INTRODUCTION:
X-ray diffraction
The diffraction of X-rays as they pass through a substance, usually forming an interference pattern that can be captured on film and used to analyze the internal structure of the substance.
The scattering of x-rays by crystal atoms, producing a diffraction pattern that yields information about the structure of the crystal. X-ray diffraction is used in x-ray crystallography .
X-ray diffraction - the scattering of X rays by the atoms of a crystal; the diffraction pattern shows structure of the crystal .
X-rays are electromagnetic radiation with typical photon energies in the range of 100 eV - 100 keV. For diffraction applications, only short wavelength x-rays (hard x-rays) in the range of a few angstroms to 0.1 angstrom (1 keV - 120 keV) are used. Because the wavelength of x-rays is comparable to the size of atoms, they are ideally suited for probing the structural arrangement of atoms and molecules in a wide range of materials. The energetic x-rays can penetrate deep into the materials and provide information about the bulk structure.
X-rays are produced generally by either x-ray tubes or synchrotron radiation. In a x-ray tube, which is the primary x-ray source used in laboratory x-ray instruments, x-rays are generated when a focused electron beam accelerated across a high voltage field bombards a stationary or rotating solid target. As electrons collide with atoms in the target and slow down, a continuous spectrum of x-rays are emitted, which are termed Bremsstrahlung radiation. The high energy electrons also eject inner shell electrons in atoms through the ionization process. When a free electron fills the shell, a x-ray photon with energy characteristic of the target material is emitted. Common targets used in x-ray tubes include Cu and Mo, which emit 8 keV and 14 keV x-rays with corresponding wavelengths of 1.54 Å and 0.8 Å, respectively. (The energy E of a x-ray photon and its wavelength is related by the equation E = hc/, where h is Planck's constant and c the speed of light) (check out this neat animated lecture on x-ray production)
In recent years synchrotron facilities have become widely used as preferred sources for x-ray diffraction measurements. Synchrotron radiation is emitted by electrons or positrons travelling at near light speed in a circular storage ring. These powerful sources, which are thousands to millions of times more intense than laboratory x-ray tubes, have become indispensable tools for a wide range of structural investigations and brought advances in numerous fields of science and technology.
METHODOLOGY:
X-Ray Diffraction Method
At Proto we use the x-ray diffraction method to measure residual stress. X-ray diffraction is presently the only portable nondestructive method that can quantitatively measure residual stress in crystalline and semi-crystalline materials. Our high speed x-ray detector technology enables measurements to be performed easily on metals and ceramics; including traditionally difficult materials such as shot peened titanium. XRD uses the coherent domains of the material (the grain structure) like a strain gage which reacts to the stress state existing in the material. Residual stress and / or applied stress expands or contracts the atomic lattice spacing (d).
How do we Measure Stress?
Actually, we measure strain and convert to stress. The d-spacings are calculated using Bragg's Law: λ = 2 d sin . If a monochromatic () x-ray beam impinges upon a sample with an ordered lattice spacing (d), constructive interference will occur at an angle . Changes in strain and thus the d-spacing translate into changes in the diffraction angle measured by the x-ray detectors. The diffraction pattern is in the shape of a cone for polycrystalline materials. The shape of the diffraction peaks can also be related to the dislocation density and coherent domain size.
Why Use Multiple Detectors?
Unlike other single detector systems. Proto uses two (2) detectors for stress measurements thus capturing both sides of the diffraction cone. This means twice as much data is collected in the same amount of time simply by virtue of the design.
Proto offers a four (4) detector system that can be used for both the four peak % retained austenite method and in multiphase stress measurements.
Proto also offers 3 and 5 detector configurations for use in Simultaneous Stress and % Retained Austenite determination. Proto adheres to SAE SP-453 Retained Austenite and Its Measurement by X-ray Diffraction and ASTM E975-84 Standard Practice for X-ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation..
Patented Fiber Optic Based Solid State Detectors
Longevity and Maintenance
Proto uses fiber optic based solid state detectors. The fiber optics allow the detector electronics to be remote from the sensing head making them suitable for measurements in harsh environments. Proto detectors are maintenance free and do not degrade with exposure to x-rays, thus less down time, better productivity and no hidden maintenance costs. Direct expose solid state detectors and position sensitive proportional counters degrade with exposure to x-rays and eventually require replacement which can be extremely costly. Because of x-ray damage, these detectors and counters must constantly be re-calibrated. In addition, some position sensitive proportional counters require periodic (bi-annual) maintenance to refill the sealed gas filled detector housing.
Speed
Proto detectors are the fastest detectors on the market today. A stress measurement can be performed in less than 0.3 seconds, an order of magnitude faster than any other detector technology commercially available. Position sensitive proportional counters can only detect one x-ray event at a time. In addition, there is dead time associated with their signal processing which slows data collection. Proto detectors have no dead time associated with them. They are multi-channel solid state detectors that collect many x-ray events simultaneously resulting in unmatched data collection speed. This is particularly important for laboratories with high throughput demands and for industrial on-line and audit station applications.
Drift
Position sensitive proportional counters can drift if there is any fluctuation in the DC bias voltage thus causing errors in peak position determination. Ambient temperature fluctuations, gas pressure and oxides on connections, to name a few, can contribute to detector instability and drift. Proto detectors are solid state, thus there is no positional drift associated with them. This means they are much more stable in harsh environments and at elevated or cold temperatures.
Detector width
Proto's wide 2 detector range, 18.7 degrees 2for the 40 mm goniometer geometry offers increased accuracy on materials with broad diffraction peaks found in hardened tool and bearing steels.
Flexibility in Residual Stress Measurement Techniques
Most systems, particularly one detector systems, offer only double exposure and multiple exposure sin ² techniques. Proto systems offer the double exposure and multiple exposure sin ² techniques as well as the single exposure technique and the multiple exposure sin ² techniques. This translates into more flexibility for characterizing samples with complicated geometries.
Flexibility in Residual Stress Analysis
With Proto equipment, unlike other diffraction systems, diffraction peaks can be fit using a number of mathematical functions including, Parabola, Gaussian, Cauchy, Pearson VII, centroid, and mid-chords. Proto also offers both the difference, and cross-correlation methods for peak position determination. This translates into both improved accuracy and flexibility.
Focusing Optics
Proto systems operate on a true center of rotation and are delivered pre-calibrated to meet exceed ASTM E915-90 "Standard Test Method for Verifying the Alignment of X-ray Diffraction Instrumentation for Residual Stress" and adhere to SAE J784a "Residual Stress Measurement by X-ray Diffraction" alignment specifications. All Proto systems operate using parafocusing optics thus eliminating the need for Sollier slits and allowing very fine positional accuracy in stress measurements inside 90 mm and 120 mm i.d. confinements (e.g. the i.d. of pipes and holes, or between parallel surfaces). The competition cannot offer access to such small holes.
Simplicity in Use, Sophistication in Results
Proto systems are easy to use and setup:
Quick change apertures allow for easy adjustment of the irradiated area and sample setup (apertures can be changed in about 2 seconds) with beam dimensions (irradiated area) available from 0.3 mm to 5.0 mm.
Sample positioning and focusing can be performed easily using the standoff pointer provided with all systems and through the collimator laser pointer which allows the user to quickly locate measurement locations. This is particularly helpful when using the Automated Stress Mapping option.
The 4-Point bending fixture and Proto strain bridge are used for quick and easy determination of the effective x-ray elastic constant for new materials as per ASTM 1426-91, "Standard Test Method for Determining the Effective Elastic Parameter for X-ray Diffraction Measurements of Residual Stress".
The Proto Portable Electro Polisher is custom manufactured specifically for x-ray diffraction work, making material removal quick and efficient.
Truly portable systems are available weighing less than 18 kg (40 lbs).
Custom systems are available for customers with special requirements.
Comprehensive turnkey systems are offered by Proto to their customers to simplify and expedite their stress measurement needs.
Continuous Research and Development and a commitment to give you the best systems in the world.
CONCLUSION:
· Other Sections▼
Abstract
Human phosphate-binding protein (HPBP) was serendipitously discovered by crystallization and X-ray crystallography. HPBP belongs to a eukaryotic protein family named DING that is systematically absent from the genomic database. This apoprotein of 38 kDa copurifies with the HDL-associated apoprotein paraoxonase (PON1) and binds inorganic phosphate. HPBP is the first identified transporter capable of binding phosphate ions in human plasma. Thus, it may be regarded as a predictor of phosphate-related diseases such as atherosclerosis. In addition, HPBP may be a potential therapeutic protein for the treatment of such diseases. Here, the purification, detergent-exchange protocol and crystallization conditions that led to the discovery of HPBP are reported.
Keywords: ABC transporters, missing gene, apoproteins, atherosclerosis, paraoxonase
· Other Sections▼HPBP was serendipitously discovered from supposedly pure PON1. The structure of HPBP (Morales et al., 2006 ) relates it to prokaryote phosphate solute-binding protein (SBP; Tam & Saier, 1993; Luecke & Quiocho, 1990), which is associated with the ATP-binding cassette transmembrane transporters (ABC transporters; Higgins, 1992). Surprisingly, the deduced HPBP sequence cannot be retrieved from the human genome or other genomic databases. HPBP is related to a family of eukaryotic proteins that are named DING owing to their four conserved N-terminal residues (Berna et al., 2002
