse proportionality of BUF to \(Z_{eq}\) of the materials. From Figure 3, we have EABF(AD3) > EABF(Adipose) > EABF(AD1) > EABF(AD2), as we have delta(%) = 100
* EABF(Material)/EABF(Adipose)), which confirms tabulated data \(Z_{eq}\)(AD3) < \(Z_{eq}\)(Adipose) < \(Z_{eq}\)(AD1) < \(Z_{eq}\)(AD2). * TEMs study: From Figure 3 and Table 4, we can see the close behave of AD1 to adipose tissue in terms of BUF and CT number at 30 kVp. However, the candidate AD2 presented a better closeness when looking into the effective atomic number (\(Z_{eff}\) & \(Z_{eff,abs}\)) and CT number at 80 and 120 kVp. Besides, the usual largest deviation of AD3 to adipose tissue was seen for all parameters. Thus, we can conclude that AD1 can be an acceptable TEM to adipose (at least for mammography imaging). Figure 2: Energy absorption buildup factor for adipose tissue as a function of: (**left** side) photon energy for fixed depths between 0.5 and 40 mfp and (**right** side) depth for fixed photon energy between 15 and 150 keV. Figure 3: Relative deviation of energy absorption buildup factor of substitute materials to adipose tissue as a function of: (**left** side) photon energy for fixed depths of 0.5, 15 and 40 mfp and (**right** side) depth for fixed photon energy of 30, 80 and 150 keV.