HOW do black holes form? Where do gamma-ray bursts originate in space? Why does matter dominate over antimatter in the universe? No one really knows. However, through the study of positrons, the antiparticles of electrons, scientists believe they may gain insight into these complex astrophysical questions.
Positrons are elementary particles whose physical properties, such as spin and mass, are the same as electrons, except that positrons have a positive charge. For years, scientists have theorized that lasers could be used to generate positrons in a laboratory by “zapping” an ultrathin micrometer-size foil target made of a high-atomic-number (high-Z) material, such as gold or tantalum. Researchers at Livermore recently showed that targets thicker than a few micrometers are a more efficient mechanism for positron generation. Using an ultraintense, short-pulse laser and millimeter-thick targets, physicists Hui Chen and Scott Wilks have produced more than 100 billion positrons—an unprecedented number of laser-generated positrons.
Particle accelerators are typically used to generate positrons for antimatter research. Livermore’s new laser-based method can generate similar numbers of positrons but in a fraction of the time—all positrons are generated in less than 100 picoseconds. With the capability of generating billions of positrons inside a small laboratory, scientists have a way of making antimatter more accessible, opening the door to new avenues of research. As a result, they may be able to uncover answers to some of the most perplexing questions about our universe. This new capability may also provide scientists with a better way to produce positronium—the short-lived bound state of an electron and a positron—which could enable the development of advanced, extremely high-powered gamma-ray lasers.
Larger Target, Greater Yield
Wilks, who designed the experiment for the LDRD project, used computer models to predict the number of positrons that would be generated as a function of the thickness of the target and the intensity of the laser. Chen, who performed and led the experiment, devised a detection scheme for positrons based on an existing electron spectrometer. They conducted their test on a laser at the Rutherford Appleton Laboratory in England. “We were allowed just one shot on the laser,” says Chen. “Unfortunately, it yielded only a hint of a positron signal.”
For their current LDRD study, Chen and Wilks improved their experimental design and detection methods. These experiments were performed in Livermore’s Jupiter Laser Facility on the Titan laser, which was completed in 2006, one year after the team’s initial experiments in England. Titan has a unique long- and short-pulse capability: A high-energy, petawatt short-pulse (subpicosecond) beam is coupled with a kilojoule long-pulse (nanosecond) beam. (See S&TR, January/February 2007, Titan Leads the Way in Laser–Matter Science.) With Titan, the team had a local, more accessible tool for proving their experimental design.
Initial experiments on Titan revealed new data on the distribution and energy of hot electrons interacting with materials. Wilks took these hot electron measurements and put them into a computational model. “The model calculated the electron distribution in the target, and how many positrons were produced in the process,” says Wilks. “After reviewing the simulation results, I realized that irradiating thicker targets would result in orders of magnitude more positrons than seen in previous experiments.”
The thicker targets increase the number of interactions that can occur inside the target. In addition, a different physical process—the Bethe–Heitler process—dominates in larger targets and promotes positron generation on a greater scale. To more accurately detect this abundance of antimatter, Chen redesigned the electron–positron spectrometers using more elaborate components to make them more sensitive to the positron signals.
Producing Particle Pairs
Within the target material, the electrons move at relativistic speeds with kinetic energies ranging from 6 to 100 megaelectronvolts. Through the Bethe–Heitler process, these high-energy electrons lose energy as they interact with the material’s nuclei, resulting in the emission of high-energy bremsstrahlung photons. These photons in turn interact with the high-Z nuclei, which enables some of the high-energy photons to split into electron–positron pairs (matter and antimatter) based on Einstein’s E = mc2 formula that relates energy and matter. The energies of the photons are proportional to the energies of the decelerating electrons as they interact with the material. The higher the energy, the more likely the bremsstrahlung photons will produce electron–positron pairs, a large fraction of which are inevitably blasted out the back of the target in a plasma jet.
The positron energy in the plasma jet was measured by two of the redesigned spectrometers positioned at various angles around the back of the target. However, the plasma jet does not contain the total amount of positrons generated, such as those still in the target. The data recorded from the spectrometers is compared with computer simulations to infer how many pairs were created overall. Chen and Wilks directly detected more than 1 million particles per laser shot. They infer that a total of about 100 billion positron particles were produced. Using targets less than 200 micrometers thick, the research team found that the positron signal fell below the detection limit of the spectrometers. The most successful results were produced using 1- to 3-millimeter-thick gold targets.
A Wealth of Possibilities
“The results of this experiment are so new, we have not even begun to investigate all the potential applications,” says Wilks. In the meantime, scientists have a new mechanism by which they may be able to unravel antimatter’s secrets. While it may be decades or longer before scientists know enough about antimatter to significantly increase their understanding of the origins of our universe, the research done by Wilks and Chen could move them one step closer to the answers.
Key Words: antimatter, astrophysics, gamma ray, high-Z material, hot electron, plasma, positron, positronium, Titan, ultraintense laser.
Lawrence Livermore National Laboratory
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