Entire Photon Energy Range Research Articles

A thermal deformation optimization method forcryogenically cooled silicon crystal monochromators under high heat load.

A method to optimize the thermal deformation of an indirectly cryo-cooled silicon crystal monochromator exposed to intense X-rays at a low-emittance diffraction-limited synchrotron radiation source is presented. The thermal-induced slope error of the monochromator crystal has been studied as a function of heat transfer efficiency, crystal temperature distribution and beam footprint size. A partial cooling method is proposed, which flattens the crystal surface profile within the beam footprint by modifying the cooling contact area to optimize the crystal peak temperature. The optimal temperature varies with different photon energies, which is investigated, and a proper cooling strategy is obtained to fulfil the thermal distortion requirements over the entire photon energy range. At an absorbed power up to 300 W with a maximum power density of 44.8 W mm-2 normal incidence beam from an in-vacuum undulator, the crystal thermal distortion does not exceed 0.3 µrad at 8.33 keV. This method will provide references for the monochromator design on diffraction-limited synchrotron radiation or free-electron laser light sources.

A versatile wide energy range spectrometer for soft X-ray FELs

The design and main characteristics of the Wide Energy range Spectrometer at TIMEX (WEST) operating at the FERMI Free Electron Laser are described. WEST covers the entire photon energy range of the FERMI source including its fundamental emission (extreme ultraviolet region, EUV) and its highest harmonics (soft X-ray region) spanning about a thousand of electrovolts from 20 to 1000 eV. The instrument is specifically designed to measure FERMI FEL spectra with good resolving power (500<RP<7000) and provide remarkable signal to noise ratio adequate for single-shot experiments. Notably, WEST is aligned in transmission geometry downstream of the TIMEX experimental beamline thus offering a valid diagnostic tool for the characterization of both linear and nonlinear spectral effects driven by the interaction between FEL radiation and samples in gaseous, liquid and solid phase. Detailed theoretical analysis of the instrument performance complemented by experimental measurements is presented. Further possible upgrades of WEST are finally considered and discussed.

Highly efficient thermal deformation optimization method for smart-cut mirrors over the entire photon energy range.

A method to optimize the notches of water-cooled white-beam mirrors over the entire photon energy range is proposed. A theoretical method is used to quantitatively evaluate the influence of the thermal load on the thermal deformation of a mirror. The result of theoretical calculations and finite-element analysis are consistent, which proves the feasibility of the method. The root mean square of the curvatures of the thermal deformation of the white-beam mirror over the entire photon energy range can be minimized. This method greatly simplifies the design work of water-cooled white-beam mirrors.

FEL Simulation of New Hard X-ray Undulator Line at PAL-XFEL

The Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) has been successfully operating as a remarkably-stable XFEL facility in the world. The hard X-ray beamline, however, has only one undulator line (HX1) with a 26-mm undulator period for which the maximum undulator parameter K is 1.87. The lowest photon energy that can be generated from the HX1 with a maximum electron beam energy at PAL-XFEL (10.5 GeV) is about 14.65 keV. When a lower photon energy than that is required by the beamline users, the electron beam energy has to be decreased, which results in a decreased accessible FEL pulse energy. Therefore, a new hard X-ray undulator line (HX2) with a higher undulator parameter K is needed to make full use of the PAL-XFEL performance in the lower photon energies by increasing the resonant electron beam energy. The undulator period of the HX2 is decided as 35 mm by using Ming Xie’s fitting formula to estimate the performance of the HX2. FEL simulations with the GENESIS code are carried out to evaluate the performance of the HX2, including the effect of the post-saturation region. The undulator tapering configuration is optimized by maximizing the FEL intensity for each case. We show that the radiation power at the end of the HX2 can be increased up to 2.5 times higher than that of the HX1 over the entire target photon-energy range of the HX2 (2 ∼ 10 keV) by utilizing an undulator with a longer period and a higher undulator parameter K.

Dielectric Parameters Study of GaAs1−xSbx Alloy from Optical Interband Transition

Numerical studies are presented for each transition of GaAsSb alloy. The dielectric function of real and imaginary parts calculation for each transition are obtained by integrating the Kramers–Kronig (KK) relation in the entire photon energy range of 0.0–6.0 eV. We observed a redshift in the optical spectra at a higher percentage of Sb. Our calculations are in agreement with the reported experimental data for a GaAsSb alloy from 0.0 eV to 6.0 eV. The results indicate that the investigated GaAsSb alloy is useful for several applications, such as photodetectors, photodiodes, and solar cells.

Vacuum-compatible photon-counting hybrid pixel detector for wide-angle x-ray scattering, x-ray diffraction, and x-ray reflectometry in the tender x-ray range.

A vacuum-compatible photon-counting hybrid pixel detector has been installed in the ultra-high vacuum reflectometer of the four-crystal monochromator beamline of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY II in Berlin, Germany. The setup is based on the PILATUS3 100K module. The detector can be used in the entire photon energy range accessible at the beamline from 1.75 keV to 10 keV. Complementing the already installed vacuum-compatible PILATUS 1M detector used for small-angle x-ray scattering (SAXS) and grazing incidence SAXS, it is possible to access larger scattering angles. The water-cooled module is located on the goniometer arm and can be positioned from -90° to 90° with respect to the incoming beam at a distance of about 200 mm from the sample. To perform absolute scattering experiments, the linearity, homogeneity, and angular dependence of the quantum efficiency, including their relative uncertainties, have been investigated. In addition, the first results of the performance in wide-angle x-ray scattering, x-ray diffraction, and x-ray reflectometry are presented.

Exclusive photoproduction of π0 up to large values of Mandelstam variables s,t, and u with CLAS

Exclusive photoproduction cross sections have been measured for the process $\gamma p \rightarrow p\pi^0(e^+e^-(\gamma))$ with the Dalitz decay final state using tagged photon energies in the range of $E_{\gamma} = 1.275-5.425$ GeV. The complete angular distribution of the final state $\pi^0$, for the entire photon energy range up to large values of $t$ and $u$, has been measured for the first time. The data obtained show that the cross section $d\sigma/dt$, at mid to large angles, decreases with energy as $s^{-6.89\pm 0.26} $. This is in agreement with the perturbative QCD quark counting rule prediction of $s^{-7} $. Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD based Generalized Parton Distribution mechanism at highest available invariant energy $s=11$ GeV$^2$. At the same time, the Regge exchange based models for $\pi^0$ photoproduction are more consistent with experimental data.

Absolute single-photoionization cross sections ofSe2+: Experiment and theory

Absolute single-photoionization cross-section measurements for ${\mathrm{Se}}^{2+}$ ions were performed at the Advanced Light Source at Lawrence Berkeley National Laboratory using the merged-beams photo-ion technique. Measurements were made at a photon energy resolution of $24\ifmmode\pm\else\textpm\fi{}3$ meV in the photon energy range 23.5--42.5 eV, spanning the ground state and low-lying metastable state ionization thresholds. To clearly resolve the resonant structure near the ground-state threshold, high-resolution measurements were made from 30.0 to 31.9 eV at a photon energy resolution of $6.7\ifmmode\pm\else\textpm\fi{}0.7$ meV. Numerous resonance features observed in the experimental spectra are assigned and their energies and quantum defects tabulated. The high-resolution cross-section measurements are compared with large-scale, state-of-the-art theoretical cross-section calculations obtained from the Dirac Coulomb $R$-matrix method. Suitable agreement is obtained over the entire photon energy range investigated. These results are an experimental determination of the absolute photoionization cross section of doubly ionized selenium and include a detailed analysis of the photoionization resonance spectrum of this ion.

Calculation of gamma-ray attenuation parameters for locally developed shielding material: Polyboron

In the present study, the mass attenuation coefficient (μm) has been calculated analytically for a locally developed shielding material, polyboron, and compared with the values obtained from the WinXCom code, a Windows version of the XCOM database at the photon energy range 0.001 MeV–20 MeV. A good agreement has been observed between these two values. The linear attenuation coefficients (μ) and relaxation lengths (λ) have also been calculated from the obtained μm values and their variations with photon energy have been plotted. For comparison, other four shielding materials- ordinary concrete, pure polyethylene, borated polyethylene and water have also been studied. The obtained result shows that μm, μ and λ strongly depends on the photon energy, chemical composition and density of the shielding materials. The values of μm and μ of polyboron have been found greater than those of pure polyethylene and borated polyethylene but less than those of ordinary concrete and water at low photon energy range; and at the intermediate photon energy range (0.125 MeV–6 MeV), all the sample materials have approximately the same μm values. It has also been noticed that polyboron has the medial relaxation length (λ) over the entire photon energy range. The total mass attenuation coefficient (μm) and linear attenuation coefficient (μ), Half Value Layer (HVL) and Tenth Value Layer (TVL) of the five sample materials for some common gamma sources have been worked out and the transmission curves have been plotted. The curves exhibit that the transmission factor of the sample materials decreases with the increase in shielding thickness. The results of this study can be utilized to comprehend the shielding effectiveness of this locally developed material.

Multilayer optics for monochromatic high-resolution X-ray imaging diagnostic in a broad photon energy range from 2 keV to 22 keV

The “Commissariat à l’énergie atomique et aux énergies alternatives” (CEA) studies and designs advanced X-ray diagnostics to probe dense plasmas produced at the future Laser MegaJoule (LMJ) facility. Mainly for X-ray imaging with high spatial resolution, different types of multilayer mirrors were developed to provide broadband X-ray reflectance at grazing incidence. These coatings are deposited on two toroidal mirror substrates that are then mounted into a Wolter-type geometry (working at a grazing angle of 0.45°) to realize an X-ray microscope. Non-periodic (depth graded) W/Si multilayer can be used in the broad photon energy range from 2keV to 22keV. A third flat mirror can be added for the spectral selection of the microscope. This mirror is coated with a Mo/Si multilayer for which the d-spacing varies in the longitudinal direction to satisfy the Bragg condition within the angular acceptance of the microscope and also to compensate the angular dispersion due to the field of the microscope. We present a study of such a so-called Göbel mirror which was optimized for photon energy of 10.35keV. The three mirrors were coated using magnetron sputtering technology by Xenocs SA. The reflectance in the entire photon energy range was determined in the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the synchrotron radiation facility BESSY II in Berlin.

Characterization of an in-vacuum PILATUS 1M detector.

A dedicated in-vacuum X-ray detector based on the hybrid pixel PILATUS 1M detector has been installed at the four-crystal monochromator beamline of the PTB at the electron storage ring BESSY II in Berlin, Germany. Owing to its windowless operation, the detector can be used in the entire photon energy range of the beamline from 10 keV down to 1.75 keV for small-angle X-ray scattering (SAXS) experiments and anomalous SAXS at absorption edges of light elements. The radiometric and geometric properties of the detector such as quantum efficiency, pixel pitch and module alignment have been determined with low uncertainties. The first grazing-incidence SAXS results demonstrate the superior resolution in momentum transfer achievable at low photon energies.

Metal-insulator transition without structural phase transition in V2O5 film

Spectroscopic ellipsometry and x-ray diffraction are used to investigate the metal-insulator transition in V2O5 films. Below and above Tc no significant change in the (001) peak is observed, but both n and k spectra undergo remarkable changes over the entire photon energy range. The SE and XRD results indicate that the V2O5 film undergoes a MIT without a structural phase transition near 280 °C. Further the MIT is confirmed by measurement of the resistance with temperature.

Structural and optical studies of GaSb implanted with iron ions

Single crystal gallium antimonide substrate was implanted with 200 keV iron ions with different ion fluences varying from 1 × 10 14 to 1 × 10 16 cm − 2 . Optical and structural properties have been investigated using the micro-Raman, Fourier transform infrared and grazing-angle X-ray diffraction measurements. Raman studies have shown the amorphization of gallium antimonide at higher fluences. Fourier transform infrared transmission studies over photon energy range 0.3 to 0.7 eV showed the change in the value of optical density over the entire photon energy range for the implanted samples as compared with the non-implanted sample. Damage profile probed by etching the sample was found to be non-uniformly distributed in the implanted sample. Grazing-angle X-ray diffraction spectra recorded at 1, 5 and 10° suggests the presence of defects and disorder in gallium antimonide after implantation.

2 E 1 Ar17+ decay and conventional radioactive sources to determine efficiency of semiconductor detectors

Although reliable models may predict the detection efficiency of semiconductor detectors, measurements are needed to check the parameters supplied by the manufacturers, namely, the thicknesses of dead layer, beryllium window, and crystal active area. The efficiency of three silicon detectors has been precisely investigated in their entire photon energy range of detection. In the zero to a few keV range, we developed a new method based on the detection of the 2E1 decay of the metastable Ar17+ 2s→1s transition. Very good theoretical knowledge of the energetic distribution of the 2E1 decay mode enables precise characterization of the absorbing layers in front of the detectors. In the high-energy range (&amp;gt;10 keV), the detector crystal thickness plays a major role in the detection efficiency and has been determined using a A241m source.

Concerning the procedure of recovering the electron energy distribution function from the soft X-ray spectrum

In the course of experiments on electron-cyclotron resonance heating of plasma in the L-2M stellarator, it was found that, at high specific heating powers, the SXR spectrum was distorted in the entire photon energy range under study (from 1.5 to 15 keV). To determine the character of this distortion, a procedure was proposed for recovering the electron energy distribution function (EEDF) from soft X-ray (SXR) measurements. The SXR spectra were analyzed for different values of the specific heating power. Numerical calculations show that the edge plasma contributes insignificantly to the chord-averaged SXR spectrum. A conclusion is drawn on the applicability and accuracy of the procedure for recovering the EEDF at high specific heating powers.

Cryogenic radiometry in the hard x-ray range

For many applications in radiometry, spectroscopy or astrophysics, absolute measurement of radiant power with low uncertainty is essential. Cryogenic electrical substitution radiometers (ESRs) are regarded as the highest-accuracy primary standard detector in radiometry, from the infrared to the ultraviolet region; in combination with tuneable monochromatized synchrotron radiation from electron storage rings, their range of operation has been extended to the soft x-ray region. ESRs are absolute thermal detectors, based on the equivalence of electrical power and radiant power that can be traced back to electrical SI units and be measured with low uncertainties. Their core piece is a cavity absorber, which is typically made of copper to achieve a short response time suitable for use with synchrotron radiation. At higher photon energies, the use of copper prevents the operation of ESRs due to increasing transmittance. A new absorber design for hard x-rays has been developed at the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II. The Monte Carlo simulation code Geant4 was applied to optimize its absorptance for photon energies of up to 60 keV, resulting in a cavity absorber with a gold base and a cylindrical shell made of copper, in combination with a thermal sensitivity of around 150 mK µW−1 and a time constant of less than 3 min, which is short compared with the lifetime of many hours for the storage ring current. The measurement of the radiant power of monochromatized synchrotron radiation was achieved with relative standard uncertainties of less than 0.2%, covering the entire photon energy range of three beamlines from 50 eV to 60 keV. Monochromatized synchrotron radiation of high spectral purity was used to calibrate silicon photodiodes against the ESR for photon energies up to 60 keV with relative standard uncertainties below 0.3%.

About the vacuum UV photoabsorption spectrum of methyl fluoride (CH 3F): the fine structure and its vibrational analysis

The vacuum UV photoabsorption spectrum of CH 3F has been recorded between 7 and 24 eV and has been analyzed in detail. Broad and structureless peaks are observed over the entire photon energy range. They are all assigned to transitions to Rydberg states, members of series converging successively to the 2e −1, 5a −1 and 1e −1 ionization limits. These features are underlying very long series of narrow and weak structures in the range of 10–13.2 eV. Through a continuum subtraction procedure, about 70 lines could clearly be identified. These have been assigned to long vibrational progressions belonging to Rydberg states corresponding to the 2e→3p and 2e→6s/5d configurations. These states are split by a Jahn–Teller distortion. Ab initio calculations lead to a Jahn–Teller stabilization energy of about 0.84 eV. The main features observed in the two bands are assigned to one single long vibrational progression of hcω≃0.160 eV (1 290 cm −1) related to the C–F Jahn–Teller inactive stretching vibration. The remaining structure is assigned to the low excitation of the Jahn–Teller active vibrational modes, i.e. the bending modes ν 5 and ν 6 characterized by hcω 5≃0.120 eV (970 cm −1) and hcω 6≃0.082 eV (660 cm −1). Leaning on the present results and assignments previously reported photoelectron spectroscopic data on CH 3F + [L. Karlsson, R. Jadrny, L. Mattsson, F.T. Chau, K. Siegbahn, Phys. Scripta 16 (1977) 225] have been reconsidered and reinterpreted.

Near-infrared transmission characteristics of GaAs implanted with high energy (100 Mev) 28Si ions

Single crystal GaAs substrates implanted with 100 MeV 28Si ions have been investigated by optical transmission over photon energy range 0.7–1.4 eV. The αx values increase with increasing dose until samples are nearly opaque ( αx>1) over the entire measurement photon energy range at a dose of 5×10 14 ions/cm 2. Damage distribution is probed by etching the sample and shows presence of an amorphous layer at a depth of 24–25 μm. Transmission recovers substantially after etching beyond 25.6 μm. Annealing experiments show that a significant damage recovery occurs over the temperature range 200–500°C with an activation energy 0.2 eV. However, some residual damage remains even after annealing at 700°C.

Excitation mechanisms and photochemistry of adsorbates with spherical symmetry.

By comparing the photo-stimulated desorption of Xe from an oxidized Si(100) surface with the photochemistry of methane on metal surfaces, we try to deduce a common concept in the excited state and the excitation mechanism responsible for the photo-induced processes. Xe atoms are desorbed from the oxidized Si(100) surface by the irradiation of photons in the range 1.16-6.43 eV. Two velocity components with average kinetic energy 0.85 and 0.25 eV are observed in the time-of-flight distributions. The fast component appears only if the photon energy exceeds approximately 3 eV, but the slow component is present over the entire photon energy range. By analogy with the photochemistry of methane on metal surfaces, the excitation mechanism responsible for the fast component is postulated to be a transition from the 5p state of Xe to the excited state originating from strong hybridization between the 6s state of Xe and the dangling bond at a surface silicon atom bonded to oxygen inserted in the dimer bond. In this scheme an excited electron is transferred from the adsorbate to the substrate, which is the reverse direction to the substrate-mediated excitation frequently assumed in surface photochemistry.

Ultra-violet range magneto-optic study of FCC-Co and Co/Pt multilayers

A magneto-optical Kerr spectroscopic apparatus for the photon energy range up to 6.8 eV has been constructed. A detailed description of the system is presented here. The spectroscopic instrument is carefully calibrated for the modulator for the entire photon energy range under consideration so as to obtain accurate Kerr rotation and ellipticity. The results of the Kerr rotation and ellipticity in 1000 Å thick face centered cubic Co films, and Co/Pt multilayers of various layer thicknesses (Pt: 3–11 Å, Co: 3 Å) are discussed in conjunction with the theoretical works.