# From Femtoseconds to Hours--Measuring Dynamics over 18 Orders of Magnitude with Coherent X-rays
## 3 Experimental
### Experimental Parameters for XPCS Experiments  
Following the estimations from the signal-to-noise ratio, an optimum is achieved for matching sizes of speckles and detector pixels. Typically, pixel sizes of hard X-ray detectors are in the range of 10 \(\upmu\)m to several 100 \(\upmu\)m (see Table 1). Thus, using a speckle size of \(s\approx\) 100 \(\upmu\)m and a wave length of \(\lambda\sim\) 0.1 nm we can estimate from Equation (18) a relation between beam size \(w\) and sample detector distance \(d_{\text{sd}}\) to detect speckles: \(d_{\text{sd}}\sim 10^{6}w\). Consequently, XPCS beamlines need a long sample-detector distance of several meters and small beam sizes in the \(\upmu\)m range. This is further supported by the coherence properties discussed in Figure 6. In particular, as the transverse coherence lengths is as well in the range of several 10 to 100 \(\upmu\)m, the beam size has to be limited to these dimensions. Slits or pinhole are frequently used to the reduce the beam size to about 10 \(\upmu\)m. With the development of different focussing schemes, compound-refractive or kinoform lenses have become a standard equipment at XPCS beamlines [96; 97]. Together with a smaller source size, larger coherence lengths are achieved that enable coherent fluxes of above \(10^{11}\) photons per second. Especially for experiments at \(q\) values probing molecular length scales the beam size is typically reduced to few \(\upmu\)m. At the same time, the sample-detector distance is chosen to several meters. A standard configuration is \(d_{\text{sd}}\approx 5\) m. Such a configuration is used at XPCS instruments at beamlines 8-ID (APS) [25; 98], ID10 (ESRF), P10 (PETRA III), 11-ID (NSLS-II) [99], XCS at LCLS [26], as well as MID (European XFEL) [27]. All of those instruments enable as well WAXS studies. In addition, a growing number of XPCS beamlines offer USAXS options, allowing for larger beamsizes that are  
\begin{table}
\begin{tabular}{l l l l l l} \hline
**Detector** & **Size (Pixels)** & **p (\(\upmu\)m)** & **f (kHz)** & **Beamlines** & **References** \\ \hline PI PI PIXIS & 1300 \(\times\) 1340 & 20 & 0.001 & P10, XCS & \\ PILATUS 2M a & 1475 \(\times\) 1679 & 172 & 0.01 / 0.1 & cSAXS (SLS) & [78] \\ Eiger 4M a & 2167 \(\times\) 2070 & 75 & 0.75 & P10, CHX & [83; 84] \\ Eiger 500k & 1030 \(\times\) 514 & 75 & 9 / 22 & ID10, P10, ID02 & [83; 84; 85] \\ Lambda 750k a & 1536 \(\times\) 512 & 55 & 1–23 & P10, 8-ID & [86] \\ Maxipix 2 \(\times\) 2 a & 512 \(\times\) 512 & 55 & 0.3 & ID10, P10 & [87; 88] \\ XSPA-500k & 1024 \(\times\) 512 & 76 & 56 / 1200 b & 8-ID & [81; 89; 90] \\ \hline Keck-PAD & 512 \(\times\) 512 & 150 & up to 10,000 c & & [91] \\ SPARTA & 128 \(\times\) 512 & 200 & up to 6500 c & & [92] \\ Timepix3 & 256 \(\times\) 256 & 55 & 2 d & & [82] \\ \hline AGIPD 1M & 1024 \(\times\) 1024 & 200 & 6500 & MID, SPB/SFX & [93] \\ ePix & 352 \(\times\) 384 & 50 & 1 & XCS, XPP & [94] \\ MPCCD a & 1024 \(\times\) 1024 & 50 & 0.06 & SACLA & [95] \\ \hline \end{tabular}
\end{table}
Table 1: Detectors used for XPCS experiments. Here, \(p\) is the pixel size in one dimension, and \(f\) the maximum frame rate. Note that different versions may exist, e.g., the data for the dual MPCCD and a single module of ePix are given. Different frame rates are connected to different operation modes or detector versions. Notes: a Different versions with various sizes and frame rates available. b Continuous and burst-mode operation. c In burst modes with different number of frames. d Full-frame repetition rate. Faster times can be accessed with the event-driven read-out offering a time resolution of 1.56 ns per single event.  
particularly suited for radiation-sensitive samples. Sample-detector distances from 16 m (CHX at NSLS-II) over 21 m (P10 at PETRA III [38]) up to 30 m (ID02 at ESRF [100]) are currently used.