M. Krumrey

Surface layer determination for the Si spheres of the Avogadro project

For the accurate determination of the Avogadro constant, two 28Si spheres were produced, whose macroscopic density, in addition to other values, must be determined. To make a contribution to the new definition of the kilogram, a relative standard uncertainty of less than 2 ? 10?8 has to be achieved. Each silicon surface is covered by a surface layer (SL). Consequently, correction parameters for the SL are determined to be applied to the mass and volume determination of the enriched spheres. With the use of a large set of surface analysing techniques, the structure of the SL is investigated. An unexpected metallic contamination existing on the sphere surface enlarges the uncertainty contribution of the correction parameters above the originally targeted value of 1 ? 10?8. In the framework of this investigation this new obstacle is resolved in two ways. A new combination of analytical methods is applied to measure the SL mass mSL and the thickness dSL, including this new contamination, with an uncertainty of u(mSL) = 14.5??g and 14.4??g, respectively, and u(dSL) = 0.33?nm and 0.32?nm for the 28Si spheres AVO28-S5 and AVO28-S8, respectively.In the second part of the work, the chemical composition of these metallic contaminations is found to be Cu, Ni and Zn silicide compounds. For the removal of this contamination, a special procedure is developed, tested and applied to the spheres to produce the originally expected surface structure on the spheres. After the application of this new procedure the use of x-ray reflectometry directly at the spheres will be possible. It is expected to reduce the uncertainty contribution due to the SL down to 1 ? 10?8.

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%.

Quantum efficiencies of gold and copper photocathodes in the VUV and X-ray range

The quantum efficiency, η, of gold and copper photocathodes has been measured in the photon-energy range from 50 eV to 10 keV. The experiments were performed with monochromatized synchrotron radiation at the radiometry laboratories of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage rings BESSY I and BESSY II. Relative standard uncertainties of η lower than 0.036 were achieved using a cryogenic electrical-substitution radiometer as the primary detector standard. The samples were cleaned, stored and examined in an ultra-high-vacuum chamber. The quantum efficiency of cleaned gold and copper was found to be reproducible within 2 parts in 102 to 3 parts in 102. A simple model assumes to be proportional to the product of absorption coefficient, μ, and photon energy, hν. The relative course of the absorption coefficient was also measured with relative standard uncertainties below 0.04. As proportionality was only locally fulfilled, a more sophisticated model for quantum efficiency was developed.

Absolute radiant power measurement for the Au M lines of laser-plasma using a calibrated broadband soft X-ray spectrometer with flat-spectral response

CEA implemented an absolutely calibrated broadband soft X-ray spectrometer called DMX on the Omega laser facility at the Laboratory for Laser Energetics (LLE) in 1999 to measure radiant power and spectral distribution of the radiation of the Au plasma. The DMX spectrometer is composed of 20 channels covering the spectral range from 50 eV to 20 keV. The channels for energies below 1.5 keV combine a mirror and a filter with a coaxial photo-emissive detector. For the channels above 5 keV the photoemissive detector is replaced by a conductive detector. The intermediate energy channels (1.5 keV < photon energy < 5 keV) use only a filter and a coaxial detector. A further improvement of DMX consists in flat-response X-ray channels for a precise absolute measurement of the photon flux in the photon energy range from 0.1 keV to 6 keV. Such channels are equipped with a filter, a Multilayer Mirror (MLM), and a coaxial detector. We present as an example the development of channel for the gold M emission lines in the photon energy range from 2 keV to 4 keV which has been successfully used on the OMEGA laser facility. The results of the radiant power measurements with the new MLM channel and with the usual channel composed of a thin titanium filter and a coaxial detector (without mirror) are compared. All elements of the channel have been calibrated in the laboratory of the Physikalisch-Technische Bundesanstalt, Germanys National Metrology Institute, at the synchrotron radiation facility BESSY II in Berlin using dedicated well established and validated methods.

Measurement of the mass energy-absorption coefficient of air for x-rays in the range from 3 to 60 keV

For the first time the absolute photon mass energy-absorption coefficient of air in the energy range of 10 to 60 keV has been measured with relative standard uncertainties below 1%, considerably smaller than those of up to 2% assumed for calculated data. For monochromatized synchrotron radiation from the electron storage ring BESSY II both the radiant power and the fraction of power deposited in dry air were measured using a cryogenic electrical substitution radiometer and a free air ionization chamber, respectively. The measured absorption coefficients were compared with state-of-the art calculations and showed an average deviation of 2% from calculations by Seltzer. However, they agree within 1% with data calculated earlier by Hubbell. In the course of this work, an improvement of the data analysis of a previous experimental determination of the mass energy-absorption coefficient of air in the range of 3 to 10 keV was found to be possible and corrected values of this preceding study are given.

Absolute Responsivity of Silicon Photodiodes in the X‐ray Range

The responsivity of several types of silicon photodiodes has been determined in the soft X‐ray and X‐ray range by comparison to two primary detector standards using monochromatized synchrotron radiation at three different beamlines of the PTB radiometry laboratory at BESSY II. The linearity and homogeneity of the responsivity have also been investigated. While a supralinearity was found for some diodes, pronounced inhomogeneities were found for other diodes especially for photon energies above the Ag K absorption edge.

Applications of non-periodic multilayer optics for high-resolution x-ray microscopes below 30 keVa)

Multilayer mirrors with enhanced bandwidth were developed with special performances for dense plasma diagnostics and mainly for high spatial resolution x-ray imaging. The multilayer coatings are designed to provide broadband x-ray reflectance at low grazing incidence angles. They are deposited onto toroidal mirror substrates. Our research is directed at the development of non-periodic (depth graded) W∕Si multilayer specifically designed for use in the 1 to 30 keV photon energy band. First, we present a study for a 5 to 22 keV x-ray spectral window at 0.45° grazing angle. The goal is to obtain a high and constant reflectivity. Second, we have modeled a broadband mirror coating for harder x-rays in the range from 10 to 30 keV, with a non-periodic structure containing 300 W∕SiC layers with periods in the range from 0.8 to 4 nm, designed for 0.35° grazing incidence angle.

High‐accuracy X‐ray detector calibration based on cryogenic radiometry

Cryogenic electrical substitution radiometers (ESRs) are absolute thermal detectors, based on the equivalence of electrical power and radiant power. Their core piece is a cavity absorber, which is typically made of copper to achieve a short response time. 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. 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 is used to calibrate silicon photodiodes against the ESR for photon energies up to ...