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Isotopes of helium
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Isotopes of helium
Helium (2He) has nine known isotopes, but only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived; the only particle-bound ones are 6He and 8He with half-lives 806.9 and 119.5 milliseconds.
In Earth's atmosphere, the ratio of 3He to 4He is 1.37×10−6. However, the isotopic abundance of helium varies greatly depending on its origin, though helium-4 is always in great preponderance. In the Local Interstellar Cloud, the proportion of 3He to 4He is 1.62(29)×10−4, which is about 120 times higher than in Earth's atmosphere. Rocks from Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to investigate the origin of rocks and the composition of the Earth's mantle. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.
Equal mixtures of liquid 3He and 4He below 0.8 K separate into two immiscible phases due to differences in quantum statistics: 4He atoms are bosons while 3He atoms are fermions. Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures as low as a few millikelvin.
A mix of the two isotopes spontaneously separates into 3He-rich and 4He-rich regions. Phase separation also exists in ultracold gas systems. It has been shown experimentally in a two-component ultracold Fermi gas case. The phase separation can compete with other phenomena as vortex lattice formation or an exotic Fulde–Ferrell–Larkin–Ovchinnikov phase.
Helium-2, 2He, is extremely unstable. Its nucleus, a diproton, consists of two protons with no neutrons. According to theoretical calculations, it would be much more stable (but still β+ decay to deuterium) if the strong force were 2% greater. Its instability is due to spin–spin interactions in the nuclear force and the Pauli exclusion principle, which states that within a given quantum system two or more identical particles with the same half-integer spins (that is, fermions) cannot simultaneously occupy the same quantum state; so 2He's two protons have opposite-aligned spins and the diproton itself has negative binding energy.
2He may have been observed. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps 2He. The team led by Alfredo Galindo-Uribarri of Oak Ridge National Laboratory announced that the discovery will help understand the strong nuclear force and provide fresh insights into stellar nucleosynthesis. Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce 18Ne, which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. The two-proton emission may proceed in two ways: the neon might eject a diproton, which then decays into separate protons, or the protons may be emitted separately but simultaneously in a "democratic decay". The experiment was not sensitive enough to establish which of these two processes was taking place.
More evidence of 2He was found in 2008 at Istituto Nazionale di Fisica Nucleare, in Italy. A beam of 20Ne ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into 18Ne nuclei. These nuclei then collided with a foil of lead. The second collision excited the 18Ne nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the 18Ne nucleus decayed into an 16O nucleus, plus two protons detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together, correlated in a quasibound 1S configuration, before decaying into separate protons much less than a nanosecond later.
Further evidence comes from Riken in Japan and Joint Institute for Nuclear Research in Dubna, Russia, where beams of 6He nuclei were directed at a cryogenic hydrogen target to produce 5H. It was discovered that the 6He can donate all four of its neutrons to the hydrogen.[citation needed] The two remaining protons could be simultaneously ejected from the target as a diproton, which quickly decayed into two protons. A similar reaction has also been observed from 8He nuclei colliding with hydrogen.
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Isotopes of helium
Helium (2He) has nine known isotopes, but only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived; the only particle-bound ones are 6He and 8He with half-lives 806.9 and 119.5 milliseconds.
In Earth's atmosphere, the ratio of 3He to 4He is 1.37×10−6. However, the isotopic abundance of helium varies greatly depending on its origin, though helium-4 is always in great preponderance. In the Local Interstellar Cloud, the proportion of 3He to 4He is 1.62(29)×10−4, which is about 120 times higher than in Earth's atmosphere. Rocks from Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to investigate the origin of rocks and the composition of the Earth's mantle. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.
Equal mixtures of liquid 3He and 4He below 0.8 K separate into two immiscible phases due to differences in quantum statistics: 4He atoms are bosons while 3He atoms are fermions. Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures as low as a few millikelvin.
A mix of the two isotopes spontaneously separates into 3He-rich and 4He-rich regions. Phase separation also exists in ultracold gas systems. It has been shown experimentally in a two-component ultracold Fermi gas case. The phase separation can compete with other phenomena as vortex lattice formation or an exotic Fulde–Ferrell–Larkin–Ovchinnikov phase.
Helium-2, 2He, is extremely unstable. Its nucleus, a diproton, consists of two protons with no neutrons. According to theoretical calculations, it would be much more stable (but still β+ decay to deuterium) if the strong force were 2% greater. Its instability is due to spin–spin interactions in the nuclear force and the Pauli exclusion principle, which states that within a given quantum system two or more identical particles with the same half-integer spins (that is, fermions) cannot simultaneously occupy the same quantum state; so 2He's two protons have opposite-aligned spins and the diproton itself has negative binding energy.
2He may have been observed. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps 2He. The team led by Alfredo Galindo-Uribarri of Oak Ridge National Laboratory announced that the discovery will help understand the strong nuclear force and provide fresh insights into stellar nucleosynthesis. Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce 18Ne, which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. The two-proton emission may proceed in two ways: the neon might eject a diproton, which then decays into separate protons, or the protons may be emitted separately but simultaneously in a "democratic decay". The experiment was not sensitive enough to establish which of these two processes was taking place.
More evidence of 2He was found in 2008 at Istituto Nazionale di Fisica Nucleare, in Italy. A beam of 20Ne ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into 18Ne nuclei. These nuclei then collided with a foil of lead. The second collision excited the 18Ne nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the 18Ne nucleus decayed into an 16O nucleus, plus two protons detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together, correlated in a quasibound 1S configuration, before decaying into separate protons much less than a nanosecond later.
Further evidence comes from Riken in Japan and Joint Institute for Nuclear Research in Dubna, Russia, where beams of 6He nuclei were directed at a cryogenic hydrogen target to produce 5H. It was discovered that the 6He can donate all four of its neutrons to the hydrogen.[citation needed] The two remaining protons could be simultaneously ejected from the target as a diproton, which quickly decayed into two protons. A similar reaction has also been observed from 8He nuclei colliding with hydrogen.