## 3 Single-Particle Imaging with X-ray Lasers
### Detector and Data Analysis  
Detector systems are critical important to all scientific experiments and SPI is not an exception. The 2017 Chemistry Nobel Prize was awarded to Cryo-EM, a powerful technology for single-particle protein crystal structures determination [176]. The current resolution is around 3 A, comparable with the X-ray crystallography [177]. About 20 years ago, the first 3D reconstruction at the sub-nanometer resolution of icosahedra virus was around 10 A. Since then, thousands of structures have been determined by the Cryo-EM at a resolution from 30 A to 3 A [3]. A breakthrough was suddenly made around 2014. Great progress was made with the direct electron detectors and single particle analysis methods [178]. From that moment, Cryo-EM has attracted much attention from biologists and chemists. Compared with the Cryo-EM, there is a long way to go for SPI. The current 3D resolution of SPI is about 10 nm. The CXI nanofocusing data showed a post-sample aperture [179] limited resolution close to 5.9 A [180]. Figure 8 shows the calculated photons number vs resolution (Q). The integrated single hit signal is higher than the background. A real-space image could not be obtained from this data due to the lack of enough hits. While the main limitation currently is hit rate, improved detectors with lower noise and easier identification of single scattered photons from the noise will be required to achieve higher resolution imaging. The SPI detector of the future will need to combine the best of dark current, detector panel size, gain, readout noise, dynamic range and quantum efficiency, with an increased readout speed. For LCLS, the maximum pulse repetition is 120 Hz. At the increased repetition rates of the European-XFEL and LCLS-II, new detectors are needed. Millions or more diffraction patterns are needed to obtain a high-resolution image. At the current hit rates of about 1%, higher repetition rate X-ray FELs will have a profound impact. The main characteristics of detectors used or planned for SPI related experiments at LCLS, LCLS-II, European-XFEL, SACLA and SwissFEL are listed in Table 1 for reference. Figure 8: Radial average of signals using hard X-rays. Elevated photon counts from the sample are visible up to an angle commensurate with 5.9 Å resolution, this being the resolution limit set by the angular acceptance of a post-sample aperture. (reproduced with permission from [180]. Copyright Springer Nature, 2016). \begin{table}
\begin{tabular}{c c c c c c c}  & **AMO, LCLS** & **CXI, LCLS** & **TXI, LCLS-II** & **SPR/SEX, European-XFEL** & **BL 2, SACLA** & **SwissFEL** \\ \hline Detector & pnCCD & CSPAD b [FOOTNOTE:b]Footnote b: The dynamic range and ASIC noise for CSPAD under high gain mode are 350 photons@8 keV and 300 e \({}^{-}\) z.m.s. For low gain mode, they’re 270 photons@8 keV and 1000 e \({}^{-}\) z.m.s [18]. \({}^{-}\) The current version is about 480 Hz. For digital domain multiplexing, the frame rate will be 2-4 kHz. For fast ADCs, the frame rate will be 10-20 kHz [18,13,14]. \({}^{-}\) See reference [18,18] for detailed information. \({}^{-}\) Keeping the single photon detection capability for X-ray photon energy higher than 6 keV [18]. \({}^{-  
Data analysis is obviously key to the final results. As XFEL beamtime is rare and precious around the world, to optimize the experimental parameters before the formal experiment and data collection period is of great importance to guarantee the success and save beamtime. Simulations based on physical theories, actual light source and instrument specifications will be a great help. The SPB/SFX instrument of the European XFEL created a start-to-end simulation (S2 Esim) software (European-XFEL, Hamburg, Germany) [189] to help researchers figure out the optimized parameters for SPI experiments based on European XFEL accelerator and undulator data bank. Recently, a powerful program was designed for SPI to demonstrate single particle imaging under ultra-shot X-ray pulses [190]. LCLS is planning to leverage these developments from the European XFEL.