# Optimization of Transmission X-ray Target for Intense Pulsed Electron Beam Accelerators
## 3 Results and Discussion  
In the simulation with tungsten and molybdenum targets, to compare the photon emission from the target, the photon fluence and dose equivalent in the center of the rear target surface were taken for analysis (Figure 4). Taking tungsten as the target material, the peak photon fluence and dose equivalent increased by over 70% compared with molybdenum. A similar trend in photon fluence and dose equivalent was exhibited with the change in the target thickness; at thin target thicknesses, the photon fluence and dose equivalent was low and rose significantly with the increase in the target thickness. In this stage, the conversion of X-ray was predominantly limited by the insufficient electron stopping in the target. With further increased target thicknesses, the photon fluence reached a maximum value and the thickness at this maximum photon emission, about 35 \(\upmu\)m for tungsten and 72 \(\upmu\)m for molybdenum, was much smaller than the CSDA range of the electrons in the target (approximately 22.6% and 28.2% of the CSDA range in tungsten and molybdenum). When the target thickness exceeded this optimal value, the output photon fluence and dose  
Figure 3: Schematic drawing of the dual-layer target model. equivalent started to decrease, but in a slow trend as there existed both X-ray emission due to the stopping of electrons in the target and X-ray absorption by the extra thickness which exceeded the optimal thicknesses. When the target thickness reached the CSDA range, the photon fluences and dose equivalent decreased by over 50% in tungsten and about 40% for molybdenum compared with the fluence at optimal target thicknesses. This means that a sacrifice in the X-ray output is made for complete electron stopping, and with a higher target atomic number comes higher output loss. This can be explained as when electrons reach the depth over the optimal thickness, a significant portion of their energy has already been deposited, as the stopping power of sub-MeV electrons reaches the maximum value soon after entering the target. At the thick region of the target, the photon emission is generally weaker than the photon absorption. In the spatial distribution of photon fluence and dose equivalent in the roz plane (Figures 5 and 6 and Figures S1 and S2 in Supplementary Materials), in addition to the photon generation by molybdenum being weaker than with tungsten, the photon distribution and its evolution trend versus the target thickness was similar on the two target materials. At a thin target thickness, the photon emission in the transmission direction was much stronger than in the reflection direction. The photon emission increased drastically with increased target thicknesses, both in the transmission and reflection directions. At the optimal thickness, the photon emission distribution in the two directions was quite close, slightly stronger in the transmission direction. With further increased target thicknesses, the photon emission in the reflection direction did not change significantly, but the transmission  
Figure 4: (**a**) Transmission photon fluence and (**b**) dose equivalent per IPEB pulse with energy spectrum in Figure 2 from tungsten and molybdenum conversion targets versus the target thickness. photons exhibited apparent attenuation due to absorption by the target. The photon fluence had a higher value at a smaller polar angle and decreased sharply approaching the xoy plane. This was induced by the angular distribution of photon emission and higher photon absorption by larger mass thickness in the polar angle near 90\({}^{\circ}\). Figure 5: Spatial distribution of photon fluence by per IPEB pulse on tungsten target with thickness of (**a**) 5 \(\upmu\)m, (**b**) 35 \(\upmu\)m (the optimal target thickness), (**c**) 70 \(\upmu\)m (2 times of the optimal target thickness), (**d**) 155 \(\upmu\)m (CSDA range).