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Massachusetts Institute of Technology (MIT) researchers are developing techniques to explore the microstructural evolution and degradation of materials exposed to radiation. The research is a step towards highlighting out the aspects of radioactive materials and radiation effects on materials, which still by large remain unknown despite decades of research and development. 

A problem with testing real-time changes in mechanical and thermal transport properties to determine the lifetime for safe use of a material in engineering systems within radiation environments is that such changes occur relatively slowly. The MIT researchers report in Applied Physics, addresses a more dynamic option to continuously monitor the properties of materials being exposed to radiation during the exposure. This provides real-time information about a material’s microstructural evolution.

At MIT’s Mesoscale Nuclear Materials Lab, researchers have been developing improvements to a technique called transient grating spectroscopy (TGS), which is sensitive to both thermal transport and elastic properties of materials. To use this type of method to monitor dynamic materials changes, they first needed to show via developing and testing new optical configurations that it’s possible to measure material properties in a time-resolved manner. TGS relies on inducing and subsequently monitoring periodic excitations on material surfaces using a laser. By pulsing the surface of a sample with a periodic laser intensity pattern, one can induce a material excitation with a fixed wavelength. These excitations manifest themselves in different ways in different systems, but the type of responses the team observed for pure metallic materials were primarily standing surface acoustic waves. This approach is generally referred to as a transient grating technique.

To help visualize this, researchers offered the imagery of flicking a drumhead, but in this case, on a solid surface where the laser does the flicking. The drum’s response depends on the condition of its structure and can therefore reveal changes in structure. These excitations' oscillation and decay are directly related to the material’s thermal and elastic properties. One can monitor these excitations by using the material excitations themselves as a diffraction grating for a probing laser. Specifically, the researchers monitor the first-order diffraction of the probing laser because its intensity and oscillation directly reflect the amplitude and oscillation of the material excitation. The signal the researchers are trying to detect is very small so it must be amplified by spatially overlapping a reference laser beam, which doesn’t contain the signal of interest, which is a process called heterodyne amplification.

According to the team, most complete measurements are made by collecting multiple measurements at different heterodyne phases (a measure of the path length difference) between the signal and reference oscillator to remove any systematic noise. Thus they added an additional probing laser path in the same compact optical configuration that allowed them to collect measurements at multiple heterodyne phases concurrently. This allows the researchers to make complete measurements in a manner constrained only by the system repetition, detection rate and desired signal-to-noise ratio of the overall final measurement.

The group’s experimental method is called Dual Heterodyne Phase Collection Transient Grating Spectroscopy (DH-TGS). MIT believs this is a significant advance because it can be used for dynamically monitoring the evolution of material systems. Because DH-TGS is a nondestructive material diagnostic method, researchers state that there are many systems one might envision studying as microstructural evolution is taking place. They are interested in the radiation damage case in particular, but other applications might include studying low-temperature phase change materials, or real-time monitoring of oxide layer formation on steel alloys.

Their next experimental iteration involves constructing a target chamber for an ion beam accelerator so that they can watch materials evolve in real time during exposure. The work presented in Applied Physics Letters was the last piece in the puzzle standing between them and realizing the overarching motivation for the project.