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Summer 2018

Radiation: A Driver of Science

Radiation’s versatility enables and inspires diverse research within the Energy Frontier Research Centers

Patricia Huestis

Radiation is energy that moves from one place to another. Radiation can take the form of waves, such as electromagnetic energy (light), or fast-moving particles, such as protons, neutrons, and electrons. Investigators within the Energy Frontier Research Centers (EFRCs) use radiation in many facets of their research. Each center approaches and uses radiation in different ways, whether it is to produce energy, probe nanoscale systems, or induce chemistry.

Every center utilizes radiation in some way in their research. Here are just six examples; each was selected to showcase a simple, direct explanation of a radiation-related process.

Utilizing radiation to power everyday society. For centers working with turning sunlight into energy, one of the main goals is to harvest solar radiation, or sunlight. Solar cells accomplish this by using specially designed materials to absorb solar radiation and transform it into electricity. This is an important research field because it allows us to create electricity by using an abundant and free source of energy: sunlight. The first step to harvesting the sun’s energy requires the absorption of solar radiation. The materials of a solar cell accomplish this on a molecular level by having the right energy gap between the valence band, where electrons are normally found, and the conduction band, where electrons have enough energy to move throughout the material.

To be absorbed by the material, the incoming radiation must have an energy greater than the material’s energy gap. Once the solar radiation is absorbed, an electron in that material is able to cross the energy gap where it can become mobile. A “hole” is created where the recently mobilized electron once was. Neighboring electrons can then move to occupy the space left behind by the excited electron, but in doing so, those electrons create new holes. The hole is, therefore, mobile, just as the electron is.

Incoming radiation (light) can be absorbed by certain materials. Excited electrons leave behind a “hole,” and both the electron and hole can move due to an applied electric field. The resulting currents are collected and used to power everyday devices. Image courtesy of Freshman404, own work, CC BY-SA 4.0

Electrons and holes travel in opposite directions within the solar panel and are driven by a strong electric field. Traveling electrons and holes are referred to as current, and by connecting metal contacts on either side of the solar cell, the current can be collected. Radiation, therefore, creates the electron-hole pair that is eventually harvested as usable electricity.

The promising nature of solar cells has led to a large investment by the DOE which is evident by the number of EFRCs researching the different facets of solar energy. For example, the Light-Material Interactions in Energy Conversion (LMI) EFRC aims to understand the fundamental aspects of how light interacts with materials to design solar cells that are able to more efficiently absorb sunlight. The Center for Excitonics (CE) EFRC is researching more efficient ways to transport energy that has already been collected by using excitons, or bound electron-hole pairs, to move energy around. For these centers, radiation inspires the research because being able to efficiently capture the sun’s energy will lead to a cleaner energy future.

Utilizing radiation to “see” tiny details. Almost all of the centers use radiation to understand materials, especially at the nanoscale level. The level is not visible to the human eye. In microscopes that use visible or sometimes ultraviolet or infrared light, photons (the particles composing light) interact with the object and then return back to the user with information about the object it interacted with, such as color, shape, and even composition. The same basic principle is at work for electron microscopes, a popular instrument for scientists trying to look at nanoscale systems.

For microscopes, the ultimate resolving power is determined by the wavelength of radiation used. Electrons have a smaller wavelength than photons at the same energy, so electrons can resolve more. Image courtesy of Patricia Huestis (adapted from University of Waikato)

Electron microscopes use electrons instead of photons as the probing radiation because electrons can reach significantly higher energy in laboratory settings. Just as with light, electrons have an associated wavelength, known as its DeBroglie wavelength. The DeBroglie wavelength is a function of energy, so the more energetic the electron is, the smaller its wavelength will be.

The ultimate resolving power of a microscope is determined by the wavelength of radiation used to probe the sample. For visible light, the smallest object that can be seen with the microscope is on the order of hundreds of nanometers, or about the size of a bacterium. For electrons, microscopes can resolve individual atoms. The incredible resolving power of electron microscopes, therefore, make them a powerful tool for scientists wanting to look at nanoscale systems.

Centers that work with electron microscopes do so because the instruments can provide valuable information about their systems. For instance, at the Integrated Mesoscale Architecture for Sustainable Catalysis (IMASC) EFRC, a center focused on improving the performance of low-cost catalysts, researchers utilize special attachments for scanning tunneling electron microscopes that allow the microscopes to image changes happening on the surface of catalysts as reactions are taking place. At the Nanostructures for Electrical Energy Storage (NEES) EFRC, researchers use electron microscopy to provide detailed information about small-scale battery electrode materials to better understand the movement of ions across electrodes, the ever-changing nature of the atomic and crystal structure, and the overall performance. For these centers, radiation assists in the research by allowing scientists to look at their systems on a fundamental level.

Studying the effects of radiation in complex systems. For some centers, radiation is not just a tool but a challenge. In fact, radiation emitted by leftovers from nuclear weapons production can alter the chemistry of the surrounding waste. Radioactive legacy waste (waste from making weapons) is a problematic entity at several sites in the United States. The chemical makeup of the waste is complicated due to the large number of elements present in different molecular forms. Radiation that is energetic enough to ionize atoms and molecules (ionizing radiation) greatly increases the complexity of the waste. Removing and eventually permanently storing the waste is, therefore, a difficult task.

Water radiolysis and the resulting products formed. After the chemical stage, the resulting products have diffused far enough away from the other radiolytically produced species that they are able to react with other species that may be in the system. Image courtesy of Patricia Huestis (adapted from Sophie Le Caër)

Ionizing radiation from radioactive fission products can cause atoms to rearrange into different molecules, thereby changing the chemical makeup. For instance, ionizing radiation can interact with a water molecule and will either excite or ionize the water molecule, leading to a process known as water radiolysis. Ionized water molecules react chemically with surrounding water molecules, leading to the creation of species that did not exist in the tanks before. Examples of such species are hydrogen peroxide (a highly reactive product) and hydrogen gas (a highly flammable gas), both of which are problematic in waste tanks. Understanding how these new products interact with what is already in the tanks is a difficult task due to the complexity of the tanks. In essence, radiation induces chemical effects that complicate already complex systems.

Several EFRCs are studying how to deal with the legacy waste. For instance, at the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) EFRC, researchers are studying how the harsh environments in the tanks coupled with ionizing radiation have aged the solids in the tanks to better understand how to remove and process the waste. The Center for Performance and Design of Nuclear Waste Forms and Containers (WastePD) EFRC is looking at how to immobilize the waste streams in different forms (such as ceramics, glass, or even metals) so that the radioactive elements can be stored away safely without risk of contamination of the storage site. While WastePD is not currently researching radiation effects, radiation is an important part of the waste environment, and WastePD’s fundamental efforts lay the groundwork for future studies that include the effects of ionizing radiation. For these centers, radiation drives the research, as knowledge of how radiation has aged materials in the tanks, as well as how it will age its permanent storage form, is essential.

Radiation is found in many forms throughout the EFRCs. It inspires, assists in, and drives the diverse research that scientists are doing.

Acknowledgments

The Energy Frontier Research Centers listed in this article are funded by the Department of Energy, Office of Science, Basic Energy Sciences.

About the author(s):

  • Patricia Huestis is a Ph.D. candidate studying physics at the University of Notre Dame under Jay LaVerne. She is a member of the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center. Her research focuses on looking at the role that ionizing radiation plays in aging materials located within waste tank environments.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.