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原子中的伪装效应让科学家头疼

时间:2011-01-15 13:49 点击:
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译文:Cloaking effect in atoms baffles scientists


Atoms called positronium inexplicably scatter off gas particles as if they were lone electrons, even though they contain an anti-electron as well. The finding hints that engineers could use the well-known scattering properties of electrons as a rule of thumb in designing future medical scanners that employ positronium. It could also help interpret puzzling astronomical observations.

"Knowledge of how positronium interacts with the surrounding medium is important, whether this is human tissue or interstellar gas," says experiment leader of University College London.

Positronium atoms are like hydrogen atoms, except the proton is replaced by a positron, the positively charged antiparticle of the electron. The atoms are unstable and their two constituent particles disappear in a puff of gamma rays within a microsecond.

When a beam of electrons or positrons flies through a gas, they scatter off the gas particles at predictable rates. Scientists guessed that positronium atoms, being twice as heavy as an electron and electrically neutral, would have very different scattering rates.

Competing effects

To test this, Laricchia's team fired positronium atoms at 1100 to 4400 kilometres per second into various gases, including hydrogen, krypton and water vapour. Curiously, the scattering rates for positronium were almost identical to that of a plain electron, as though the positron's influence was somehow "cloaked".

, a theorist at Queen's University Belfast who studies matter-antimatter interactions, says the result will be tough to explain mathematically because the scattering process is so complex.

"When positronium hits an atom, the electrons in the atom don't know what to do – do they stick with the nucleus, or try to chase after the positron?" he asks. "There's a lot of competition going on and theoretically it's difficult to model that accurately."

Limiting damage

Laricchia speculates that the electron might play a dominant role in each positronium scattering by getting closer to a gas particle, on average, than the positron does.

Whatever the reason for the strange results, they could have important consequences for medical PET (positron emission tomography) scanning. PET scanners pick up gamma rays emitted by positronium formed when a radioactive tracer that emits positrons is injected into the body.

Knowing the positronium scattering rates should clarify how the particles deposit energy along their tracks as they collide with molecules in tissue. "That's required to limit damage to healthy tissue," Laricchia told New Scientist. It would also help refine estimates of how far positronium travels, allowing tumour volumes to be measured more accurately.

Positronium also forms in space. Understanding its interaction with interstellar gas clouds could prove crucial for determining the positions of some in the Milky Way.


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