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Although there are nine known isotopes of helium (He) (standard atomic mass: 4.002602(2) u), only helium-3 () and helium-4 () are stable. All radioisotopes are short-lived, the longest-lived being 6He with a half-life of 806.7 milliseconds. The least stable is , with a half-life of 7.6×10−22 seconds, although it is possible that has an even shorter half-life. In the Earth's atmosphere, there is one atom for every million atoms.〔 〕 However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of is around a hundred times higher.〔 〕 Rocks from the 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 and below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: atoms are bosons while atoms are fermions).〔 〕 Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins. == Helium-2 (diproton) == Helium-2 or , also known as a diproton, is an extremely unstable isotope of helium that consists of two protons without any neutrons. According to theoretical calculations it would have been much more stable (although still beta decaying to deuterium) had the strong force been 2% greater.〔R. A. W. Bradford, ''J. Astrophys. Astr.'' (2009) 30, 119–131 (The Effect of Hypothetical Diproton Stability on the Universe )〕 Its instability is due to spin/spin interactions in the nuclear force, and the Pauli exclusion principle, which forces the two protons to have anti-aligned spins and gives the diproton a negative binding energy.〔''Nuclear Physics in a Nutshell'', C. A. Bertulani, Princeton University Press, Princeton, N.J., 2007, Chapter 1, ISBN 978-0-691-12505-3.〕 There may have been observations of . In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps a nucleus.〔(Physicists discover new kind of radioactivity ), in (physicsworld.com ) Oct 24, 2000〕〔(Decay of a Resonance in 18Ne by the Simultaneous Emission of Two Protons ), ''Physical Review Letters'' vol. 86, pp. 43–46 (2001), by J. Gómez del Campo, A. Galindo-Uribarri et al.〕 The team led by Alfredo Galindo-Uribarri of the Oak Ridge National Laboratory announced that the discovery will help scientists understand the strong nuclear force and provide fresh insights into the creation of elements inside stars. 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 that the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce , which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. There are two ways in which the two-proton emission may proceed. The neon nucleus might eject a 'diproton'—a pair of protons bound together as a nucleus—which then decays into separate protons. Alternatively, the protons may be emitted separately but at the same time—so-called 'democratic decay'. The experiment was not sensitive enough to establish which of these two processes was taking place. More evidence of was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy.〔("New Form of Artificial Radioactivity" ) ''Inside Physics Research—Science News Update'' Number 865 #2, May 29, 2008 by Phil Schewe〕〔G. Raciti ''et al.'', ''Physical Review Letters'' 100, 192503–06 (2008) "Experimental Evidence of 2He Decay from 18Ne Excited States"〕 A beam of ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into nuclei. These nuclei then collided with a foil of lead. The second collision had the effect of exciting the nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the nucleus decayed into an 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 JINR in Dubna, Russia, where beams of nuclei were directed at a cryogenic hydrogen target to produce . It was discovered that the nucleus can donate all four of its neutrons to the hydrogen. The two remaining protons could be simultaneously ejected from the target as a nucleus, which quickly decayed into two protons. A similar reaction has also been observed from nuclei colliding with hydrogen. is an intermediate in the first step of the proton-proton chain reaction. The first step of the proton-proton chain reaction is a two-stage process; first, two protons fuse to form a diproton: : followed by the immediate beta-plus decay of the diproton to deuterium: : with the overall formula: : R. A. W. Bradford has considered the hypothetical effect of this isotope on Big Bang and stellar nucleosynthesis.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Isotopes of helium」の詳細全文を読む スポンサード リンク
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