Studies of photon induced desorption of surface gas within an aluminium vacuum chamber using an X-ray source

The role of photon induced desorption of gases arising from hard X-rays has been investigated within a vacuum chamber of aluminium alloy (Extrudal) as proposed for the new European LEP accelerator. The radiation was derived from a commercial 100 kV X-ray unit, and was introduced into the test chamber of length around 1 m by means of an end window formed of aluminium foil 25 ωm thick. In this manner a photon flux density of up to some 10⁹ photons cm² s^?1 irradiated a total surface area of around 4 × 10³ cm². The mean energy of the radiation was continuously variable in the range 9–30 keV. Measurements of desorption efficiency appropriate to each species of desorbed gas have been carried out with chamber pressures at around 1.5 × 10^?8 and 5 × 10^?11 torr attained, respectively, with evacuation at room temperature and following bakeout at 150°C. Studies have further been extended to assess the contribution of the pretreatment of the chamber by glow discharge cleaning in argon gas. In all cases the desorbed gas species under irradiation are predominantly H₂ and CO₂ which contribute in excess of 80% of the total desorption. The relative preponderance of these species is influenced by the treatments of bakeout and glow discharge cleaning, although the total desorption efficiency at around 10^?1 mol photon^?1 is not strongly affected by these actions. The desorption efficiencies of all species and at all stages of preparation of the chamber are found to decrease with increasing mean energy of the radiation, a factor of reduction of one third being observed over the range of energies available in the test.     

Determination of the ionization potentials of security-relevant substances with single photon ionization mass spectrometry using synchrotron radiation

Several ionization potentials (IPs) of security relevant substances were determined with single photon ionization time of flight mass spectrometry (SPI-TOFMS) using monochromatized synchrotron radiation from the "Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung" (BESSY). In detail, the IPs of nine explosives and related compounds, seven narcotics and narcotics precursors, and one chemical warfare agent (CWA) precursor were determined, whereas six IPs already known from the literature were verified correctly. From seven other substances, including one CWA precursor, the IP could not be determined as the molecule ion peak could not be detected. For these substances the appearance energy (AE) of a main fragment was determined. The analyzed security-relevant substances showed IPs significantly below the IPs of common matrix compounds such as nitrogen and oxygen. Therefore, it is possible to find photon energies in between, whereby the molecules of interest can be detected with SPI in very low concentrations due to the shielding of the matrix. All determined IPs except the one of the explosive EGDN were below 10.5 eV. Hence, laser-generated 118 nm photons can be applied for detecting almost all security-relevant substances by, e.g., SPI-TOFMS.     

Direct observation of X-ray induced atomic motion using scanning tunneling microscope combined with synchrotron radiation

X-ray induced atomic motion on a Ge(111)-c(2 x 8) clean surface at room temperature was directly observed with atomic resolution using a synchrotron radiation (SR)-based scanning tunneling microscope (STM) system under ultra high vacuum condition. The atomic motion was visualized as a tracking image by developing a method to merge the STM images before and after X-ray irradiation. Using the tracking image, the atomic mobility was found to be strongly affected by defects on the surface, but was not dependent on the incident X-ray energy, although it was clearly dependent on the photon density. The atomic motion can be attributed to surface diffusion, which might not be due to core-excitation accompanied with electronic transition, but a thermal effect by X-ray irradiation. The crystal surface structure was possible to break even at a lower photon density than the conventionally known barrier. These results can alert X-ray studies in the near future about sample damage during measurements, while suggesting the possibility of new applications. Also the obtained results show a new availability of the in-situ SR-STM system.     

Polarization-dependent vacuum-ultraviolet reflectometry using elliptically polarized synchrotron radiation

In the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the Berlin electron-storage ring BESSY II, a procedure has been developed to investigate the dependence of vacuum-ultraviolet reflection on polarization. It is based on characterizing the elliptically polarized synchrotron radiation at PTB's normal-incidence monochromator beamline for reflectometry by means of polarization-sensitive photodetectors. For this purpose, the polarization dependency in the detector responsivity was determined at a small, low, solid angle of acceptance for the synchrotron radiation, i.e., within the orbital plane of the storage ring where the degree of linear polarization is known to be almost 100%. Our method allows the polarization dependence of reflection samples to be measured with relative standard uncertainties ranging from 2.4% to 11% in the spectral range between 60 and 160 nm. The method has been applied to the optimization of polarizing mirrors at the Lyman-alpha wavelength of 121.6 nm.     

X-ray techniques using synchrotron radiation in materials analysis

The extraordinary properties of SR have opened up new horizons for all techniques which use vacuum ultraviolet light and X-rays. In this article, some applications of SR in materials science have been discussed. X-ray absorption spectroscopy is a local, atom specific probe of the geometrical and electronic structure of the absorbing species. As an example, the lattice site location and the valence of Co in a doped high-T_c, superconductor YBa₂Cu_2–5Co_0–5O_7–4 has been discussed.     

Ion induced gas desorption problems in the ISR

Not least among the many stringent conditions which must be satisfied to achieve the proper operation of the Intersecting Storage Rings at CERN are those pertaining to its 2 km of ultrahigh vacuum. Apart from the direct influence of the residual gas molecules through nuclear and Coulomb scattering on the stored beam lifetimes (of the order of days during which the stored protons should travel the astronomical distance of some light days!) there are also indirect processes which additionally involve adsorption/desorption phenomena on the vacuum chamber wall. The beam of high energy protons creates low energy secondary ions in the residual gas. These ions, generated in the space charge potential of 1 kV (10 A)^?1 of proton beam, are accelerated by this potential onto the stainless steel vacuum chamber wall where a variety of adsorption/desorption phenomena may be stimulated. Desorbed molecules are fed back into the residual gas resulting in the possibility of creating an unstable run-away pressure increase which can finally destroy the stored beam. This danger depends on the local parameters of the vacuum system, and most critically, on the state of the vacuum chamber surface.The paper summarises recent observations in this field and describes the results of investigations aimed at producing cleaner surfaces in vacuum chambers. These investigations, which have included high temperature bakeouts, ion bombardment scrubbing by glow discharge, oxidation, etc, will show that under favourable conditions, net negative desorption coefficients can be obtained which turn the Intersecting Storage Rings into a large ion pump. Results, supported and compared with measurements using Auger and Sims surface analysis, are presented mainly from the empirical point of view with the emphasis on practical implications for machines where the surfaces of the vacuum system are subjected to ion bombardment.     

Photoionization studies of atoms and molecules using synchrotron radiation

Photoionization studies of free atoms and molecules have undergone considerable development in the past decade, in large part due to the use of synchrotron radiation. The tunability of synchrotron radiation has permitted the study of photoionization processes near valence- and core-level ionization thresholds for atoms and molecules throughout the Periodic Table. A general illustration of these types of study will be presented, with emphasis on a few of the more promising new directions in atomic and molecular physics being pursued with synchrotron radiation.     

Calculations for the availability of photoneutron using synchrotron radiation

The availability of the neutrons due to photonuclear reactions has been discussed by using synchrotron radiation with the beryllium targets. The superconducting wiggler with the magnetic field of approximately 10 T, which is installed into an 8 GeV class storage ring, can emit intense and high-energy photons to produce neutrons. By using MCNPX, the simulations were performed for the conceptual design of the neutron beamline to estimate the available intensity and to investigate the shield conditions. The results were discussed in comparison with other research reactors.     

Residual stresses in particle-reinforced ceramic composites using synchrotron radiation

Residual stresses were determined in particle-reinforced ceramic composites using synchrotron based x-ray diffraction. The baseline Si₃N₄ and the Si₃N₄-TiN composites were processed by turbomilling, pressure casting, and isopressing. They were then continuously sintered to full density, under a pressureless, flowing nitrogen atmosphere. The flexural strength, fracture toughness, and residual stress were measured for as-machined samples and following quenching in water from 1000°C, 1100°C, and 1200°C. The residual stresses for both the baseline Si₃N₄ and the Si₃N₄-TiN composites were determined from the (441) and (531) reflections, by applying the 2-sin² method. The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the mechanical properties of each material. In both the baseline Si₃N₄ and Si₃N₄-TiN composites, after thermal shocking, the compressive residual stresses were developed in directions both parallel and perpendicular to the sample surface. The residual compressive stresses for the Si₃N₄-TiN composites were much higher than the baseline Si₃N₄. As a result, both fracture toughness and flexural strength of the Si₃N₄-TiN composites were improved. In addition, the addition of the TiN appears to improve both the strength and toughness of the baseline Si₃N₄.     

Focusing of synchrotron radiation

Focusing of the synchrotron radiation from a storage ring using a convex lens is geometrically analysed and tested. The source radiation is supposed to have a bivariate normal distribution in its phase space both vertically and horizontally. Its modification caused by a lens is calculated as a function of distances among the source, the lens and the image plane. It is shown that the horizontal image becomes sharpest when the source is focused on the image plane. The vertical image, however, is not sharpest under this condition. The vertical distribution has more information than the horizontal; we can derive the orbit dependence of the vertical profile and the angle distribution of the radiation changing focusing.     

Three-dimensional microtomography with synchrotron radiation using planar integral projection data

An investigation of the possibility of reconstructing a three-dimensional object on a microscopic scale using the planar-integral technique with a synchrotron radiation source is presented. It is shown that the proposed planar-integral technique offers a realistic approach for 3-D tomography on a microscopic scale aimed towards spatial resolution within the μm range.     

UHV-type synchrotron radiation reflectometer

We designed a reflectometer for grazing-incidence X-ray measurements with a rotational feedthrough consisting of three welded bellows and a circle-type goniometer. This apparatus does not require differential pumping and is suitable for ultra-high-vacuum applications. With this reflectometer, we successfully performed high-precision synchrotron radiation X-ray reflectivity measurements on a thin iron-film under an ultra-high-vacuum condition.     

Characterization of damage structure in ion implanted SiC using high photon energy synchrotron ellipsometry

The optical properties of ion implantation induced disorder in SiC have been investigated in the photon energy range of 5-9 eV using spectroscopic ellipsometry (SE). The most characteristic interband transitions of SiC are located between 5 and 8 eV. This photon energy region is extremely important for the sensitive characterization of lattice order in SiC. The dielectric function of the disordered layer has been calculated taking into account the surface overlayer consisting of oxide and roughness using complementary characterization tools. The dielectric function of the damaged region has been analyzed using different techniques like second derivative analysis and effective medium approximation (EMA) based on reference dielectric functions. The disorder determined by SE has been verified by Rutherford backscattering spectrometry combined with channelling (RBS/C). Using derivative lineshape analysis combined with simulations, the track size can be estimated. The results can give insight into the effect of the decreasing characteristic size of the unchanged crystalline regions on the optical properties. We created near-surface damage using heavy ions, since the penetration depth of light at photon energies around the direct interband transitions is very small (in the range of 10 nm). We used 100-keV Xe at fluences ranging from a slight damage to full amorphization (between 2.0 × 10¹³ cm^− 2 and 1.6 × 10¹⁴ cm^− 2).     

Luminescence studies of Ce:YAG using vacuum ultraviolet synchrotron radiation

Photoluminescence spectrum of Ce:YAG single crystal was studied employing vacuum ultraviolet (VUV) synchrotron radiation. Intrinsic absorption edge at about 52,000 cm⁻¹ was observed in the absorption spectrum. From the VUV excitation spectrum, the energy of the highest d-component of 53,191 cm⁻¹ (188 nm) for the Ce³⁺ ions in YAG was obtained at 300 K. The disappearance of the third 5d level at 37,735 cm⁻¹ (265 nm) in absorption and excitation spectra in our samples may be due to the impurity Fe³⁺ ions absorption.     

Dispersive characteristics of one-dimensional Fresnel zone plates measured using synchrotron radiation

This paper presents the dispersive characteristics of a Ta film one-dimensional Fresnel zone plate (FZP) measured using synchrotron radiation as a light source. Spectral spatial distributions of the FZP linear monochromator output beam are recorded on an X-ray film. The observed distributions are well explained by the FZP's chromatic aberration. The spectral bandwidth of the monochromator output beam decreases proportional to the slit width and saturates at approximately the value limited by the size of the focused image of the synchrotron radiation source.