Positronium scatters like an electron

Nov 05, 2010 by Lin Edwards report
Image credit: Charles Darwin University

(PhysOrg.com) -- Positronium atoms have been found to scatter off gas particles in the same way as lone electrons, a finding which could help astronomers interpret some of their more puzzling observations, and which may have applications in medicine.

Positronium (Ps) atoms are extremely unstable hydrogen-like atoms containing a positron (the electron's positively charged antimatter partner) and an electron. They are formed in large quantities whenever positrons interact with matter, but disintegrate in less than a microsecond to produce gamma rays. An earlier PhysOrg article described the first observations of positronium molecules, which consist of two atoms of positronium.

Scientists have known that when electrons or positrons are fired through a gas, they are scattered off the gas particles at predictable rates. Researchers from the department of Physics and Astronomy at the University College London in the UK, expected positronium atoms to have different scattering rates since they are electrically neutral and double the mass of electrons or positrons. They were surprised to find their scattering rates were almost identical to those produced by bare electrons moving at the same velocity, as if the influence of the positron was somehow cloaked.

The researchers made their discovery by firing positronium atoms into various gases, including krypton, , and , at speeds of up to 4400 kilometers per second and impact energies of 250 eV.

Leader of the team, Gaetana Laricchia, said knowing how positronium interacts with the surrounding medium, whether it is interstellar gas or human tissue, is important, and the results may be useful for positron emission tomography (PET) scans used in medicine. In PET scanning a radioactive glucose tracer is injected into the human body. The tracer emits positrons and they in turn briefly produce positronium atoms that decay almost instantly to produce , which the scanner measures.

Lariccia said knowing more about the positronium scattering rates should make it possible to refine estimates of the distance traveled by positronium, which would allow tumor volumes to be measured with more accuracy. It could also help clarify how positronium deposit their energy as they collide with molecules in the tissue, which may help to limit damage to healthy tissues.

The findings, published in the journal Science, may also be useful to astronomers since positronium is known to form in space, and an understanding of how it interacts with interstellar gas clouds could help determine the positions of some mysterious positron sources in our galaxy.

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More information: Electron-Like Scattering of Positronium, S. J. Brawley et al, Science 5 November 2010: Vol. 330. no. 6005, p. 789. DOI:10.1126/science.1192322

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User comments : 9

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genastropsychicallst
1 / 5 (13) Nov 05, 2010
Positron scatter too real round Uranus, lambda after less crook Saturnus.
KwasniczJ
1 / 5 (10) Nov 05, 2010
If positrons would really scatter in the same way like electrons through remote clouds, it would mean, these clouds are composed of antimatter..
RigorMan
4 / 5 (2) Nov 05, 2010
The Science article looks more like sensational news than a real research paper: no details on the detection system, it is not said if they scan for gamma coincidence with E > 6.8 eV (it would mean that the Ps breaks up, the electon scatters and the positron goes somewhere else). In addition in the paper they refer to TOTAL cross section, not to SCATTERING alone, except in the title. This is a quite substantial difference! It entails inelastic excitations of the target, break-up, electon transfer (and why-not positronium transfer).
I'm very skeptical about these results!
Jarek
5 / 5 (1) Nov 05, 2010
Such separate behavior of electron (and positron) suggests that the distance between them is quite large - it's probably about ortho-positornium in which magnetic repelling make it more stable (142ns lifetime to 125ps for para-positronium).
I would say that they are hold together on relatively large distances (with magnetically synchronized spins) by the same phenomenon which holds other pairs of fermions, like Cooper pairs or nucleons in nucleus - that they are connected by quant of magnetic field/fluxon going through their spin axes (4th section of http://arxiv.org/abs/0910.2724 ).
Slotin
1 / 5 (4) Nov 06, 2010
Such separate behavior of electron (and positron) suggests that the distance between them is quite large.

This doesn't explain, why positronium scatters in the same way, like lone electron - which means, positive charge of positrons is suppressed in such mixture.

My feeling is, it's the consequence of the asymmetric arrangement of the experiment - inside of cloud of antimatter the positronium would be scattered like positron, instead and it's consequence of the fact, anihillation starts from surface to interior of positronium, so that the prevailing particle tends to remain at the very end. I'm opened to believe in weak charge asymmetry of positron, but such bias should be compensated first in experiment in symmetric arrangement.
Jarek
5 / 5 (1) Nov 07, 2010
Slotin, I think positrons from positronium are not scattered because they are practically immediately annihilated after breaking coupling with electron.
The problem of this experiment is that electron's charge isn't screened by positron's - they are coupled into positronium, but behave like two separate charged particles - so they are relatively distant but still somehow joined in stable, but relatively (to scattering) weak way, untrue?
So the real question is why they are still coupled on large distances - what synchronize their spins in opposite (repelling) positions so that they cannot rotate them (to attracting one) and immediately collapse and annihilate like para-positronium.
Such rotation looks as the basic way to destabilize ortho-positronium and produces one of three photons of its decay.
This magnetic coupling of fermions becomes natural when we accept that magnetic field is quantified not only in superconductors, like in this news: http://www.physor...923.html
Slotin
1 / 5 (4) Nov 07, 2010
The problem of this experiment is that electron's charge isn't screened by positron's. I think positrons from positronium are not scattered because they are practically immediately annihilated after breaking coupling with electron.
Isn't such explanation the whole story, after then? Why to discuss things like "why they are coupled on large distances", which has nothing to do with experimental results discussed, IMHO? Or maybe I didn't notice something?

The purpose of my remark was to make discussion more coherent and focused on problem of interpretation of experimental results.

Electron is coupled with positron at relatively large distances, because they're lightweight particles. Muonium would be smaller accordingly - but it's classical, well known physics. I don't see any apparent problem here.
Jarek
5 / 5 (1) Nov 07, 2010
No it's not the end of the story, because it's important that electrons behave as separate charged particles - their charge isn't screened by positron's ... while still being coupled, against Coulomb attraction they stay on relatively large distance - because (anti)parallel spins respells (attract) with force proportional to 1/r^4 ...
But so why they don't just rotate into more energetically preferable anti-parallel alignment and annihilate?
And here is where we need something to stabilize them in parallel alignment - some additional magnetic coupling ...
Slotin
1 / 5 (3) Nov 07, 2010
Nope - the question why antiparallel positronium annihilates slower is indeed interesting, but not related to question why it scatters like electron, because during such scattering the spin orientation wasn't studied. You're confusing two different aspects of positronium behavior: one depends on spin orentation of particles inside of positronium, the second one doesn't.

I have no problem with some magnetic coupling, but with such approach you could solve all problems of the whole world in the single moment - which is why I'm calling such thinking incoherent and off topic.

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