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Billions of Positrons Created for Antimatter Research
A team of Livermore researchers has developed an improved method for creating a large number of positrons in a laboratory, opening the door to new areas of antimatter research. Led by physicist Hui Chen in the Physical and Life Sciences Directorate, the team used a short-pulse, ultraintense laser to irradiate a millimeter-thick gold target. “Previously, we concentrated on making positrons using paper-thin targets,” says physicist Scott Wilks, who designed and modeled the experiment using computer codes. “Recent simulations showed that millimeter-thick gold would produce far more positrons. We were excited to see so many of them.”

In the experiment, the laser ionizes and accelerates electrons, which are driven right through the gold target. The electrons interact with the gold nuclei, which serve as a catalyst to create positrons. Because the laser concentrates energy in space and time, it produces positrons more rapidly and in greater density than ever before in a laboratory.

Particles of antimatter are almost immediately annihilated by contact with normal matter and converted to pure energy (gamma rays). Normal matter and antimatter are thought to have been in balance in the very early universe, but because of an “asymmetry,” the antimatter decayed or was annihilated. Scientists have yet to determine why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter, and what might be possible if antimatter could be harnessed.

Laser production of antimatter is not entirely new. Livermore researchers detected about 100 particles in experiments 10 years ago on the since-decommissioned Nova petawatt laser. With a better target and a more sensitive detector, Chen’s team directly detected more than 1 million particles per laser shot. From that sample, the scientists infer that about 100 billion positron particles were produced in total.

Chen presented the team’s results in November 2008 at the American Physical Society’s Division of Plasma Physics meeting.
Contact: Hui Chen (925) 423-5974 (chen33@llnl.gov).

Mechanical Response without Heat or Electricity
Laboratory researchers in collaboration with the Institut für Angewandte und Physikalische Chemie of the Universität Bremen and the Institut für Nanotechnologie of the Forschungszentrum Karlsruhe (both in Germany) have found a new method that directly converts chemical energy into a mechanical response without generating heat or electricity first. The team led by chemist Juergen Biener from Livermore’s Nanoscale Synthesis and Characterization Laboratory took a sample of nanoporous gold and alternately exposed it to ozone and carbon monoxide. Using the oxidation of carbon monoxide by ozone as a driver, the team achieved reversible macroscopic strain amplitudes of up to 0.5 percent.

The experiment is based on surface-chemistry-induced changes of the surface stress at a metal–gas interface, which in turn cause the sample to contract and expand. “Like nature’s muscles, our actuator directly converts chemical energy into a mechanical response without generating heat or electricity first,” says Biener.

The team selected nanoporous gold for several reasons. It has remarkable catalytic properties and can sustain high stresses. In addition, its open-cell foam morphology allows for mass transport. The research showed that ozone exposure leads to oxygen absorption, which triggers sample contraction by modifying the surface stress. Exposure to carbon monoxide then restores the original sample dimensions by removing the absorbed oxygen.

Using molecular dynamics simulations, the team independently tested the effect of surface stress on the equilibrium shape of nanoporous gold and its structural building blocks. “These simulations allowed us to understand the stress–strain response of nanoporous gold,” says Biener. He adds that the team’s research, which appeared in the November 30, 2008, issue of Nature Materials, could be applied to developing a new generation of chemical-driven sensor and actuation devices.
Contact: Juergen Biener (925) 422-9081 (biener2@llnl.gov).


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