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The Laboratory
in the News

Gamma-ray detector on Mercury voyage
Scientists and engineers from Lawrence Livermore, the Space Sciences Laboratory at the University of California (UC) at Berkeley, and the Applied Physics Laboratory at Johns Hopkins University designed a high-resolution gamma-ray detector for use on the Mercury MESSENGER spacecraft. MESSENGER (short for Mercury Surface, Space Environment, Geochemistry, and Ranging) was launched on August 3, 2004, and will conduct an in-depth study of Mercury, the planet closest to the Sun. Its voyage will include three flybys of Mercury in 2008 and 2009 and a yearlong orbit of the planet starting in March 2011. As it orbits the planet, MESSENGER will use the detector to measure characteristic gamma-ray emissions from Mercury’s crust as well as solar winds and cosmic rays.
The Livermore team’s role in the project was to ensure that the spacecraft’s gamma-ray spectrometer could withstand the Sun’s heat reflected from the surface of Mercury. To do that, the team combined a rugged, encapsulated germanium gamma-ray detector with a miniature cryocooler and a multilayered thermal shield. The cryocooler and shield maintain the detector at a temperature of less than 90 kelvins, ensuring that the spectrometer operates correctly.
The detector is based on technology originally developed by Lawrence Livermore and Lawrence Berkeley national laboratories for CryoFree/25—a handheld, mechanically cooled detector that can detect gamma rays from radioactive material. (See S&TR, September 2003, Portable Radiation Detector Provides Laboratory-Scale Precision in the Field.) The MESSENGER detector is cooled by a low-power, compact cryocooler, which eliminates the need for liquid nitrogen yet allows the detector to attain the high-level energy resolution needed for accurate measurements.
More information on the MESSENGER voyage is available online at messenger.jhuapl.edu.
Contact: Norm Madden (925) 423-1934 (madden8@llnl.gov).

Regional change in climate affected gorge formation
Livermore scientist Robert Finkel is part of a collaboration that is studying the geologic history of the Susquehanna and Potomac rivers to better understand how each river’s gorge was formed. The team, which includes researchers from the University of Vermont, the U.S. Geological Survey, and the University of Maryland, is measuring the beryllium-10 content in bedrock terraces in these rivers, both of which drain into the Atlantic Ocean.
Comparing the two rivers may help scientists better understand how regional climate changes affect geologic processes. The Susquehanna has been glaciated, but the Potomac has not. Yet, each river has formed a steep bedrock gorge. The Susquehanna narrows as it travels south into Pennsylvania, forming the 5-kilometer-long, 1-kilometer-wide Holtwood Gorge. The Potomac River drops 20 meters as it passes over Great Falls, Virginia, through the 3-kilometer-long, 75- to 125-meter-wide Mather Gorge.
Finkel, who works in the Laboratory’s Center for Accelerator Mass Spectrometry, analyzed the samples for beryllium-10, a rare isotope that forms when cosmic rays hit surface rock and sediment. Measuring the beryllium-10 content allowed the team to determine the age of each terrace and then calculate how quickly the rivers had cut through the bedrock.
The team’s research indicates that a period of cold, stormy, and unstable climate, which began about 35,000 years ago, led to a pulse of incision, or rock cutting, in both rivers. This increase in the cutting rate created the steep bedrock gorges. Because the Holtwood and Mather gorges formed at about the same time and in the same manner, the researchers conclude that regional climate change, not simply glacial meltwater, caused the gorges to form. The team’s results were published in the July 23, 2004, issue of Science.
Contact: Robert C. Finkel (925) 422-2044 (rfinkel@llnl.gov).

Sample size changes mechanical properties of metal
Livermore engineer Jeff Florando collaborated on an Air Force Research Laboratory project to develop a technique for more accurately measuring the mechanical properties of micrometer-size samples. In experiments with the new technique, the team found that reducing a specimen to a few micrometers (millionths of a meter) or less affects the mechanisms by which the sample deforms. The finding is important because micrometer-size materials are commonly used to miniaturize electronic devices and other equipment.
The team’s technique combines a focused ion-beam microscope and a nanoindentation system to create micrometer-size samples and measure each sample’s mechanical properties under compression. According to Florando, this technique can be used to create samples in almost any inorganic material. In characterization studies of nickel, the team found that the material’s strength changes dramatically when the sample size is reduced.
The project, which was led by Michael D. Uchic of the Air Force Research Laboratory in Dayton, Ohio, was funded by the Air Force Office of Scientific Research, the Defense Advanced Research Projects Agency, the Department of Energy, and the National Science Foundation. Florando worked on this project when he was studying with William D. Nix of Stanford University. The team’s research is presented in the August 13, 2004, issue of Science.
Contact: Jeff Florando (925) 422-0698 (florando1@llnl.gov).

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UCRL-52000-04-10 | October 8, 2004