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The interactions of ionic liquids (ILs) with organic solutes are of interest to many applications involving the use of ILs as task-specific solvents, such as in sensing devices or catalysis. However, currently little is known about these specific interactions. The interactions of two structurally-similar organic analytes with different functional groups (alkyne and cyanide) in the IL [C2C1Im][MeSO4] are studied via density functional theory. By analysing the type and strength of the interactions between the organic solutes and ionic liquid using explicit solvation, the study aims to better understand of the possible noncovalent interactions between them. Implicit solvation models were also adopted to obtain solvation energies from solvent continuum. The understanding of IL-solute interactions and solubilities are critical in many applications such as improving the analytical performance of ILs-based sensors., Comment: 20 pages, 10 figures

The European XFEL is an extremely brilliant Free Electron Laser Source with a very demanding pulse structure: trains of 2700 X-Ray pulses are repeated at 10 Hz. The pulses inside the train are spaced by 220 ns and each one contains up to $10^{12}$ photons of 12.4 keV, while being $\le 100$ fs in length. AGIPD, the Adaptive Gain Integrating Pixel Detector, is a hybrid pixel detector developed by DESY, PSI, and the Universities of Bonn and Hamburg to cope with these properties. It is a fast, low noise integrating detector, with single photon sensitivity (for $\text{E}_{\gamma} \ge 6$ keV) and a large dynamic range, up to $10^4$ photons at 12.4 keV. This is achieved with a charge sensitive amplifier with 3 adaptively selected gains per pixel. 352 images can be recorded at up to 6.5 MHz and stored in the in-pixel analogue memory and read out between pulse trains. The core component of this detector is the AGIPD ASIC, which consists of $64 \times 64$ pixels of $200 {\mu}\text{m} \times 200 {\mu}\text{m}$. Control of the ASIC's image acquisition and analogue readout is via a command based interface. FPGA based electronic boards, controlling ASIC operation, image digitisation and 10 GE data transmission interface AGIPD detectors to DAQ and control systems. An AGIPD 1 Mpixel detector has been installed at the SPB experimental station in August 2017, while a second one is currently commissioned for the MID endstation. A larger (4 Mpixel) AGIPD detector and one to employ Hi-Z sensor material to efficiently register photons up to $\text{E}_{\gamma} \approx 25$ keV are currently under construction., Comment: submitted to the proceedings of the ULITIMA 2018 conference, to be published in NIM A

The Adaptive Gain Integrating Pixel Detector (AGIPD) is an x-ray imager, custom designed for the European x-ray Free-Electron Laser (XFEL). It is a fast, low noise integrating detector, with an adaptive gain amplifier per pixel. This has an equivalent noise of less than 1 keV when detecting single photons and, when switched into another gain state, a dynamic range of more than 10$^4$ photons of 12 keV. In burst mode the system is able to store 352 images while running at up to 6.5 MHz, which is compatible with the 4.5 MHz frame rate at the European XFEL. The AGIPD system was installed and commissioned in August 2017, and successfully used for the first experiments at the Single Particles, Clusters and Biomolecules (SPB) experimental station at the European XFEL since September 2017. This paper describes the principal components and performance parameters of the system., Comment: revised version after peer review

We studied the properties of chromium compensated GaAs when coupled to charge integrating ASICs as a function of detector temperature, applied bias and x-ray tube energy. The material is a photoresistor and can be biased to collect either electrons or holes by the pixel circuitry. Both are studied here. Previous studies have shown substantial hole trapping. This trapping and other sensor properties give rise to several non-ideal effects which include an extended point spread function, variations in the effective pixel size, and rate dependent offset shifts. The magnitude of these effects varies with temperature and bias, mandating good temperature uniformity in the sensor and very good temperature stabilization, as well as a carefully selected bias voltage., Comment: revised version after 2nd peer review

CdTe is increasingly being used as the x-ray sensing material in imaging pixel array detectors for x-rays, generally above 20 keV, where silicon sensors become unacceptably transparent. Unfortunately CdTe suffers from polarization, which can alter the response of the material over time and with accumulated dose. Most prior studies used long integration times or CdTe that was not of the hole-collecting Schottky type. We investigated the temporal response of hole-collecting Schottky type CdTe sensors on timescales ranging from tens of nanoseconds to several seconds. We found that the material shows signal persistence on the timescale of hundreds of milliseconds attributed to the detrapping of a shallow trap, and additional persistence on sub-microsecond timescales after polarization. The results show that this type of CdTe can be used for time resolved studies down to approximately 100 ns. However quantitative interpretation of the signal requires careful attention to bias voltages, polarization and exposure history., Comment: revised version after peer-review

Several charge integrating CMOS pixel front-ends utilizing charge removal techniques have been fabricated to extend dynamic range for x-ray diffraction applications at synchrotron sources and x-ray free electron lasers (XFELs). The pixels described herein build on the Mixed Mode Pixel Array Detector (MM-PAD) framework, developed previously by our group to perform high dynamic range imaging. These new pixels boast several orders of magnitude improvement in maximum flux over the MM-PAD, which is capable of measuring a sustained flux in excess of 10$^{8}$ x-rays/pixel/second while maintaining sensitivity to smaller signals, down to single x-rays. To extend dynamic range, charge is removed from the integration node of the front-end amplifier without interrupting integration. The number of times this process occurs is recorded by a digital counter in the pixel. The parameter limiting full well is thereby shifted from the size of an integration capacitor to the depth of a digital counter. The result is similar to that achieved by counting pixel array detectors, but the integrators presented here are designed to tolerate a sustained flux >10$^{11}$ x-rays/pixel/second. Pixel front-end linearity was evaluated by direct current injection and results are presented. A small-scale readout ASIC utilizing these pixel architectures has been fabricated and the use of these architectures to increase single x-ray pulse dynamic range at XFELs is discussed briefly., Comment: 6 pages, 7 figures

Pixel Array Detectors (PADs) consist of an x-ray sensor layer bonded pixel-by-pixel to an underlying readout chip. This approach allows both the sensor and the custom pixel electronics to be tailored independently to best match the x-ray imaging requirements. Here we present characterizations of CdTe sensors hybridized with two different charge-integrating readout chips, the Keck PAD and the Mixed-Mode PAD (MM-PAD), both developed previously in our laboratory. The charge-integrating architecture of each of these PADs extends the instantaneous counting rate by many orders of magnitude beyond that obtainable with photon counting architectures. The Keck PAD chip consists of rapid, 8-frame, in-pixel storage elements with framing periods $<$150 ns. The second detector, the MM-PAD, has an extended dynamic range by utilizing an in-pixel overflow counter coupled with charge removal circuitry activated at each overflow. This allows the recording of signals from the single-photon level to tens of millions of x-rays/pixel/frame while framing at 1 kHz. Both detector chips consist of a 128$\times$128 pixel array with (150 $\mu$m)$^2$ pixels., Comment: Revised version after 2nd peer-review

Using the multi-channel Transient Current Technique the currents induced by electron-hole pairs, produced by a focussed sub-nanosecond laser of 660 nm wavelength close to the Si-SiO2 interface of p+n silicon strip sensors have been measured, and the charge-collection efficiency determined. The laser has been operated in burst mode, with bursts typically spaced by 1 ms, each consisting of 30 pulses separated by 50 ns. In a previous paper it has been reported that, depending on X-ray-radiation damage, biasing history and humidity, situations without charge losses, with hole losses, and with electron losses have been observed. In this paper we show for sensors before and after irradiation by X-rays to 1 MGy (SiO2), how the charge losses change with the number of electron-hole pairs generated by each laser pulse, and the time interval between the laser pulses. This allows us to estimate how many additional charges in the accumulation layers at the Si-SiO2 interface have to be trapped to significantly change the local electric field, as well as the time it takes that the accumulation layer and the electric field return to the steady-state situation. In addition, results are presented on the change of the pulse shape caused by the plasma effect for high charge densities deposited close to the Si-SiO2 interface.

A systematic experimental study of the main challenges for silicon-pixel sensors at the European XFEL is presented. The high instantaneous density of X-rays and the high repetition rate of the XFEL pulses result in signal distortions due to the plasma effect and in severe radiation damage. The main parameters of X-ray-radiation damage have been determined and their impact on p+n sensors investigated. These studies form the basis of the optimized design of a pixel-sensor for experimentation at the European XFEL.

Single photon sensitivity is an important property of certain detection systems. This work investigated the single photon sensitivity of the Adaptive Gain Integrating Pixel Detector (AGIPD) and its dependence on possible detector noise values. Due to special requirements at the European X-ray Free Electron Laser (XFEL) the AGIPD finds the number of photons absorbed in each pixel by integrating the total signal. Photon counting is done off line on a thresholded data set. It was shown that AGIPD will be sensitive to single photons of 8 keV energy or more (detection efficiency $\gg$ 50%, less than 1 count due to noise per 10$^6$ pixels). Should the final noise be at the lower end of the possible range (200 - 400 electrons) single photon sensitivity can also be achieved at 5 keV beam energy. It was shown that charge summing schemes are beneficial when the noise is sufficiently low. The total detection rate of events is increased and the probability to count a single event multiple times in adjacent pixels is reduced by a factor of up to 40. The entry window of AGIPD allows 3 keV photons to reach the sensitive volume with approximately 70% probability. Therefore the low energy performance of AGIPD was explored, finding a maximum noise floor below 0.035 hits/pixel/frame at 3 keV beam energy. Depending on the noise level and selected threshold this value can be reduced by a factor of approximately 10. Even though single photon sensitivity, as defined in this work, is not given, imaging at this energy is still possible, allowing Poisson noise limited performance for signals significantly above the noise floor., Comment: revised version after peer review

The European XFEL, currently under construction, will produce a coherent X-ray pulse every 222 ns in pulse trains of up to 2700 pulses. In conjunction with the fast 2D area detectors currently under development, it will be possible to perform X-ray Photon Correlation Spectroscopy (XPCS) experiments on sub-microsecond timescales with non-ergodic systems. A case study for the Adaptive Gain Integrating Pixel Detector (AGIPD) at the European XFEL employing the intensity autocorrelation technique was performed using the detector simulation tool HORUS. As optimum results from XPCS experiments are obtained when the pixel size approximates the (small) speckle size, the presented study compares the AGIPD (pixel size of (200 $\upmu$m)$^2$) to a possible apertured version of the detector and to a hypothetical system with (100 $\upmu$m)$^2$ pixel size and investigates the influence of intensity fluctuations and incoherent noise on the quality of the acquired data. The intuitive conclusion that aperturing is not beneficial as data is 'thrown away' was proven to be correct for low intensities. For intensities larger than approximately 1 photon per (100 $\upmu$m)$^2$ aperturing was found to be beneficial, as charge sharing effects were excluded by it. It was shown that for the investigated case (100 $\upmu$m)$^2$ pixels produced significantly better results than (200 $\upmu$m)$^2$ pixels when the average intensity exceeded approximately 0.05 photons per (100 $\upmu$m)$^2$. Although the systems were quite different in design they varied in the signal to noise ratio only by a factor of 2-3, and even less in the relative error of the extracted correlation constants. However the dependence on intensity showed distinctively different features for the different systems., Comment: Re-Revised version after 2nd peer review process

Charge multiplication (CM) in p$^+$n epitaxial silicon pad diodes of 75, 100 and 150 $\upmu$m thickness at high voltages after proton irradiation with 1 MeV neutron equivalent fluences in the order of $10^{16}$ cm$^{-2}$ was studied as an option to overcome the strong trapping of charge carriers in the innermost tracking region of future Super-LHC detectors. Charge collection efficiency (CCE) measurements using the Transient Current Technique (TCT) with radiation of different penetration (670, 830, 1060 nm laser light and $\alpha$-particles with optional absorbers) were used to locate the CM region close to the p$^+$-implantation. The dependence of CM on material, thickness of the epitaxial layer, annealing and temperature was studied. The collected charge in the CM regime was found to be proportional to the deposited charge, uniform over the diode area and stable over a period of several days. Randomly occurring micro discharges at high voltages turned out to be the largest challenge for operation of the diodes in the CM regime. Although at high voltages an increase of the TCT baseline noise was observed, the signal-to-noise ratio was found to improve due to CM for laser light. Possible effects on the charge spectra measured with laser light due to statistical fluctuations in the CM process were not observed. In contrast, the relative width of the spectra increased in the case of $\alpha$-particles, probably due to varying charge deposited in the CM region., Comment: 11 pages, accepted by NIMA

The mobility of electrons and holes in silicon depends on many parameters. Two of them are the electric field and the temperature. It has been observed previously that the mobility in the transition region between ohmic transport and saturation velocities is a function of the orientation of the crystal lattice. This paper presents a new set of parameters for the mobility as function of temperature and electric field for <111> and <100> crystal orientation. These parameters are derived from time of flight measurements of drifting charge carriers in planar p^+nn^+ diodes in the temperature range between -30{\deg}C and 50{\deg}C and electric fields of 2x10^3 V/cm to 2x10^4 V/cm., Comment: revised version after peer review

In silicon sensors high densities of electron-hole pairs result in a change of the current pulse shape and spatial distribution of the collected charge compared to the situation in presence of low charge carrier densities. This paper presents a detailed comparison of numerical simulations with time resolved current measurements on planar silicon sensors using 660~nm laser light to create different densities of electron hole pairs., Comment: Revised Version after peer-review