Quantum Metrology Division

Technical Highlights

• X-Ray Fluorescence/Auger Electron Coincidence Spectroscopy Using Tunable Synchrotron Radiation. In a joint effort with colleagues from Argonne and Daresbury Laboratories, we measured coincidence spectra between K fluorescent x-rays and L_2,3-M_2,3M_2,3 Auger electrons in atomic argon following K-shell excitation by tunable synchrotron radiation. Coincidence spectroscopy has again proven to be a powerful tool in picking out distinct processes in order to understand the decay dynamics of atomic relaxation after creation of a deep core hole. In the figure we show electron spectra recorded in the energy range of the L_2,3-M_2,3M_2,3 Auger transitions. Panel a of Figure 3 shows a normal spectrum, measured at an excitation energy 32.7 eV below the K-edge. This spectrum results from direct photoionization of the L_2,3 subshells and has been extensively studied both experimentally and theoretically. In Figure 3b, an x-ray/electron coincidence spectrum is shown, recorded using excitation energy 10 eV above the K-edge. The same structures appear at the same electron energies, but the relative intensities of the lines are modified when L_2,3-vacancies are produced via radiative decay of initial K-vacancies. The spectrum in Figure 3c results from resonant excitation of the K-shell electron to the 4p Rydberg orbital, which primarily acts as a spectator electron in the de-excitation of the K-vacancy. The lines of this spectrum display strong resonance shifts, mainly due to the change in binding energy of the 4p electron when going from a singly- to doubly-ionized atomic core. The lines are also broadened by multiplet effects through interaction with the 4p electron, and there are additional structures on the low energy side due to shake-up of the 4p electron. In addition to fundamental studies of atomic inner-shell processes, electron/x-ray coincidence spectroscopy using tunable synchrotron radiation is potentially useful for materials science applications.
  
  

  
  
  Figure 3. Argon L_2,3-M_2,3M_2,3 Auger electron spectra: A - normal spectrum recorded 32.7 eV below the K-edge; B - electron/x-ray coincidence spectrum recorded 10 eV above the K-edge; C - electron/x-ray coincidence spectrum recorded on the 1s → 4p resonance.

• Primary Thin film Metrology by X-Ray Diffraction. X-ray reflectometry has long been a method of choice for obtaining structural information about surfaces, thin films, and multilayers. Examples include the measurement of optical constants, surface crystallography, surface roughness, etc. Long-standing interest in diffraction physics led us to look for a unified description of x-ray reflection from crystals and multilayers. We have developed design procedures, precision deposition facilities, and x-ray diffractometry techniques needed to support this program. In the course of this effort, theoretical formulation, atomic optical data, and simulation software were improved. As a result of this program, we acquired techniques that precisely evaluate film thickness, density, composition, and interface structures. It has recently become clear that these developments are well poised to address specific needs of the semiconductor industry for thin film reference materials and interfacial characterization.
• Electronic Image Registration in the Mammographic Spectrograph and its Rapid Commercialization. Previously, we designed and patented a Laue-case spectrometer for determination of the high voltage on mammographic x-ray sources. A second patent was granted on 1/15/95 for a curved-crystal variant. In addition to the high voltage on the x-ray tube, the device also provides spectral information and, through an on-axis pinhole camera, focal spot imaging. This year an exclusive license has been granted to the Radcal Corporation (Monrovia, CA) for commercial development. Working under a CRADA with Radcal, we have improved design and functionality. In place of photographic film, a CCD dental sensor has been mounted in the image plane of the spectrometer, providing real-time, solid-state registry of spectra. This sensor, from the Schick Corporation, has ideally-sized 48 m pixels, a large area format (37 mm × 25 mm), phosphor conversion of x-rays, and using multi-pinned phasing (MPP) it can operate inexpensively at room temperature with adequate sensitivity. The spectrometer has been shortened by curving the analyzing crystal to a six inch radius of curvature. The spectral throughput of a curved silicon crystal has been doubled with an air-abrasive treatment which induces a lattice spacing gradient. A variety of other crystals have been tested and categorized by image quality, d-spacing, integrated reflectivity, elasticity, and cost. The spectrometer is mounted on a translation stage in a tiltable shroud allowing for easy setup and alignment in a clinical setting. The imaging spectrometer together with a notebook computer for display of spectra, is now of a size and weight that can be easily carried into clinical settings by regulatory personnel.