# Optimization of Transmission X-ray Target for Intense Pulsed Electron Beam Accelerators
## 3 Results and Discussion

Table 2: The thickness of the molybdenum-titanium target (in \(\upmu\)m) with mass densities of 10.22 and 4.54 g/cm\({}^{3}\) correspondingly. Figure 8: Transmission X-ray spectrum per IPEB primary electron on (**a**) tungsten and (**b**) molybdenum target with the thickness of CSDA range. stronger photon attenuation existed than with a thin conversion layer target with 50% less in the total photon counts, especially in low energy photons. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **a** & **b** & **c** & **d** & **e** \\ \hline W + Ti & 1.987 & 2.112 & 2.491 & 2.926 & 3.240 \\ Mo + Ti & 1.560 & 1.670 & 1.749 & 1.924 & 1.984 \\ \hline \end{tabular}
\end{table} 
Table 4: Dose equivalent after the dual-layer target per pulse (in Sv) with target parameters in Tables 1 and 2. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **a** & **b** & **c** & **d** & **e** \\ \hline W + Ti & 2.023 & 2.170 & 2.663 & 3.298 & 3.727 \\ Mo + Ti & 1.653 & 1.741 & 1.957 & 2.259 & 2.367 \\ \hline \end{tabular}
\end{table} 
Table 3: Photo fluence after the dual-layer target per pulse (in \(10^{12}\) photons/cm\({}^{2}\)) with target parameters in Tables 1 and 2. Figure 9: Spatial distribution of photon fluence by IPEB on tungsten-titanium dual-layer target with thicknesses combination (subfigure (**a–e**) corresponding to the duplicate item titles in Table 1). Figure 11: Dose equivalent distribution per IPEB pulse on dual-layer targets with parameters of Table 3.  
Figure 10: Spatial distribution of dose equivalent by IPEB on tungsten-titanium dual-layer target with thicknesses combination (subfigure **(a–e)** corresponding to the duplicate item titles in Table 1). By the above analysis, the combination of a thin conversion target at optimal photon emission thickness and a thick titanium window is an optimized choice for maximum X-ray output. When considering more issues, such as target heating by IPEBs, some problems in the actual design need to be solved. The optimal conversion target is relatively thin under the thermal shocks by IPEBs. It is verified by experimental and numerical means that under the irradiation of IPEBs, a temperature rise of several tens of K [26] can be induced in the target and the consequent thermal stress is an important cause of target damage. The situation of conversion layer damage can be monitored by online monitoring, such as with a semiconductor X-ray detector [46], and in tests with a titanium electron window, a window with a thickness within 100 \(\upmu\)m can well secure the vacuum in the vacuum chamber [23], ensuring that, with the dual-layer target, the working vacuum can be kept even after conversion layer damage. However, frequent maintenance of the window system should undoubtedly be avoided in practical applications. In this sense, the use of a thicker conversion layer may be better for higher system reliability in long-term operation. Further, with a thicker target, such as with 80 \(\upmu\)m tungsten and 137 \(\upmu\)m molybdenum, the energy deposition in the titanium window (Figure 13 and Figure S7 in Supplementary Materials) is over two orders lower than in the conversion layer and the titanium window heating by IPEBs is negligible, while with a thin conversion layer at the optimized conversion layer thicknesses, the titanium window may endure heating at high IPEB output repetition and extra cooling may be required. Figure 12: Transmission photons spectrum by per IPEB electron on tungsten-titanium and molybdenum-titanium dual-layer targets with parameters in (**a**) Table 1 and (**b**) Table 2.