How to build a gamma-ray laserwith antimatter hybrid
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All the elements in the periodic table consist of atoms with a nucleus of positively charged protons, orbited by the same number of negatively charged electrons. Positronium , symbol Ps, is different. It consists of an electron and apositron orbiting each other (seediagram) . A positron is the electron's antimatter counterpart.Though positively charged like the proton,it has just 0.0005 timesits mass. Positronium"atoms" survive less than a millionth of a second before the electron and positron annihilate in a burst of gamma rays.
In principle, positroniumcouldbe used to make agamma raylaser. It would produce a highly energetic beam of extremely short wavelength that could probe tiny structures including the atomic nucleus - the wavelength of visible light ismuch too long tobe of any use for this.
The trouble is that this means assembling a dense cloud of positronium in a quantum state known asa Bose-Einstein condensate (BEC). How to do this without the dense cloud of positronium in a quantum state known asa Bose-Einstein condensate (BEC). How to do this without the positronium annihilatinginthe process was unclear.
Now a team led by Christoph Keitel of the Max Planck Institute forNuclear Physics in Heidelberg, Germany, suggests that ordinary lasers could be used to slowthe annihilation. The trick is to tune the lasers to exactlythe energy needed to boost the positronium into a higherenergy state, in which the electron and positron orbit farther from one another. That makes them much less likely to annihilate ( arxiv.org/abs/1112.1621).
The positronium will eventually lose energy by emitting photons andreturn to the annihilation-prone state. But the team calculatesthat about half the excited positronium atoms can survive for 28 millionths of asecond on average, 200 times as long as unexcited ones.
This may be long enough to assemble the BEC cloud. In a BEC, positronium atoms behave in lockstep, so whenone annihilates itself, the rest follow suit, producing a burst of laser radiation madeof gamma rays.
It may sound like a lot of work, but one thing makes the task easier. Ordinary atoms can only form a BEC when cooled gradually to within a fraction of a degree of absolute zero. By contrast, due to quantum effects, positronium will form a BEC at close to room temperature.
Where mirror, dark andanti-matter meet Half a century after it was first made, positronium could find uses. As well as powering a gammaray laser (see main story), itmight put the strange theoryof mirror matter to the test.
The idea that every particle has an identical- but so far undetectable - mirror partner was dreamed up to explain baffling asymmetries in the emission of electrons from radioactive atoms.Mirror matter has alsobeen touted as a candidate for the mysterious dark matter that makes up 80 per cent of the universe.
The theory says that particles of ordinary matter might very occasionally transform into their mirror-reversed versions, effectively disappearingfrom view. Positronium normally ends its life byhurling out a flurry of gamma rays. If the mirror world exists, positronium might sometimes turn into mirror matter and vanish without these emissions.
The idea could be tested by trapping positronium in a chamber and keeping track of how much energy it gives off as gamma rays. If the amount is smaller than expected based on the number of positronium atomsthat entered the chamber, then some of it may be turning into mirror matter. New calculations by Sergei Demidov of the Institutefor Nuclear Research in Moscow,Russia, and colleagues indicate this should happen often enough to be detectable( arxiv.org/abs/1111.1072).
Paolo Crivelli of the Swiss Federal Institute of Technology in Zurich is leading the development of one such experiment ( arxiv.org/abs/1005.4802). The existing AEgIS antimatter experiment at CERN near Geneva, Switzerland, could also be modified for this purpose.
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