# Towards a Counting Point Detector for Nanosecond Coherent X-ray Science
## 1 Introduction  
Synchrotron sources have become an indispensable tool for probing complex processes in versatile choices of condensed matter and chemical and biological materials. During the past two decades, aided by highly collimated X-ray beams available at 3 rd-generation light sources, notable breakthroughs have been made in measurement techniques that exploit a large transversely coherent flux, such as X-ray photon correlation spectroscopy (XPCS), coherent diffraction imaging [1; 2; 3], diffraction microscopy [4], and ptychography [5]. The pulsed nature of the X-ray sources has also enabled time-domain science to understand the structures and dynamics of transient systems that had been previously considered inaccessible [6; 7]. For instance, recent studies reported measuring coherent X-ray diffraction patterns (speckles) from atomic-scale ordering in amorphous systems, such as liquid and glasses [8; 9; 10], which opens up possibilities for measuring dynamics at the relevant length scales [11; 12; 13; 14; 15]. In time-domain methods such as XPCS, the intensity fluctuations of these speckles reflect the change in the sample dynamics, of which characteristic time scales can be extracted from intensity-to-intensity time correlation functions. However, typical detectors that are used for such measurements carry an intrinsic frame rate of kHz, which defines the experimental time resolution limit. Therefore, the time-correlation experiments today are mostly limited to studying model systems with slow dynamics. Ever-increasing demands to measure smaller and faster features in fundamental science and industrial applications have instigated the development of high brightness and transverse coherence of the beam beyond what is available at current storage-ring based sources. The recent emergence of low-emittance storage ring light sources based on a multi-bend achromat lattice, known as fourth-generation storage rings (4 GSR), deliver orders of magnitude more brilliant X-ray radiation than that previously available from 3 rd-generation light sources [16; 17; 18; 19]. The high spatial coherence of the beam promises the possibility toprobe fast microscopic dynamics via X-ray photon correlation spectroscopy. However, such light sources are expected to employ a very dense-fill pattern (electron pulse structure) to reduce the charge density per electron pulse. As a result, they are expected to operate at very high repetition rates of hundreds of MHz, which allows only a few nanoseconds between subsequent X-ray pulses that inevitably carry much fewer X-ray photons per pulse as compared to 3 rd-generation light sources. Ultimately, the successful implementation of time-resolved measurements such as XPCS or stroboscopic (pump-probe) diffraction at next-generation light sources requires both the sufficiently fast detection of individual X-ray pulses and the correct analysis of registered X-ray photons [20,21]. Previously, such a task has been enabled by using state-of-art mechanical choppers or the high-speed electronic gating of one-dimensional [22,23] or two-dimensional detectors [24,25]. Here, the use of mechanical choppers is not a viable option as they only operate at the limited opening times of hundreds of nanoseconds [26,27]. On the other hand, the use of electronic gating requires a detection scheme that is capable of recovering its active state before the arrival of the subsequent X-ray pulses. These recoveries typically take several tens of nanoseconds and are even slower for hard X-rays due to the long X-ray penetration depth into the sensors. Therefore, time-resolved measurements using dense fill patterns would be extremely difficult to implement, if not impossible, using current detector technology. A potential solution for these challenges is collecting X-rays via single photon counting with appropriate dead-time corrections. Fundamental excitation and subsequent relaxation processes in condensed matter systems evolve at very fast speeds.