New way to produce antimatter-containing atom discovered

July 11, 2011, University of California - Riverside
Researchers Tomu H. Hisakado, Harry Tom, Allen Mills and David Cassidy have found a new way to produce positronium. Credit: M. Kelley, UCR Strategic Communications

( -- Physicists at the University of California, Riverside report that they have discovered a new way to create positronium, an exotic and short-lived atom that could help answer what happened to antimatter in the universe, why nature favored matter over antimatter at the universe's creation.

Positronium is made up of an electron and its twin, the positron. It has applications in developing more accurate Positron or and in fundamental physics research.

Recently, antimatter made headlines when scientists at , the European Organisation for Nuclear Research, trapped antihydrogen atoms for more than 15 minutes. Until then, the presence of antiatoms was recorded for only fractions of a second.

In the lab at UC Riverside, the first irradiated samples of silicon with laser light. Next they implanted positrons on the surface of the silicon. They found that the frees up silicon that then bind with the positrons to make positronium.

"With this method, a substantial amount of positronium can be produced in a wide temperature range and in a very controllable way," said David Cassidy, an assistant project scientist in the Department of Physics and Astronomy, who performed the research along with colleagues. "Other methods of producing positronium from surfaces require heating the samples to very . Our method, on the other hand, works at almost any temperature – including very low temperatures."

Cassidy explained that when positrons are implanted into materials, they can sometimes get stuck on the surface, where they will quickly find electrons and annihilate.

"In this work, we show that irradiating the surface with a laser just before the positrons arrive produces electrons that, ironically, help the positrons to leave the surface and avoid annihilation," said Allen Mills, a professor of physics and astronomy, in whose lab Cassidy works. "They do this by forming positronium, which is spontaneously emitted from the surface. The free positronium lives more than 200 times longer than the surface , so it is easy to detect."

Study results appear in the July 15 issue of Physical Review Letters.

The researchers chose silicon in their experiments because it has wide application in electronics, is robust, cheap and works efficiently.

"Indeed, at very low temperatures, silicon may be the best thing there is for producing positronium, at least in short bursts," Cassidy said.

The researchers' eventual goal is to perform precision measurements on positronium in order to better understand antimatter and its properties, as well as how it might be isolated for longer periods of time.

Cassidy and Mills were joined in the research by Harry Tom, a professor and the chair of physics and astronomy, and Tomu H. Hisakado, a graduate student in Mills's lab.

In the near future, this research team hopes to cool the positronium down to lower energy emission levels for other experimental uses, and create also a "Bose-Einstein condensate" for positronium – a collection of positronium atoms that are in the same quantum state.

"The creation of a Bose-Einstein condensate of positronium would really push the boundaries of what is possible in terms of real precision measurements," Cassidy said. "Such measurements would shed more light on the properties of antimatter and may help us probe further into why there is asymmetry between matter and antimatter in the universe."

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1 / 5 (4) Jul 11, 2011
Wouldn't this destroy itself really quickly?

Positrons are positive, electrons are negative, they should attract.
3.7 / 5 (3) Jul 11, 2011
Depends on if its ortho or para positornium (spin alignments) mean half life is on the order of 10^-7 s and 10^-10 s respectively
2 / 5 (4) Jul 11, 2011
Is it better to ask why positronium (or positron) is unstable, rather than asking why there is asymmetry between matter and antimatter?
2.5 / 5 (4) Jul 11, 2011
I think the real question here is could they produce antimatter cheaply and in large enough quantities to power spaceships and other vehicles? Or is this just a slight improvement from before and it's still just wishful thinking?
3.7 / 5 (3) Jul 11, 2011
Why does positronium leave the surface? It should be as chemically reactive as atomic hydrogen and make a silane like bond with the silicon surface. Or is the silicon surface not pure silicon?

Making a Bose Einstein condensate with particles that decay so fast is a tall order. Has a condensate been made from atomic hydrogen yet? Or from antihydrogen? Both of these would be easier as there is an indefinite time to cool and condense.
not rated yet Jul 12, 2011
i think the laser knocks of the electrons and their momentum carries them away from the surface while forming positronium in midair??
5 / 5 (1) Jul 12, 2011
Anti-hydrogen (an anti-proton and a positron in a bound state) is more interesting in some ways than the positronium described here. It is now possiblwe to keep it for up to 15 minutes, so there is time to measure its properties, and compare with ordinary hydrogen.
not rated yet Jul 12, 2011
Out of curiosity, how/where are they getting positrons?
3 / 5 (2) Jul 12, 2011
Is it better to ask why positronium (or positron) is unstable,

It isn't unstable (in fact it is indefinitely stable in a complete vacuum or in a world made of antimatter - just like electrons are here). A positron just gets annihilated as soon as it hits anything made of regular matter.

I think the real question here is could they produce antimatter cheaply and in large enough quantities to power spaceships and other vehicles

No. The energy cost to produce positrons (and especially anti-hydrogen) is much, much, MUCH higher than the energy that is stored in them for duture matter-antimatter reactions.

Why does positronium leave the surface?

Positronium should be electrically neutral so there's no real point for it to stick around. If it's created from electrons that were already leaving the silicon substrate (because of excitation by the laser) then the net impulse will be away from the surface.
not rated yet Jul 12, 2011
Out of curiosity, how/where are they getting positrons?

The article doesn't mention that, which is a "sin" of omission. However, a few ways are known:
"Positrons are produced at the facility in two ways: either from pair production in a target or from the decay of 22Na."

not rated yet Jul 12, 2011
A lot of posters above appear to be confusing this discussion (of positronium?) with that of anti-hydrogen. Anti-hydrogen is a genuine molecule comprised of an anti-electron bound with an anti-proton, and could theoretically potentially form a stable gas provided it never encounters any normal matter. What this article discusses is the "joining" of an anti-electron with an electron, which, according to what I used to know, should immediately anihilate each other and produce a pair of xray photons of equivalent energy to their mass.

Is this team claiming they've figured out how to get anti-electrons (positrons) to join with negative electrons? Having opposite charges, I'd think that's trivial.
not rated yet Jul 12, 2011
Thanks David!
not rated yet Jul 12, 2011

It isn't unstable (in fact it is indefinitely stable in a complete vacuum or in a world made of antimatter - just like electrons are here). A positron just gets annihilated as soon as it hits anything made of regular matter.

Where can we find a complete vacuum?
not rated yet Jul 17, 2011
why don't they just use dilithium crystals? kind of seems obvious.

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