## 5 The SPB/SFX Instrument
### Requirements  
These experiments require X-ray instrumentation in a traditional forward scattering geometry to collect diffraction at angles up to those commensurate with atomic resolution. Crystallography requires photon energies up to about 16 keV--beyond the Selenium edge--to aid in anomalous diffraction measurements for structure determination. On the low energy side, single particle imaging, which deals with typically very low diffraction signals, requires as low a photon energy that permits the desired resolution for the system under study. Furthermore, mitigation of radiation damage requires optimization of the beam power, that is, to carefully trade highest pulse energy versus shortest pulse duration.  
X-ray FEL serial crystallography and imaging experiments are primarily performed in a mode that is destructive to the sample. The goal is to illuminate the specimen with as many X-ray photons per pulse as possible, to maximize the scattered signal from each particle. To do so, one must have an optical system that is highly transmissive, as well as focusing to a spot size that is comparable to the size of the sample(s) under investigation. The \"small\" crystals used in serial crystallography at XFELs tend to be around 1 \(\upmu\)m diameter in size, with some variation larger or smaller. Relevant biological single particles range from biomolecules some tens of nanometers across to large viruses up to 500 nm in diameter. To accommodate this wide range of sample sizes, the SPB/SFX instrument plans to deliver two different focal spots--a 1 \(\upmu\)m-scale focus and a 100 nm-scale focus. Coherent diffraction imaging of individual particles further requires precise knowledge of the wavefront of the incident X-ray pulse leading to stringent requirements on the selection of the X-ray optical components and their performance.  
Of particular importance for the experiments to be performed at SPB/SFX will be the performance of the large area detectors. The two primary requirements are a very high dynamic range and single photon sensitivity. An ideal 2D detector for serial crystallography should have a high dynamic range much higher than the four [31] or five [32] orders of magnitude presently achievable, as the intensities of individual Bragg peaks can vary enormously and these intensities must be determined very accurately for successful phasing and structure determination (though nevertheless a detector with 10\({}^{4}\) or smaller dynamic range can be successfully used). Single photon sensitivity is important for detecting weaker scattering, such as from single non-crystalline particles or weak Bragg peaks. The detectors should be compatible with detection at the 4.5 MHz intra-train repetition rate to ensure collection of as much images as possible within a meaningful time frame during which particles can be injected. This requirement leads to the further need for stringent data reduction techniques to avoid a data deluge. Finally the detector(s)' mechanical design must be compatible with the instrument. This means a pixel size that is not too large (\(\leq\)200 \(\upmu\)m), a number of pixels commensurate with the number of resolution elements desired in any given structure (i.e., \(\geq\)1 MPixel for \(\sim\)200 linear resolution elements) and an operation that is ideally compatible with the sample environment (for the upstream interaction region at SPB/SFX this means in vacuum operation). The detector is required to operate in vacuum and be placed as close as 129 mm from the upstream interaction region, resulting in a better than 2 A geometrical resolution limit for 9 keV photon energy. It can also be placed downstream as far as 6 m from the interaction region, allowing for appropriate sampling of diffraction data from samples as large as almost 1 \(\upmu\)m at the lowest energies.