Phase Contrast Analysis (PCA)

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Overview
Nondestructive Evaluation through Phase Contrast Analysis (PCA)

- Advanced Radiography Method for Nondestructive Evaluation
- Compact, Mono-Energetic X-Ray Source: previously accessible only at synchrotron facility
- Tune-able Energy: superior imaging through optimization of both absorption contrast and phase contrast
- Low Radiation Emissions: machine operates in shirt-sleeve environment
- NDE Applications:

 

Hidden Aluminum Corrosion Detection
Composite and Composite/Metal Defects Imaging
Metal and Ceramic Void/Impurity Detection
Materials Research- X-Ray Diffraction and X-Ray Fluorescence


Monochromatic X-Ray Beams
Traditional synchroton (x-ray) technology

PCA

Synchrotron light, or the x-rays produced by a synchrotron, has, and continues to, fundamentally change the way that researchers examine materials. Key characteristics of a synchrotron that drive this innovation are the intensity of the x-ray beams and the fact that these x-rays are of one energy level, rather than the spectrum that a conventional x-ray tube produces. Major limitations of synchrotrons that preclude their widespread adoption to industry, including for non-destructive testing (NDT), are: synchrotron beams are at fixed locations, extremely expensive, and difficult to obtain beam time on. The ability to create an x-ray beam similar to that of a synchrotron in a more convenient and cost effective form factor will fundamentally change many industries, including NDT.

Laser Compton Scattering (LCS)
LCS enables similar x-ray beam technology used with synchrotron without the same costly infrastructure

Laser Compton Scattering Technology

Using a process known as Laser Compton Scattering (LCS), it is possible to create an x-ray beam very similar to that of a synchrotron without requiring the same costly infrastructure. These beams, when incorporated into well designed imaging systems, have the potential to enable insitu inspections of large structures.

An LCS source uses both a high energy laser and a linear accelerator (linac) to create x-rays that are nearly the same energy or wavelength. Whether or not the machine produces a true monochromatic, or one wavelength, x-ray beam is largely irrelevant to non-destructive evaluation. Monochromatic or—more properly—narrowband x-ray imaging has shown great promise for improving the quality of images.

.In an LCS source, electrons are accelerated to near the speed of light in the linac and focused to a small focal spot at the interaction zone (IZ). The light from the laser is compressed in time (this shortens the duration of the pulse) and then reflected back along the path of the electron beam coming out of the linac. It too is focused to a small spot at the IZ, where the electron beam and the laser beam collide head-on.

The interaction between the electrons and high energy photons results in the electron giving up part of its energy and a photon containing much higher energy being emitted in the opposite direction, so that the overall momentum of the system is conserved. This process is known as the inverse-Compton effect. These higher energy photons, x-rays, exit the machine in the direction that the electron beam was headed.

Since the electron beam is tunable, meaning that the energy can be adjusted, the emitted x-rays are tunable. Since the laser photons are all at the same frequency, the x-rays are all at (or very near) the same wavelength.

 

LCS Beam Characteristics
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X-ray beams produced by LCS have many other characteristics of interest that enable unique uses. Brief descriptions of some of these are listed below. The energy level of the x-rays is directly related to the wavelength or frequency- in this discussion, energy level is used.

Monochromatic: The LCS source is not truly monochromatic, but does produce x-rays with a narrow band of energies. The fact that the x-rays are near one energy level gives the beam unique characteristics. For example, x-rays that do not contribute to or degrade the quality of the image can be excluded. By analyzing the materials of interest to be imaged, the energy of x-rays can be chosen to provide the greatest contrast. This feature is additionally important because radiation dose is also dramatically reduced with a mono-energetic source, making them safer to use.

Variable Energy Level: The ability to change the energy level of the x-ray beam allows the energy to be chosen to best suit the material or materials of the object being imaged. The energy level can be chosen such that it provides the maximum contrast between two materials. Furthermore, by comparing the measured absorption of materials over a range of energy levels, combined with thickness information, it is possible to make identification of the constituents being imaged.

Energy Level Extents: The maximum and minimum energy level obtainable for an x-ray intended for imaging materials of interest is important. The energy must be sufficiently high to penetrate an object. High energy x-rays can also cause scatter which degrades image quality, so there are limits to how high of an energy level will be useful.

Pulse Width: The pulse width is the duration of the pulse. A short pulse is of interest because it would effectively freeze moving objects in an image, preventing movement caused blurring. State of the art LCS systems are being designed to generate pulses in the pico-second time frame which opens the door to opportunities of still-imaging of rapidly moving objects (for example turbo machinery components at operating speeds). Additionally, due to the pulsed nature of the beam, the corresponding radiation dose levels are dramatically lower than most conventional x-ray systems. Operation can occur, even with today's lab systems, in a shirt sleeve environment with no vaulting requirements.

Repeat Rate: The repeat rate is the frequency that x-ray pulses can be created. This is an important factor when considering the effective flux (number of photons over a given amount of time) of the beam.

Flux: The amount of photons produced in a given amount of time (the "brightness" of the beam). With a pulsed source, this can be expressed in terms of photons per pulse. The effective flux can be calculated from the flux per pulse and the repeat rate. The flux of a beam is important when imaging highly attenuative materials. When a high percentage of incident photons are absorbed, the amount of incident photons becomes increasingly important in determining the time to acquire an image.

Interaction Zone: The interaction zone is the area where light photons and electrons collide, creating x-ray photons. The size of the interaction zone determines the apparent source size. With very small source sizes, phase contrast effects can be taken advantage of in addition to absorption contrast, to further enhance images. This is a key attribute to effectively image composite materials.