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| X-ray and gamma-ray wavelengths are measured with respect to optically-calibrated crystal spacings by absolute goniometry. The systematics of x-ray transition energies provide an important testing ground for the theoretical calculations of the energy levels of atoms containing inner-shell vacancies. Specific transitions play a prominent role in analysis of crystal structures and the determination of fundamental physical constants. Similarly, in the case of gamma-ray wavelengths there are several cases in which their numerical values have direct impact on elementary particle properties, such as the neutron mass, as well as the wider range of transition families that reflect the intimate connection between nuclidic masses and gamma-ray derived binding energy estimates. |
Vacuum Double-Crystal Spectrometer:A vacuum double-crystal spectrometer with high-resolution angle encoders is used to measure x-ray wavelengths with energies from 1 keV to 20 keV to high precision. Absolute accuracy is obtained by a calibration chain linking the lattice spacing of the diffraction crystals to the definition of the meter via x-ray optical interferometry. The measurement sequence involves measuring profile scans in the dispersive and non-dispersive geometries and determining the Bragg angle, and hence wavelength, from the measured angle between plus- and minus-scan diffraction features. Wavelength measurements complement efforts of a new wavelength tables project as well as provide secondary energy standards for other experiments both here at NIST and around the world. An example of the latter is a measurement of |
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Figure 1. Photo of index-of-refraction experiment that employs a thin silicon lamella in the second crystal position (larger view 201 k). |
One of the largest systematic corrections in the crystal diffraction method involves the angular shift due to the index of refraction of the crystal. Different schemes are being undertaken in our laboratory to measure directly the index of refraction at soft x-ray energies. Each employs diffraction from more than one set of planes of a crystal that has been specially aligned and cut. The experiments are designed to maximize the refraction effect and use knowledge of the relative orientation of diffraction planes as an internal gauge. One such experiment is shown here that employs a thin silicon lamella in the second crystal position. It has been cut such that the Si(220) planes are both parallel and perpendicular to the surface. By measuring the diffraction profile from this crystal both in transmission and reflection, one has a determination of the index of refraction correction at that given wavelength: it is the deviation of the two measured profiles from 90 degrees since there is no refractive shift in the transmission case. | ||
E > 20 keV, two-crystal transmission spectrometerWe have developed a versatile two-crystal transmission spectrometer for x-ray measurements above 20 keV. The instrument is in a room containing a 400 keV, 20 mA tungsten x-ray unit. The primary purpose of this x-ray unit is radiography, but it can be used 1) as a source of characteristic x-ray transitions, 2) as a bremsstrahlung source for absorption measurements, and 3) as an excitation source for producing fluorescence radiation. Other lower energy x-ray sources such as Cu, Mo, and Ag can be set up at this instrument to provide the radiation that may be needed for a particular problem.The spectrometer has several unique features which make it particularly well suited for precision wavelength measurements of characteristic lines, for absorption edge measurements, and for crystal diffraction studies. These features include: 1) diffraction crystals whose lattice spacing is known in terms of the meter with a relative uncertainty of 5 × 10-8 (See lattice comparison), 2) sensitive angle interferometers with a resolution of 1 × 10-9 rad, and 3) calibration of the angle interferometers based on measurements with an optical polygon. 
Schematic of NIST high-energy two crystal facility The layout of the experimental facility is shown in the figure. The spectrometer, collimators, and detector are located on a 1.2 m × 3 m steel table which is supported on air springs for vibration isolation. The spectrometer is stationary, the source rotates around the first crystal, and the detector rotates around the second crystal. There are Soller collimators between the source and first crystal, between the two crystals (not shown), and between the second crystal and the detector. The spectrometer control electronics are located in an adjacent room that also houses the tungsten x-ray tube power supply. A personal computer running LabVIEW software is the heart of the data logging system. Precision wavelength measurements for energies > 20 keV are the main thrusts of the scientific program associated with this facility. For example, samples of high-Z elements such as Pb, Th, and U have been excited by florescence and the emitted characteristic transitions measured. Absorption edge profiles for Er and Pb have been recorded by placing foils in the bremsstrahlung radiation emitted by the tungsten x-ray tube. Although wavelength measurements are emphasized, the facility is quite flexible and we do not hesitate to configure the source, inter-crystal, and detector regions to accommodate a particular measurement. In addition, the usual flat crystals used in transmission can be replaced with other crystal geometries. For example, in a recent measurement of a diffraction grating, a copper x-ray source was used with channel cut crystals, a diffraction grating between the crystals, and a fixed detector behind the second crystal. Finally, this spectrometer and the gamma-ray spectrometer at the ILL are very similar, providing us with a NIST-based test bed for the gamma-ray measurements. |
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