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Antiprotonic helium
Antiprotonic helium is a three-body atom composed of an antiproton and an electron orbiting around a helium nucleus. It is thus made partly of matter, and partly of antimatter. The atom is electrically neutral, since both electrons and antiprotons have a charge of −1, whereas helium nuclei have a charge of +2. The antiprotonic helium atom is called an atomcule, as it has a structure like an atom, but has two nuclei like a molecule.
== Production ==
These exotic atoms can be produced by simply mixing antiprotons with ordinary helium gas; the antiproton spontaneously removes one of the two electrons contained in a normal helium atom in a chemical reaction, and then begins to orbit the helium nucleus in the electron's place. This will happen in the case of approximately 3% of the antiprotons introduced to the helium gas. The antiproton's orbit, which has a large principal quantum number and angular momentum quantum number of around 38, lies far away from the surface of the helium nucleus. The antiproton can thus orbit the nucleus for tens of microseconds, before finally falling to its surface and annihilating. This long lifetime is seen only in the case of antiprotons mixed in helium; in all other substances, antiprotons annihilate a million times faster, in less than a picosecond.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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