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Laser Pulse Produces Positrons
Nov 2008
LIVERMORE, Calif., Nov. 18, 2008 -- More than 100 billion particles of antimatter have been created by using a short-pulse, ultraintense laser to irradiate a gold sample the size of the head of a push pin. The antimatter, also known as positrons, shoots out of the target in a cone-shaped plasma "jet."

This new ability to create a large number of positrons in a small laboratory opens the door to several avenues of antimatter research, including an understanding of the physics underlying various astrophysical phenomena such as black holes and gamma ray bursts. Antimatter research also could reveal why more matter than antimatter survived the Big Bang at the start of the universe.

"We've detected far more antimatter than anyone else has ever measured in a laser experiment," said Hui Chen, a Lawrence Livermore National Laboratory researcher who led the experiment. "We've demonstrated the creation of a significant number of positrons using a short-pulse laser."
Physicist Hui Chen sets up targets for the antimatter experiment at the Jupiter laser facility. (Photo: Lawrence Livermore National Laboratory)
Chen and her colleagues used a short, ultraintense laser to irradiate a millimeter-thick gold target. "Previously, we concentrated on making positrons using paper-thin targets," said Scott Wilks, who designed and modeled the experiment using computer codes. "But recent simulations showed that millimeter-thick gold would produce far more positrons. We were very excited to see so many of them."

In the experiment, the laser ionizes and accelerates electrons, which are driven right through the gold target. On their way, the electrons interact with the gold nuclei, which serve as a catalyst to create positrons. The electrons give off packets of pure energy, which decays into matter and antimatter, following the predictions by Einstein's famous equation that relates matter and energy. By concentrating the energy in space and time, the laser produces positrons more rapidly and in greater density than ever before in the laboratory.

"By creating this much antimatter, we can study in more detail whether antimatter really is just like matter, and perhaps gain more clues as to why the universe we see has more matter than antimatter," said Peter Beiersdorfer, a lead Livermore physicist working with Chen.

Particles of antimatter are almost immediately annihilated by contact with normal matter, and converted to pure energy (gamma rays). There is considerable speculation as to 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. Normal matter and antimatter are thought to have been in balance in the very early universe, but due to an "asymmetry" the antimatter decayed or was annihilated, and today very little antimatter is seen.

Over the years, physicists have theorized about antimatter, but it wasn't confirmed to exist experimentally until 1932. High-energy cosmic rays impacting Earth's atmosphere produce minute quantities of antimatter in the resulting jets, and physicists have learned to produce modest amounts of antimatter using traditional particle accelerators. Antimatter similarly may be produced in regions like the center of the Milky Way and other galaxies, where very energetic celestial events occur.

The presence of the resulting antimatter is detectable by the gamma rays produced when positrons are destroyed when they come into contact with nearby matter. Laser production of antimatter isn't entirely new either. Livermore researchers detected antimatter about 10 years ago in experiments on the since-decommissioned Nova petawatt laser -- about 100 particles. But with a better target and a more sensitive detector, this year's experiments directly detected more than 1 million particles. From that sample, the scientists infer that around 100 billion positron particles were produced in total.

Until they annihilate, positrons (antielectrons) behave much like electrons (just with an opposite charge), and that's how Chen and her colleagues detected them. They took a normal electron detector (a spectrometer) and equipped it to detect particles with opposite polarity as well.

"We've entered a new era," Beiersdorfer said. "Now, that we've looked for it, it's almost like it hit us right on the head. We envision a center for antimatter research, using lasers as cheaper anti-matter factories."

Chen is presenting her work this week at the American Physical Society's Division of Plasma Physics meeting in Dallas.

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