|
In nuclear physics, the island of stability is the prediction that a set of heavy isotopes with a near magic number of protons and neutrons will temporarily reverse the trend of decreasing stability in elements heavier than Uranium. Although predictions of the exact location differ somewhat, Klaus Blaum expects the island of stability to occur in the region near the isotope 300Ubn.〔 Quoting: ''“We expect (island of stability ) at around element 120,” says Blaum, “and to be more precise, in a nucleus with around 180 neutrons.”''〕 Estimates about the amount of stability on the island are usually around a half-life of minutes or days, with "some optimists" expecting half-lives of millions of years.〔(【引用サイトリンク】 url = http://www.physorg.com/news173028810.html )〕 Although the theory has existed since the 1960s, the existence of such superheavy, relatively stable isotopes has not been demonstrated. Like the rest of the superheavy elements, the isotopes on the island of stability have never been found in nature, and so must be created in an artificial nuclear reaction to be studied. However, scientists have not found a way to carry out such a reaction. ==Theory and origin== The possibility of an "island of stability" was first proposed by Glenn T. Seaborg in the late 1960s.〔(【引用サイトリンク】title=The Island of Stability? )〕 The hypothesis is based upon the nuclear shell model, which implies that the atomic nucleus is built up in "shells" in a manner similar to the structure of the much larger electron shells in atoms. In both cases, shells are just groups of quantum energy levels that are relatively close to each other. Energy levels from quantum states in two different shells will be separated by a relatively large energy gap. So when the number of neutrons and protons completely fills the energy levels of a given shell in the nucleus, the binding energy per nucleon will reach a local maximum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not possess filled shells. A filled shell would have "magic numbers" of neutrons and protons. One possible magic number of neutrons for spherical nuclei is 184, and some possible matching proton numbers are 114, 120 and 126 – which would mean that the most stable spherical isotopes would be flerovium-298, unbinilium-304 and unbihexium-310. Of particular note is Ubh-310, which would be "doubly magic" (both its proton number of 126 and neutron number of 184 are thought to be magic) and thus the most likely to have a very long half-life. (The next lighter doubly magic spherical nucleus is lead-208, the heaviest known stable nucleus and most stable heavy metal.) Recent research indicates that large nuclei are deformed, causing magic numbers to shift. Hassium-270 is now believed to be a doubly magic deformed nucleus, with deformed magic numbers 108 and 162. It has a half-life of 3.6 seconds. Isotopes have been produced with enough protons to plant them upon an island of stability but with too few neutrons to even place them upon the island's outer "shores". It is possible that these elements possess unusual chemical properties and, if they have isotopes with adequate lifespans, would be available for various practical applications (such as particle accelerator targets and as neutron sources as well). In particular, the very small critical masses of transplutonic elements (possibly as small as grams) implies that if stable elements could be found, they would enable small and compact nuclear bombs either directly or by serving as primaries to help ignite fission/fusion secondaries; this possibility motivated much of the early research and multiple nuclear tests by the United States (including Operation Plowshare) and the Soviet Union aimed at producing such elements.〔pp. 129–133, ''The physical principles of thermonuclear explosives, inertial confinement fusion, and the quest for fourth generation nuclear weapons'' (Andre Gsponer and Jean-Pierre Hurni 2009)〕 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「Island of stability」の詳細全文を読む スポンサード リンク
|