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Quarterly the Actinide Research 4th quarter 2001 TheLos Actinide Alamos National Research Laboratory N u c l e a r M Quarterlya t e r i a l s R e s e a r c h a n d T e c h n o l o g y a U.S. Department of Energy Laboratory Researchers Use Transmission Electron Microscopy to Observe Helium Bubbles in Plutonium Los Alamos, Livermore, and Aldermaston In This Issue Collaborate on Plutonium-Aging Study Mark Wall of Lawrence Livermore National The ability to directly image self-irradiation damage accumulation Laboratory uses in plutonium is critical to understanding aging. Scientists know that Livermore’s powerful helium is building up in plutonium metal during self-irradiation. transmission electron What they don’t know is what is happening to that helium over time, microscope to image and how it ultimately affects the behavior of plutonium over long periods. a sample. Los Alamos Researchers from Los Alamos and Lawrence Livermore national researchers are collaborating with laboratories and the Atomic Weapons Establishment at Aldermaston Livermore and the 4 in the United Kingdom are col- Shock-Wave Research Atomic Weapons laborating on a study to observe Establishment at on Plutonium Yields the microstructural effects New Information Aldermaston, United caused by the formation of Kingdom, to study the Concerning Dynamic helium atoms and vacancies microstructural effects Material Properties during self-irradiation. of the buildup of Recent studies using helium in aging 6 Livermore’s state-of-the-art plutonium. Recent A Personal Perspective Transmission Electron Microscopy research has revealed on Issues in Science (TEM) facility have revealed the the existence of minute and Technology in existence of minute—approxi- bubbles too tiny to be seen with conventional NMT Division mately 1 nanometer TEM instruments. in diameter—spheri- Photo courtesy of Lawrence 9 cally shaped Livermore National Laboratory Publications and bubbles, which are Invited Talks too tiny to be seen with conventional TEM instruments. Scientists pre- sume the tiny bubbles formed from the mi- gration and 10 coalescence Newsmakers of many helium-filled vacancy clusters, which occur as pluto- nium ages. continued on page 2 Nuclear Materials Technology/Los Alamos National Laboratory 1 Actinide Research Quarterly Helium Bubbles Contributors to this The existence of bubbles in plutonium has third law: For every article are: Thomas been seen before in heated samples. The fact action there is an Zocco (NMT-6); that researchers saw bubbles in materials un- equal and opposite Mark Wall, Adam der approximately room-temperature storage reaction. This move- Schwartz and Bill conditions is somewhat surprising, according ment may cause sig- Wolfer (Lawrence to Los Alamos researcher Tom Zocco of nificant damage to the Livermore National Manufacturing Systems (NMT-6). surrounding atomic Laboratory); and “It implies that helium and helium vacancy arrangement. This two-dimensional Paul Roussel clusters are mobile at room temperature and Imagine a three- representation shows the crystalline lattice (Atomic Weapons can cluster, forming the bubbles,” said Zocco. dimensional, periodic and two types of Establishment, “The formation of bubbles can have a variety atomic arrangement— damage caused by the Aldermaston, United of effects on the mechanical and physical a lattice—of pluto- radioactive decay Kingdom). properties of plutonium metals and alloys, nium atoms in a process. Each red dot Also contributing to which can possibly affect the long-term aging crystalline structure. represents an atom. this project are: Mary of our stockpile.” When any atom radio- In the figure on the left, Esther Lucero and Zocco was one of the first researchers to actively decays, the an interstitial atom (the Michael Ramos successfully use TEM for the microstructural resulting uranium black dot) is displaced (NMT-16). analysis of plutonium metal and alloys. As atom and helium and squeezed between part of this collaboration, he has developed a nucleus fly apart, hit- other atoms. In the Figures: L. Kim Nguyen Gunderson sample matrix and supplied prepared and ting other atoms as figure on the right, a (IM-1) vacancy is created by aged plutonium materials for examination. they travel through the alpha release and the lattice. Both the uranium-235 recoil. uranium and helium atoms generate sub- stantial damage within the atomic arrange- ment of the crystalline structure, which results in defects or discontinuities in this normally periodic arrangement. The defects are prima- rily of two forms: vacancies or missing atoms in the lattice, or interstitial atoms, which are atoms squeezed between other regularly spaced atoms. The amount of damage produced is directly related to the mass and energy of each moving This schematic Livermore is finishing the sample preparation, particle. The uranium atom is large and does illustrates the performing the TEM operations, and provid- not travel far and deposits its kinetic energy radioactive decay ing image simulations. Researchers from over a short distance. This causes significant process of a Aldermaston also are providing material damage to the lattice and creates thousands of plutonium-239 atom. and expertise. displaced plutonium atoms. The alpha particle Radioactive materials are made up of atoms The alpha particle (or helium ion), on the releases and the that are inherently unstable and decay over uranium-235 atom other hand, is very energetic and travels recoils. varying periods of time to form more stable farther through the lattice. But because the atomic elements. For example, the unstable alpha particle is relatively small, it creates a plutonium-239 isotope decays by the process lower number of lattice defects and less over- of alpha emission. When the alpha particle is all damage. emitted, the loss of protons and neutrons from Because of the high local stresses created the plutonium-239 atom transmutes it to a from squeezing the atoms into abnormal uranium-235 ion. This uranium ion rapidly re- positions, most of them quickly return to coils during the alpha release, as in Newton’s the vacancies created when they were 2 Nuclear Materials Technology/Los Alamos National Laboratory 4th quarter 2001 displaced from their The technique for imaging these original positions in small voids is called the defocus or the lattice. This “Fresnel fringe” imaging technique. “self-healing” Depending on the amount and process returns most direction of defocus, the small of the atoms to their voids or bubbles will visually original, uniformly appear as small white or black spots spaced positions. with surrounding black or white However, the defects that do not self-heal fringes, respectively. The diameter ultimately result in the buildup of excess dam- of these circular fringes will vary age in the material. It is this lattice damage and with defocus, and is not easily its long-term accumulation that is of interest to related to the true diameter of the researchers investigating the aging or self- voids or bubbles. irradiation damage phenomena in radioactive For example, when measuring materials. (For more details on the aging ef- small (less than 1 nanometer) voids fects in plutonium, see The Actinide Research or bubbles, the diameter of the cen- Quarterly, 4th Quarter 1999.) tral bright or dark spots may be in After the alpha particle comes to rest in the error as much as 50 percent from the lattice, it rapidly attracts free electrons from its true diameter. Therefore, it is neces- surroundings to become a helium atom. This sary to perform image simulations process occurs at a pace that creates approxi- to correctly interpret the actual size. mately 29 helium atoms per year for every After the under- or over-focused 1 million atoms of plutonium. This may not images are collected, they can be seem like a significant amount, but over a processed and analyzed. Through period of years the accumulation of helium careful control of the processing becomes substantial and potentially can bring parameters, image-processing soft- about significant changes in macroscopic ware quickly identifies and mea- physical properties. sures the bubbles in a variety of The remaining defects (vacancies and ways, such as bubble density, mean diameter, At the top is a raw interstitials) that survive the self-healing pro- area, aspect ratio, and roundness. By measur- (as-captured) digital cess coexist with the helium atoms, forming ing or estimating the thickness of the TEM Transmission Electron complex interactive relationships. Helium at- specimen and counting the number of bubbles Microscopy (TEM) oms may readily combine with nearby vacan- in each image, researchers can calculate the image. The image on the bottom has been cies to form helium-filled vacancies, which true bubble density. processed and shows diffuse randomly until they meet and bind Through the use of complex image identified and measured with other similar species, creating a bubble simulation techniques, Livermore researchers bubbles. The existence nucleus. The bubble nucleus grows as it cap- are determining how Fresnel contrast images of bubbles in plutonium tures additional helium-filled vacancies of bubbles appear and change as a function of has been seen before in moving through the lattice. defocus and bubble position in the TEM heated samples. The Larger voids or bubbles, and/or those sample. This may require correction factors for fact that bubbles were having associated strain fields, are readily bubble size, to account for distortions pro- seen in materials under observable in conventional TEM. However, the duced from the many imaging effects. approximately room- imaging and observation of very small voids The researchers also are modeling helium temperature storage (less than 2 nanometers in diameter) or small bubble nucleation and growth. By coupling conditions is somewhat surprising, according to bubbles that are in equilibrium (no strain) experiments and modeling, they hope to researchers. with the surrounding lattice are difficult and develop a good correlation between bubble require the use of a TEM with a highly formation and age.
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