Wednesday, August 13, 2008

No Surprise to a Radiation Physicist

The e-mail note for "Physical Review Focus 11 August 2008" carried the news story entitled "Antimatter Bounces off Matter" (online Focus story is given in [1]). The story tells that in the August Physical Review A, a team of Italian researchers reports that a good fraction of a low energy antimatter beam directed at a normal matter wall will bounce right back.

Fans of science fiction know that the meeting of matter and antimatter results in the annihilation of both, accompanied by a release of a tremendous amount of energy. Therefore, the above result, based on a new analysis of 12-year-old data, is reported to be "surprising even to most physicists, though it is explained by basic textbook principles."

I have read the full story [1] and the abstract of the paper [2] written by the Italian physicists. The antimatter in their experiment was a beam of low energy antiprotons, and they observed a large fraction of the beam was reflected by an aluminum wall at the end of the apparatus. They made a Monte Carlo simulation of the antiproton path in aluminum, and found that the observed reflection occurred primarily via a multiple Rutherford scattering on Al nuclei.

The phenomenon is the backscattering of antiprotons from a thick layer of matter. Radiation physicists well know the same phenomenon for the beams of electrons, protons, ions and positrons (note that positrons are the antiparticles of electrons). I'm one of those physicists, and especially studied the backscattering of electrons and ions. Thus the story came as no surprise to me.

The abstract concludes with the sentence, "These results contradict the common belief according to which the interactions between matter and antimatter are dominated by the reciprocally destructive phenomenon of annihilation." I would like to say, "It is rather surprising that such a common belief has been held not only by fans of science fiction but also by high-energy physicists."

Antiprotons having some energy and passing far from nuclei behave like ordinary particles with a unit negative charge, and suffer the same amount of Rutherford scattering as that protons do; they annihilate with protons only when they come quite close to nuclei. This should be a common belief about antiproton beams instead of the one described by the Italian team.

  1. Antimatter Bounces Off Matter, Physical Review Focus, 11 August 2008.
  2. A. Bianconi, et al., Experimental evidence of antiproton reflection by a solid surface, Phys. Rev. A Vol. 78 (issue of August 2008).

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