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לקריאה נוספת: מאמר מקיף בעברית על חומר ואנטי-חומר
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Antimatter, a substance that often features in science fiction, is routinely created at the CERN particle
physics laboratory in Geneva, Switzerland, to provide us with a better understanding of atoms and
molecules. Now, RIKEN scientists, as part of a collaborative team with researchers from Denmark,
Japan, the United Kingdom and Hungary, have shown that antiprotons—particles with the same
mass as a proton but negatively charged—collide with molecules in a very different way from
their interaction with atoms1. The result sets an important benchmark for testing future
atomic-collision theories.
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RIKEN scientist Yasunori Yamazaki explains that to assess such collisions: “We shot the simplest
negatively charged particles, slow antiprotons, at the simplest molecular target, molecular hydrogen.”
Slow antiprotons are a unique probe of atoms and molecules because their negative charge does
not attract electrons—thereby simplifying theoretical modelling. Further, slower projectile speeds
mean longer-lasting, stronger interactions and avoid the need for complicated relativistic
calculations.
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The scientists at CERN created antiprotons by firing a beam of high-speed protons into a block of the
metal iridium. Then, in a facility known as the Antiproton Decelerator, they used magnets to focus the
antiprotons before applying strong electric fields to slow them down to approximately 10% of the speed
of light. Yamazaki and his colleagues trapped and cooled these antiprotons to 0.01% of the velocity
of light before accelerating them one by one to the desired velocity (Fig. 1). They then slammed
antiprotons into a gas of molecular deuterium—a pair of bound hydrogen atoms each with a nucleus
comprising one proton and one neutron—and used sensitive equipment to detect the remnants of
the collision.
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Figure 1: A schematic diagram of the antiproton decelerator at CERN that is used to smash antiprotons
and molecular hydrogen molecules to together so that the remaining particles can be analyzed to
provide insight to their interactions.
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Yamazaki and the team found that the likelihood of the ionization of the deuterium molecules scales
linearly with the antiproton velocity. This is contrary to what is expected for the atomic target, hydrogen.
“This was a big surprise, and it infers that our understanding of atomic collision dynamics, even at a
qualitative level, is still in its infancy,” says Yamazaki. The team suggests that molecular targets provide
a mechanism for suppressing the ionization process. As an antiproton approaches one of the protons in
the molecule, the presence of the second proton shifts the orbiting electron cloud. The slower the
antiproton, the more time the electron has to adjust, and hence the smaller the chance of ionization.
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The team now hopes to investigate how ionization depends on the antiproton–target distance and the
orientation at the moment of collision.
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