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Register a new userPhotoemission from the gas phase using soft x-ray fs pulses: An investigation of the space-charge effects
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An experimental and computational investigation of the space-charge effects occurring in ultrafast photoelectron spectroscopy from the gas phase is presented. The target sample CF$_3$I is excited by ultrashort (100 fs) far-ultraviolet radiation pulses produced by a free-electron laser. The modification of the energy distribution of the photoelectrons, i.e. the shift and broadening of the spectral structures, is monitored as a function of the pulse intensity. The experimental results are compared with computational simulations which employ a Barnes-Hut algorithm to calculate the effect of individual Coulomb forces acting among the particles. In the presented model, a survey spectrum acquired at low radiation fluence is used to determine the initial energy distribution of the electrons after the photoemission event. The spectrum modified by the space-charge effects is then reproduced by $N$-body calculations that simulate the dynamics of the photoelectrons subject to the individual mutual Coulomb repulsion and to the attractive force of the positive ions. The employed numerical method accounts for the space-charge effects on the energy distribution and allows to reproduce the complete photoelectron spectrum and not just a specific photoemission structure. The simulations also provide information on the time evolution of the space-charge effects on the picosecond scale. Differences with the case of photoemission from solid samples are highlighted and discussed. The presented simulation procedure, although it omits the analysis of angular distribution, constitutes an effective simplified model that allows to predict and account for space-charge effects on the photoelectron energy spectrum in time-resolved photoemission experiments with high-intensity pulsed sources.
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Large liquid argon time projection chambers (LArTPCs), especially those operating near the surface, are susceptible to space charge effects. In the context of LArTPCs, the space charge effect is the build-up of slow-moving positive ions in the detector primarily due to ionization from cosmic rays, leading to a distortion of the electric field within the detector. This effect leads to a displacement in the reconstructed position of signal ionization electrons in LArTPC detectors (spatial distortions), as well as to variations in the amount of electron-ion recombination experienced by ionization throughout the volume of the TPC. We present techniques that can be used to measure and correct for space charge effects in large LArTPCs by making use of cosmic muons, including the use of track pairs to unambiguously pin down spatial distortions in three dimensions. The performance of these calibration techniques are studied using both Monte Carlo simulation and MicroBooNE data, utilizing a UV laser system as a means to estimate the systematic bias associated with the calibration methodology.
The spectrum of laser-plasma generated X-rays is very important, it characterizes electron dynamics in plasma and is basic for applications. However, the accuracies and efficiencies of existing methods to diagnose the spectrum of laser-plasma based X-ray pulse are not very high, especially in the range of several hundred keV. In this study, a new method based on electron tracks detection to measure the spectrum of laser-plasma produced X-ray pulses is proposed and demonstrated. Laser-plasma generated X-rays are scattered in a multi-pixel silicon tracker. Energies and scattering directions of Compton electrons can be extracted from the response of the detector, and then the spectrum of X-rays can be reconstructed. Simulations indicate that the energy resolution of this method is approximately 20% for X-rays from 200 to 550 keV for a silicon-on-insulator pixel detector with 12 $rm mu$m pixel pitch and 500 $rm mu$m depletion region thickness. The results of a proof-of-principle experiment based on a Timepix3 detector are also shown.
The use of strongly bent crystals in spectrometers for pulses of a hard x-ray free-electron laser is explored theoretically. Diffraction is calculated in both dynamical and kinematical theories. It is shown that diffraction can be treated kinematically when the bending radius is small compared to the critical radius given by the ratio of the Bragg-case extinction length for the actual reflection to the Darwin width of this reflection. As a result, the spectral resolution is limited by the crystal thickness, rather than the extinction length, and can become better than the resolution of a planar dynamically diffracting crystal. As an example, we demonstrate that spectra of the 12 keV pulses can be resolved in 440 reflection from a 20 micron thick diamond crystal bent to a radius of 10 cm.
We have investigated the radiation damage effects on a CCD to be employed in the Japanese X-ray astronomy mission including the Monitor of All-sky X-ray Image (MAXI) onboard the International Space Station (ISS). Since low energy protons release their energy mainly at the charge transfer channel, resulting a decrease of the charge transfer efficiency, we thus focused on the low energy protons in our experiments. A 171 keV to 3.91 MeV proton beam was irradiated to a given device. We measured the degradation of the charge transfer inefficiency (CTI) as a function of incremental fluence. A 292 keV proton beam degraded the CTI most seriously. Taking into account the proton energy dependence of the CTI, we confirmed that the transfer channel has the lowest radiation tolerance. We have also developed the different device architectures to reduce the radiation damage in orbit. Among them, the ``notch CCD, in which the buried channel implant concentration is increased, resulting in a deeper potential well than outside, has three times higher radiation tolerance than that of the normal CCD. We then estimated the charge transfer inefficiency of the CCD in the orbit of ISS, considering the proton energy spectrum. The CTI value is estimated to be 1.1e-5 per each transfer after two years of mission life in the worse case analysis if the highest radiation-tolerant device is employed. This value is well within the acceptable limit and we have confirmed the high radiation-tolerance of CCDs for the MAXI mission.
Printed circuit boards (PCBs) are widely used in most electrical and electronic equipments or products. Hazardous substances such as Pb, Hg, Cd, etc, can be present in high concentrations in PCBs and the degradation and release of these substances poses a huge threat to humans and the environment. To investigation the chemical composition of PCBs in domestic market of China, a practical micro-focus X-ray fluorescence system is setup to make the elements analysis, especially for detecting hazardous substances. Collimator is adopted to focus the X-ray emitted from X-ray tube. BRUKER X-ray detector with proportional counter is used to detect the emitted fluorescence from the PCB samples. Both single layer PCB samples and double layers PCB samples made of epoxy glass fiber are purchased from the domestic market of China. Besides, a MC55 wireless communication module made by SIEMENS in Germany is used as the reference material. Experimental results from the fluorescence spectrums of the testing points of PCB samples show that, hazardous substances, mainly Pb and Br, are detected from the welding pads and substrates. In addition, statistical data about the average relatively amount of the main substances in testing points are also illustrated. It is verified that micro-XRF screening offers a simple and quick qualitative measurement of hazardous substances in PCBs.
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