Hard X-ray operation of X-ray gas monitors at the European XFEL.

X-ray gas monitors (XGMs) are operated at the European XFEL for non-invasive single-shot pulse energy measurements and average beam-position monitoring. The underlying measurement principle is the photo-ionization of rare gas atoms at low gas pressures and the detection of the photo-ions and photo-electrons created. These are essential for tuning and sustaining self-amplified spontaneous emission (SASE) operation, machine radiation safety, and sorting single-shot experimental data according to pulse energy. In this paper, the first results from XGM operation at photon energies up to 30 keV are presented, which are far beyond the original specification of this device. Here, the Huge Aperture MultiPlier (HAMP) is used for single-shot pulse energy measurements since the standard X-ray gas monitor detectors (XGMDs) do not provide a sufficient signal-to-noise ratio, even at the highest operating gas pressures. A single-shot correlation coefficient of 0.98 is measured between consecutive XGMs operated with HAMP, which is as good as measuring with the standard XGMD detectors. An intra-train non-linearity of the HAMP signal is discovered, and operation parameters to mitigate this effect are studied. The upper repetition rate limit of HAMP operation at 2.25 MHz is also determined. Finally, the possibilities and limits for future XGM operation at photon energies up to 50 keV are discussed.

Machine Learning Nucleation Collective Variables with Graph Neural Networks.

The efficient calculation of nucleation collective variables (CVs) is one of the main limitations to the application of enhanced sampling methods to the investigation of nucleation processes in realistic environments. Here we discuss the development of a graph-based model for the approximation of nucleation CVs that enables orders-of-magnitude gains in computational efficiency in the on-the-fly evaluation of nucleation CVs. By performing simulations on a nucleating colloidal system mimicking a multistep nucleation process from solution, we assess the model's efficiency in both postprocessing and on-the-fly biasing of nucleation trajectories with pulling, umbrella sampling, and metadynamics simulations. Moreover, we probe and discuss the transferability of graph-based models of nucleation CVs across systems using the model of a CV based on sixth-order Steinhardt parameters trained on a colloidal system to drive the nucleation of crystalline copper from its melt. Our approach is general and potentially transferable to more complex systems as well as to different CVs.

Development of a photoelectron spectrometer for hard x-ray photon diagnostics.

The development and characterization of an angle-resolved photoelectron spectrometer, based on the electron time-of-flight concept, for hard x-ray photon diagnostics at the European Free-Electron Laser, are described. The instrument is meant to provide users and operators with pulse-resolved, non-invasive spectral distribution diagnostics, which in the hard x-ray regime is a challenge due to the poor cross-section and high kinetic energy of photoelectrons for the available target gases. We report on the performances of this instrument as obtained using hard x-rays at the PETRA III synchrotron at DESY in multibunch mode. Results are compared with electron trajectory simulations. We demonstrate a resolving power of 10 eV at incident photon energies up to at least 20 keV.

Influence of Fiber Content and Dosing Position on the the Mechanical Properties of Short-Carbon-Fiber Polypropylene Compounds.

The properties of short-fiber-reinforced composites depend on the fiber length of the reinforcing fibers. This fiber length is typically influenced by processing to different extents. In this work, we investigate the influence of processing, i.e., the influence of residence time achieved via different dosing points in compounding, and the fiber content on the fiber length and mechanical properties of short-carbon-fiber-reinforced polypropylene. We found that, with increasing fiber content, the fiber length decreases from 900 to 300 µm after compounding and from 500 to 250 µm after injection molding. Additionally, a decrease in residence time in the compounder leads to an increase in the fiber length of approx. 300 µm compared to the longer residence time. This is later reduced by the injection molding step, but the longer fibers are still longer in the final molded test specimen, thus resulting in a 5-10% increased tensile strength and elastic modulus as well as an some increase in impact strength. As the injection molding step showed considerable fiber length reduction (down to 250 µm), further investigations of injection molding should be undertaken to preserve fiber length better for the increased performance of these composites.

Training algorithm matters for the performance of neural network potential: A case study of Adam and the Kalman filter optimizers.

One hidden yet important issue for developing neural network potentials (NNPs) is the choice of training algorithm. In this article, we compare the performance of two popular training algorithms, the adaptive moment estimation algorithm (Adam) and the extended Kalman filter algorithm (EKF), using the Behler-Parrinello neural network and two publicly accessible datasets of liquid water [Morawietz et al., Proc. Natl. Acad. Sci. U. S. A. 113, 8368-8373, (2016) and Cheng et al., Proc. Natl. Acad. Sci. U. S. A. 116, 1110-1115, (2019)]. This is achieved by implementing EKF in TensorFlow. It is found that NNPs trained with EKF are more transferable and less sensitive to the value of the learning rate, as compared to Adam. In both cases, error metrics of the validation set do not always serve as a good indicator for the actual performance of NNPs. Instead, we show that their performance correlates well with a Fisher information based similarity measure.

Hard x-ray single-shot spectrometer at the European X-ray Free-Electron Laser.

The European X-ray Free-Electron Laser Facility in Germany delivers x-ray pulses with femtosecond pulse duration at a repetition rate of up to 4.5 MHz. The free-electron laser radiation is created by the self-amplified spontaneous emission (SASE) process, whose stochastic nature gives rise to shot-to-shot fluctuations in most beam properties, including spectrum, pulse energy, spatial profile, wavefront, and temporal profile. Each spectrum consisting of many spikes varies in width and amplitude that appear differently within the envelope of the SASE spectrum. In order to measure and study the SASE spectrum, the HIgh REsolution hard X-ray single-shot (HIREX) spectrometer was installed in the photon tunnel of the SASE1 undulator beamline. It is based on diamond gratings, bent crystals as a dispersive element, and a MHz-repetition-rate strip detector. It covers a photon energy range of 3 keV-25 keV and a bandwidth of 0.5% of the SASE beam. The SASE spikes are resolved with 0.15 eV separation using the Si 440 reflection, providing a resolving power of 60 000 at a photon energy of 9.3 keV. The measured SASE bandwidth is 25 eV. In this paper, we discuss the design specifications, installation, and commissioning of the HIREX spectrometer. The spectral results using Si (110), Si (111), and C (110) crystals are presented.

X-ray photon diagnostics at the European XFEL.

The European X-ray Free-Electron Laser (European XFEL) (Altarelli et al., 2006; Tschentscher et al., 2017), the world's largest and brightest X-ray free-electron laser (Saldin et al., 1999; Pellegrini et al., 2016), went into operation in 2017. This article describes the as-built realization of photon diagnostics for this facility, the diagnostics commissioning and their application for commissioning of the facility, and results from the first year of operation, focusing on the SASE1 beamline, which was the first to be commissioned. The commissioning consisted of pre-beam checkout, first light from the bending magnets, X-rays from single undulator segments, SASE tuning with many undulator segments, first lasing, optics alignment for FEL beam transport through the tunnel up to the experiment hutches, and finally beam delivery to first users. The beam properties assessed by photon diagnostics throughout these phases included per-pulse intensity, beam position, shape, lateral dimensions and spectral properties. During this time period, the machine provided users with up to 14 keV photon energy, 1.5 mJ pulse energy, 300 FEL pulses per train and 4.5 MHz intra-bunch train repetition rate at a 10 Hz train repetition rate. Finally, an outlook is given into the diagnostic prospects for the future.

Commissioning of a photoelectron spectrometer for soft X-ray photon diagnostics at the European XFEL.

Commissioning and first operation of an angle-resolved photoelectron spectrometer for non-invasive shot-to-shot diagnostics at the European XFEL soft X-ray beamline are described. The objective with the instrument is to provide the users and operators with reliable pulse-resolved information regarding photon energy and polarization that opens up a variety of applications for novel experiments but also hardware optimization.

Operation of X-ray gas monitors at the European XFEL.

X-ray gas monitors (XGMs) are operated at the European XFEL for non-invasive single-shot pulse energy measurements and average beam position monitoring. They are used for tuning and maintaining the self-amplified spontaneous emission (SASE) operation and for sorting single-shot experimental data according to the pulse-resolved energy monitor data. The XGMs were developed at DESY based on the specific requirements for the European XFEL. In total, six XGM units are continuously in operation. Here, the main principle and experimental setup of an XGM are summarized, and the locations of the six XGMs at the facility are shown. Pulse energy measurements at 0.134 nm wavelength are presented, exceeding 1 mJ obtained with an absolute measurement uncertainty of 7-10%; correlations between different XGMs are shown, from which a SASE1 beamline transmission of 97% is deduced. Additionally, simultaneous position measurements close to the undulator and at the end of the tunnel are shown, along with the correlation of beam position data simultaneously acquired by an XGM and an imager.

Femoral stem subsidence in cementless total hip arthroplasty: a retrospective single-centre study.

PURPOSE AND HYPOTHESIS: Subsidence is a known reason for early failure of total hip arthroplasty (THA). In particular, cementless THA might be vulnerable to migration. The present study analysed femoral stem subsidence after primary cementless THA. Prosthetic and anatomical risk factors for early femoral stem subsidence were evaluated. METHODS: Two hundred thirty-one consecutive patients who underwent primary cementless THA in a single centre were retrospectively analysed. Post-operative results were evaluated in consideration of prosthetic and anatomical properties in correlation with subsidence on standing pelvic anteroposterior radiographs. Stem type and design, demographic data, BMI, canal flare index (CFI) and canal fill ratio (CFR) were evaluated. RESULTS: The subsidence rate was significantly higher in collarless femoral stems [3.1 mm (SD 2.8 mm) vs. 1.9 mm (SD 1.5 mm); p = 0.013] while the anatomical type of the proximal femur as described by the canal flare index did not influenced subsidence (p = 0.050). Also, the canal fill ratio showed no significant correlation with subsidence at any level. CONCLUSIONS: In the present study, stem subsidence was significantly higher in the collarless group compared to collared stems. No anatomical parameter (CFI and CFR) could be identified as risk factor for subsidence. Neither age nor BMI influenced subsidence in this cohort. Still, subgroup analysis indicated a sex-dependent role of BMI. Prospective studies of large cohorts should address the problem of subsidence in the future. LEVEL OF EVIDENCE: Retrospective therapeutic study, Level IV.

Complications in hip arthroscopy: necessity of supervision during the learning curve.

PURPOSE: The aim of this study was to determine whether the learning curve of arthroscopic treatment of femoroacetabular impingement (FAI) could be verified by analyzing the complication rate of this procedure. Additionally, it was investigated whether supervision by an experienced surgeon leads to a steeper learning curve (lower number of complications) when starting to perform arthroscopic FAI treatment. METHODS: The complications occurring in 317 consecutive patients treated with the sole diagnosis of FAI were analyzed. 256 patients (collective A) were treated by surgeon A between June 2005 and January 2010. Sixty-one patients (collective B) were treated by surgeon B between August 2008 and December 2009. From January to June 2008, surgeon B performed many hip arthroscopies under supervision of surgeon A. Complications were recorded in a central complication register. Statistic analysis of the complication rates was performed using Fischer's exact T test. RESULTS: Subdividing collective A chronologically into thirds a significant decline of complications (p = 0.0044) was found with growing experience of the surgeon. Comparing the first 61 patients of both surgeons a significantly lower complication rate was discovered in the patients of surgeon B (p = 0.0375). In total there were 21 complications (6.6 %; CI 4.4-9.9 %). The observed complication rate was 7.0 % in collective A and 4.9 % in collective B. CONCLUSION: The learning curve can be comprehended by the distribution of complications in collective A. Having spent 6 months performing under supervision of surgeon A, surgeon B has a lower complication rate than surgeon A when comparing the first 61 patients each surgeon operated on. This implies that surgeon B benefits from the experience of surgeon A. According to this analysis, beginners in arthroscopic FAI treatment should be taught at a specialized centre to reduce the number of complications.

Short-keeled cemented tibial components show an increased risk for aseptic loosening.

BACKGROUND: The choice of implant design plays an important role for primary fixation of a TKA. Short-keeled tibial components allow implantation through a smaller approach with less femorotibial subluxation. QUESTIONS/PURPOSES: The purpose of this study was to detect early implant failure resulting from aseptic loosening after cemented short-keeled and standard tibial baseplate implantation. METHODS: Between 2008 and 2010, a group of 160 consecutive patients (with 80 standard and with 80 short-keeled tibial trays) received cemented TKAs. At 1-year followup, patients were examined clinically and radiographs were analyzed regarding aspects of radiolucency. The components were divided into five zones on each radiographic view and the measurements of the 10 zones were added. RESULTS: The mean sum of radiolucencies was increased significantly with the short-keeled baseplates. In the current study, short-keeled tibial trays revealed a revision rate of 6.3% after 1-year followup. In contrast, none of the standard tibial baseplates were revised. CONCLUSIONS: The implantation of cemented, short-keeled tibial components is linked to an increased rate of early loosening. Therefore, the indication for cemented, short-keeled component implantation should be reviewed. LEVEL OF EVIDENCE: Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Prevalence of Bartonella henselae and Borrelia burgdorferi sensu lato DNA in ixodes ricinus ticks in Europe.

Bartonella spp. can cause persistent bloodstream infections in humans and animals. To determine whether Bartonella henselae is present in questing Ixodes ricinus ticks, we analyzed the prevalence of B. henselae DNA among tick stages compared to the prevalence of DNA from Borrelia burgdorferi sensu lato, the pathogen most frequently transmitted by ticks. B. henselae DNA was present with a prevalence of up to approximately 40% in tick populations sampled in four European sites (Eberdingen, Germany; Klasdorf, Germany; Lembach, France; and Madeira, Portugal). The odds of detecting B. henselae DNA in nymphal ticks was approximately 14-fold higher than in adult ticks. No tick was found to be coinfected with B. henselae and B. burgdorferi sensu lato. Taken together, our data indicate that ticks might serve as a vector for the transmission of B. henselae to humans.

Analysis of a novel insect cell culture medium-based growth medium for Bartonella species.

Human- and animal-pathogenic Bartonella species are fastidious and slow-growing bacteria difficult to isolate and cultivate. We describe a novel, easy-to-prepare liquid medium for the fast and reliable growth of several Bartonella spp. that does not affect bacterial protein expression patterns or interactions with host cells.