Focus: Matter-Antimatter Molecules
Positronium is an “exotic” atom made of matter and antimatter: an electron and a positron (anti-electron) bound together without a nucleus. Now, in a dense but short-lived gas of such atoms, researchers have observed interactions between them–and may have produced the first positronium molecules, each composed of two atoms. Reported in the 4 November PRL, the experiments represent another step toward creating new kinds of “exotic matter,” whose properties physicists are eager to study, and that could ultimately lead to a gamma ray laser.
Positronium is the simplest in a series of more complex forms of matter that physicists expect can be made by putting electrons and positrons together. Since researchers have already made positronium atoms and ions, the next step would be to coax the atoms to interact through collisions, and ultimately to form molecules. While there are always occasional collisions in positronium gas, researchers haven’t succeeded in making it dense enough for frequent collisions–enough to affect the gas’s properties.
But a team of physicists led by Allen Mills of the University of California at Riverside may have done just that. They collected and compressed positrons in a magnetic trap and then fired super-intense positron pulses at a thin film of “nano-porous” silica, a material riddled with myriad microscopic pores. Positrons hitting the film liberate electrons and can bind with them to make positronium atoms. These atoms live for a brief time as tiny gas clouds, trapped within the material’s pores, until the electrons and positrons inevitably annihilate with one another in a burst of gamma-rays. Mills and his colleagues detected these gamma rays to measure the rate of annihilation, or “decay,” and probe the underlying physics.
After first looking at low-density positronium, the team used spatially compressed pulses to produce high-density positronium, whose atoms are more likely to collide with one another. They reasoned that more collisions should lead to a higher decay rate. That’s because positronium atoms are born in either the fast-decaying spin-0 state or the long-lasting (hundreds of nanoseconds) spin-1 state, depending in part on the initial alignment of the electron and positron spins. But collisions between two spin-1 atoms can transform them into the faster-decaying state.
Mills and his colleagues found a higher decay rate with the denser pulses–clear evidence, they say, of frequent positronium collisions, an important step toward making molecules. They were surprised, however, that the decay rate was four times as high as expected based on the simplest understanding of the collisions.
The extra-high rate may have resulted from positronium atoms concentrating in large cracks and other imperfections that weren’t accounted for in the calculation, which assumed a perfect, nano-porous material. The atoms might seek these larger spaces to escape the nano-pores, whose cramped quarters tend to raise the energy of the atoms. But another possibility is that some pairs of positronium atoms were joining together to form molecules. The experiments cannot distinguish between these two causes, but Mills thinks further studies, perhaps using other materials, should help.
Ultimately, Mills and his colleagues hope to produce positronium atoms with sufficient density to generate a Bose-Einstein condensate (BEC), an ultracold state with weird quantum properties. Using the BEC, they might someday make a gamma ray source that sounds like science fiction: an electron-positron annihilation laser. “This is important and exciting work,” says Cliff Surko of the University of California at San Diego, “and this approach isn’t going to run out of gas anytime soon.”
Mark Buchanan is a freelance science writer based in Wales and Normandy, France.
P. M. Platzman and A. P. Mills, Jr., “Possibilities for Bose Condensation of Positronium,” Phys. Rev. B 94, 454 (1994)