Albert Henins

Remeasurement of γ-Ray reference lines

Abstract Over the last several years, we have established a new and significantly improved measurement chain connecting γ-ray lines with visible reference standards. This chain has three links: First, a Si lattice repeat distance is determined by combined X-ray and optical interferometry. Second, this crystal calibration is transferred to other specimens suited to γ-ray diffraction. Finally, these crystals are used in a transmission double-crystal instrument to determine γ-ray wavelengths via the Bragg-Laue equation. To obtain the required precision and accuracy, the spectrometer is fitted with angle interferometers of considerable sensitivity (∼0.05 marcsec) and these are calibrated by summing to closure the external interfacial angles of a 72-sided optical polygon. This report focuses on the apparatus and methodology of the third step but includes descriptions of the first two also. Results are presented from tests of closure using cascade-crossover relations and inter-order comparisons. Energies for γ-transitions in 198Au, 192Ir, 170Tm, 169Yb, together with the X-ray transition W Kα1 are included. γ-Ray wavelength accuracies are estimated to be near 0.5 ppm.

A curved crystal spectrometer for energy calibration and spectral characterization of mammographic x‐ray sources

Clinical efficacy of diagnostic radiology for mammographic examinations is critically dependent on source characteristics, detection efficiency, image resolution and applied high voltage. In this report we focus on means for evaluation of source-dependent issues including noninvasive determination of the applied high voltage, and characterization of intrinsic spectral distributions which in turn reflect the effects of added filtration and target and window contamination. It is shown that a particular form of x-ray curved crystal spectrometry with electronic imaging can serve to determine all relevant parameters within the confines of a standard clinical exposure.

Enhanced x-ray resolving power achieved behind the focal circles of Cauchois spectrometers

Maintaining high resolving power is a primary challenge in hard x-ray spectroscopy of newly developed bright and transient x-ray sources such as laser-produced plasmas. To address this challenge, the line widths in x-ray spectra with energies in the 17 keV to 70 keV range were recorded by positioning the detectors on and behind the focal circles of Cauchois type transmission-crystal spectrometers. To analyze and understand the observed line widths, we developed a geometrical model that accounts for source broadening and various instrumental broadening mechanisms. The x-ray sources were laboratory Mo or W electron-bombarded anodes, and the spectra were recorded on photostimulable phosphor image plates. For these relatively small x-ray sources, it was found that when the detector was placed on or near the focal circle, the line widths were dominated by the effective spatial resolution of the detector. When the detector was positioned beyond the focal circle, the line widths were determined primarily by source-size broadening. Moreover, the separation between the spectral lines increased with distance behind the focal circle faster than the line widths, resulting in increased resolving power with distance. Contributions to line broadenings caused by the crystal thickness, crystal rocking curve width, geometrical aberrations, and natural widths of the x-ray transitions were in all cases smaller than detector and source broadening, but were significant for some spectrometer geometries. The various contributions to the line widths, calculated using simple analytical expressions, were in good agreement with the measured line widths for a variety of spectrometer and source conditions. These modeling and experimental results enable the design of hard x-ray spectrometers that are optimized for high resolving power and for the measurement of the x-ray source size from the line widths recorded behind the focal circle.

The deuteron binding energy and the neutron mass

Abstract A new value for the deuteron binding energy of S(d)=2.38817007(42)×10−3 u is reported based on an absolute wavelength determination of the 2.2 MeV n–p capture gamma-ray using a crystal diffraction spectrometer. A new more precise value for the neutron mass mn=1.00866491637(82) u is obtained by summing this binding energy and the 2 H− 1 H mass difference.

A high energy resolution X-ray spectroscopy synchrotron radiation beamline for the energy range 800–5000 eV

A beamline for X-ray spectroscopy of atomic and molecular gases and condensed matter has been designed and installed at the National Synchrotron Light Source. The beamline is UHV compatible to allow windowless operation for improved flux at low photon energies. A double axis crystal monochromator is employed with a collimating premirror and a focusing postmirror. Paris of beryl, quartz or silicon crystals define an energy band width of <0.4 eV at an arbitrary energy above 0.8 keV. The premirror acts as a tuneable low-pass filter to minimize heat loading on the first monochromator crystal. At the present operating parameters of NSLS, a flux of 109–1013 photons/s of highly monochromatic X-rays can be focused onto a 1 mm diameter spot. Initial experimental results are presented.

Position-sensitive X-ray detector

Abstract We have constructed a single anode wire position sensitive gas proportional detector with a fwhm of 80 μm for a highly collimated monochromatic beam of 8 keV photons over a useful range of 38 mm, operated with 600 Torr Xe/CH4. The transfer function of the detector has been measured and explained by the theoretical charge distribution induced on the cathode.

The GAMS4 flat crystal facility

Abstract A high-resolution flat-crystal gamma-ray facility, GAMS4, has been constructed at the high-flux reactor at the Institut Laue-Langevin (ILL), Grenoble, France. The facility is located at the exit of a through-tube equipped to place sources in a neutron flux of ≈5.5×10 14 n cm −2 s −1 . Considerable care has been exercised to provide a precision gamma-ray metrology station including vibration isolation, temperature control, environmental monitoring, background suppression, and remote and automatic spectrometer control. The diffraction angles are controlled and measured interferometrically and absolute angle calibration is established with a relative uncertainty of ≈3×10 −7 . The crystals are nearly perfect specimens of Si and Ge whose lattice spacings are measured in meters and have a relative uncertainty of ≈5×10 −8 . The resolution obtained with this facility closely approaches dynamical diffraction predictions. Examples of measurements performed using this facility include (1) absolute wavelengths standards and neutron binding energies, (2) lifetimes of nuclear exited states, (3) determination of interatomic potentials, and (4) crystal structure factors. GAMS4 is a scheduled ILL instrument with beamtime being awarded through the ILL proposal process.

A high-energy x-ray spectrometer diagnostic for the OMEGA laser

A new x-ray diagnostic has been commissioned at the OMEGA laser facility at the University of Rochester. It is a transmission curved crystal spectrometer designed primarily to characterize the hot-electron energy distribution of laser generated plasmas by registering the continuum x-ray spectrum produced by these hot plasmas from 12 to 60 keV. The diagnostic package is assembled in a linear configuration to ride in a standard instrument insertion module. The instrument consists of a nosecone with a blast shield, spectrometer, electronic imager, drive electronics, and battery. The instrument is connected to the external diagnostic processor and control unit by trigger and data fiber optic cables. Time integrated spectra from various targets have been registered with high sensitivity from single shots of the OMEGA laser.

Certification of Standard Reference Material 660b

The National Institute of Standards and Technology (NIST) certifies a suite of Standard Reference Materials (SRMs) to address specific aspects of the performance of X-ray powder diffraction instruments. This report describes SRM 1976b, the third generation of this powder diffraction SRM. SRM 1976b consists of a sintered alumina disc, approximately 25.6 mm in diameter by 2.2 mm in thickness, intended for use in the calibration of X-ray powder diffraction equipment with respect to line position and intensity as a function of 2 θ -angle. The sintered form of the SRM eliminates the effect of sample loading procedures on intensity measurements. Certified data include the lattice parameters and relative peak intensity values from 13 lines in the 2 θ region between 20° and 145° using Cu Kα radiation. A NIST-built diffractometer, incorporating many advanced and unique design features was used to make the certification measurements.

Noninvasive high-voltage measurement in mammography by crystal diffraction spectrometry.

Wavelength dispersive crystal diffraction spectrometry has been applied to the measurement of the accelerating voltage on an x-ray source in a prototype experiment in the mammographic source. The results indicate that this noninvasive approach can yield determinations of such voltages within 0.1 kV, a level of imprecision that appears adequate for high-level standardization of such potentials.