Resonance between beams of x-ray waves and electronic transitions from bound atomic orbitals leads to a phenomenon known as anomalous scattering. This effect can be exploited in x-ray crystallographic studies on biological macromolecules by making diffraction measurements at selected wavelengths associated with a particular resonant transition. In this manner the problem of determining the three-dimensional structure of thousands of atoms is reduced to that of initially solving for a few anomalous scattering centers that can then be used as a reference for developing the entire structure. This method of multiwavelength anomalous diffraction has now been applied in a number of structure determinations. Optimal experiments require appropriate synchrotron instrumentation, careful experimental design, and sophisticated analytical procedures. There are rich opportunities for future applications.

https://science.sciencemag.org/content/sci/254/5028/51.full.pdf

Determination of Macromolecular Structures from Anomalous Diffraction of Synchrotron Radiation

Accurate x-ray scattering techniques to measure the physical properties of dense plasmas have been developed for applications in high energy density physics. This class of experiments produces short-lived hot dense states of matter with electron densities in the range of solid density and higher where powerful penetrating x-ray sources have become available for probing. Experiments have employed laser-based x-ray sources that provide sufficient photon numbers in narrow bandwidth spectral lines, allowing spectrally resolved x-ray scattering measurements from these plasmas. The backscattering spectrum accesses the noncollective Compton scattering regime which provides accurate diagnostic information on the temperature, density, and ionization state. The forward scattering spectrum has been shown to measure the collective plasmon oscillations. Besides extracting the standard plasma parameters, density and temperature, forward scattering yields new observables such as a direct measure of collisions and quantum effects. Dense matter theory relates scattering spectra with the dielectric function and structure factors that determine the physical properties of matter. Applications to radiation-heated and shock-compressed matter have demonstrated accurate measurements of compression and heating with up to picosecond temporal resolution. The ongoing development of suitable x-ray sources and facilities will enable experiments in a wide range of research areas including inertial confinement fusion,more » radiation hydrodynamics, material science, or laboratory astrophysics.« less

X-ray Thomson scattering in high energy density plasmas

We describe the application of photostimulable storage phosphor imaging plates to autoradiography of samples labeled with 32P, 14C and 35S Imaging plates can detect a 32P-labeled sample at an exposure level of 1 disintegration/mm2, and 14C and 35S at 25 disintegrations/mm2 Compared to X-ray film, imaging plates offer increased sensitivity of 15- to 250-fold for 32P and 20- to 100-fold for 14C and 35S Resolution is in the 03 mm range and is therefore suitable for any gel or blot application

Autoradiography using storage phosphor technology.

Ribonuclease H digests the RNA strand of duplex RNA.DNA hybrids into oligonucleotides. This activity is indispensable for retroviral infection and is involved in bacterial replication. The ribonuclease H from Escherichia coli is homologous with the retroviral proteins. The crystal structure of the E. coli enzyme reveals a distinctive alpha-beta tertiary fold. Analysis of the molecular model implicates a carboxyl triad in the catalytic mechanism and suggests a likely mode for the binding of RNA.DNA substrates. The structure was determined by the method of multiwavelength anomalous diffraction (MAD) with the use of synchrotron data from a crystal of the recombinant selenomethionyl protein.

Structure of ribonuclease H phased at 2 A resolution by MAD analysis of the selenomethionyl protein.

The erasable phosphor imaging plate developed for medical radiography has found new uses in the laboratory. X-ray diffraction, protein crystallography and autoradiography have all benefited from this technology transfer from the clinic.

Imaging plate illuminates many fields

A conceptually new integrating area (250 mm×200 mm or larger) detector system, originally developed for diagnostic radiography, has been used for synchrotron radiation experiments. In this detector system, an X-ray image is temporarily stored as a distribution of F-centers in a photostimulable phosphor (BaFBr:Eu 2+ ) screen. The stored image is read out by measuring the intensity of fluorescence ( λ ∼390 nm) stimulated by a HeNe laser beam scanned over the surface of the screen. The detector has 100% detective quantum efficiency for 8–17 keV X-rays, a spatial resolution better than 0.2 mm (fwhm) in both directions, a dynamic range of 1:10 5 and no counting rate limitation. Diffraction patterns from muscle and a protein crystal were recorded in several tens times less exposure than that of high-sensitivity X-ray films.

A new type of X-ray area detector utilizing laser stimulated luminescence

We have developed two kinds of imaging plate neutron detectors (IP-ND), where the neutron converters, 6 Li (tentatively, Nat Li was used) and Gd were mixed with photostimulated luminescence (PSL) materials on a flexible plastic support, and the dynamic range and spatial resolution of our IP-ND were successfully obtained as 1:10 5 and less than 0.2 mm, respectively, which are comparable to the ones of X-ray. The distinctive features of the IP-ND, where the IP-NDs of different neutron detection efficiency and converters were positively used, were found and discussed. The application of the IP-ND for neutron radiography was demonstrated.

An imaging plate neutron detector

A new readout system for a BaFBr: Eu2+ photostimulable phosphor screen (imaging plate) was constructed by modifying a drum scanner, with a design optimized for X-ray diffraction and scattering applications. An effort was made to achieve a high detective quantum efficiency below 20 keV, a small pixel size (25 μm × 25 μm), a low quantization noise (0.22%) using 12-bit A/D converters, and the capability to cover an inherent dynamic range (1:105) of the photostimulated luminescence by using two photomultiplier tubes. This system is being used in several synchrotron radiation experiments: Laue diffraction of protein crystals, small angle diffraction from a single muscle fiber, powder diffraction from crystals in a diamond anvil cell, and time-resolved small-angle X-ray scattering from a synthetic polymer during stretching.

Design and performance of an imaging plate system for X-ray diffraction study

An integrating x-ray area detector that operates on the basis of laser-stimulated luminescence was used in a diffraction study of muscle contraction. The area detector has a dynamic range of 1 to 10(5), a sensitivity about 60 times greater with approximately 1/300 as much fog background as x-ray film. It is erasable and reusable but, like film, can integrate at a practically unlimited counting rate. The high sensitivity and wide dynamic range of the detector resulted in a sufficient reduction in the exposure time to make possible the recording of a clear x-ray diffraction pattern, with up to 2.0-nanometer axial spacing, from a contracting frog skeletal muscle in as little as 10 seconds with synchrotron radiation. During the isometric contraction of the muscle, most of the actin diffraction lines increased in intensity without noticeable changes in their peak positions. Changes also occurred in diffraction intensities from the myosin heads. The results indicate that during contraction the structure of the actin filaments differs from that in the rigor state, suggesting a possible structural change in the actin subunits themselves; the myosin heads during contraction retain the axial periodicity of the myosin filament and become aligned in a more perpendicular manner to the actin filaments.

https://science.sciencemag.org/content/sci/237/4811/164.full.pdf

Laser-stimulated luminescence used to measure x-ray diffraction of a contracting striated muscle

A new series of experimental imaging plate neutron detectors (IP-NDs) were made, where the composition of the respective IP-NDs, containing a photostimulable BaFBr:Eu 2+ phosphor and a neutron converter material, Gd 2 O 3 or 6 LiF, were varied systematically. The different conversion processes between natural abundant Gd and 6 Li caused distinct imaging properties. The imaging steps and the factors governing the image quality are considered in the same way as X-ray radiography, and the quantum noise is estimated by the effective neutron absorption in these IP-NDs when they are read by a BAS2000 image reader. From this point of view, Gd is superior to 6 Li, but 6 Li is expected to be more suited to imaging than Gd in higher γ -ray to neutron ratios because of its higher amplification rate in the conversion and excitation process.

Junji Miyahara

Papers

A new type of X-ray area detector utilizing laser stimulated luminescence

An imaging plate neutron detector

Design and performance of an imaging plate system for X-ray diffraction study

Laser-stimulated luminescence used to measure x-ray diffraction of a contracting striated muscle

Imaging performance of imaging plate neutron detectors