ALBERT

Notes: An X-ray spectrometer for simultaneous position, intensity and energy determinations suitable for Laue diffraction applications is described. The foil-mask spectrometer consists of a series of metal foils of varying composition and thickness which are used to modulate the energy distribution of an incident X-ray source. Three modes of operation are described: a high-resolution spectrometer for measurement of nearly monochromatic X-rays, an intensity discriminator for partitioning the intensity from a small number of spatially overlapped monochromatic X-ray sources, and a low-resolution spectrometer for polychromatic X-rays with broad spectral features. The first mode of operation is designed to allow the energy of monochromatic Laue reflections to be measured with a resolution suitable for determination of unit-cell parameters. The second mode of operation is designed to allow the intensity of each component in a spatial region containing overlapping orders or spatially overlapped reflections to be discriminated for use in refinements or space-group assignment. The third mode of operation is described for completeness. The theory behind each mode of operation is described. The energy resolution of the spectrometer improves with the square root of the intensity of the incident beam. It also increases linearly with the change in energy with respect to transmission efficiency of a particular foil. In theory, the resolution of the spectrometer can readily exceed 50 eV over a wide range of energies depending on the foils used and the incident X-ray photon flux. Determinations of the energies of Mo Kα and Cu Kα radiation using a first-generation ten-foil spectrometer gave values of 17.5 ± 0.1 and 8.08 ± 0.05 keV, respectively. Treatment of random error shows good correspondence with a Poisson model. The use of this spectrometer is demonstrated using a sample of tetraphenylphosphonium tetrachlorooxomolybdenum(V). Comparison of predicted and observed energies shows good agreement over a wide range of energies. The ratio of predicted to measured energy for the first 50 measurements was 0.9918±0.0344. Up to three components of a position having harmonic overlap were separated. This work demonstrates the feasibility of using Laue diffraction to completely determine the crystal structure of a molecule without recourse to monochromatic methods.

Notes: The use of energy-resolved area detection of Laue diffraction patterns for the determination of unit-cell parameters and systematic absences is demonstrated. Seven different crystals having previously known unit cells were re-examined using Laue diffraction methods. These crystals included four different crystal systems including cubic, orthorhombic, tetragonal and monoclinic cells. The crystals had cell sizes ranging from 179.4 to 4588.3 Å3. Comparison of known and re-determined cells showed good agreement (ratio of known to measured cells = 0.987 ± 0.18). A single procedure was suitable for all unit-cell determinations. The accuracy of the method is presently limited by the quality of the available energy measurements. Some of the crystals represent space groups containing systematic absences normally obscured by harmonic overlap when using the Laue method. These include absences due to 21 screw axes (h, k or 1 = 2n + 1) and cell centering (h + k = 2n + 1). All systematic absences were identified using a combination of multiple linear regression with either stepwise elimination or stepwise inclusion and an F test for assignment of systematic absence. The methods are discussed in detail and simulations are used to evaluate critical tolerances for future systems.

Notes: Charge-injection devices (CIDs) are versatile detectors having a number of features which recommend them for use in the imaging of X-ray diffraction patterns. They have a flexible nondestructive readout allowing for analysis of image quality during data collection and rapid readout of selected portions of the device. CIDs have full-well capacities in the range of 106 charge carriers giving them a high dynamic range for both direct and indirect imaging of X-rays. CIDs have peak quantum efficiencies in the optical region over 50% allowing for their incorporation into indirect detection systems. Rapid random single-pixel address allows for their use as single X-ray photon counters with energy discrimination. Three types of position-sensitive detectors for X-rays have been developed using CIDs. Two CID formats, the CID 17PPRA (388 × 256) and the CID 38SG (512 × 512), were incorporated into systems performing indirect imaging, direct imaging and single X-ray photon counting with energy discrimination. Indirect images of the Laue diffraction patterns from tetraphenylphosphonium tetrachlorooxomolybdenum(V) and natural MoS2 were collected using a phosphor sheet to convert X-rays into optical photons which were detected with the CID 38SG. Directly detected images of spots from the Laue diffraction pattern of MoS2 were recorded with the CID 17PPRA. Single photon counting with energy discrimination is demonstrated with the CID 17PPRA using a reflection from the Laue diffraction pattern of MoS2. Useful information could be obtained from a single pixel at read rates over 7 kHz. Complete energy-dispersive analysis suitable for determination of space groups from Laue diffraction is currently limited due to incomplete charge collection and/or split events.