|
Uranium (U) is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes (uranium-238 and uranium-235) that have long half-life and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234. The average atomic mass of natural uranium is 238.02891(3) u. Other isotopes such as uranium-232 have been produced in breeder reactors. Naturally occurring uranium is composed of three major isotopes, uranium-238 (99.2739–99.2752% natural abundance), uranium-235 (0.7198–0.7202%), and uranium-234 (0.0050–0.0059%). All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.4683 years (close to the age of the Earth). Uranium-238 is an α emitter, decaying through the 18-member uranium series into lead-206. The decay series of uranium-235 (historically called actino-uranium) has 15 members that ends in lead-207. The constant rates of decay in these series makes comparison of the ratios of parent to daughter elements useful in radiometric dating. Uranium-233 is made from thorium-232 by neutron bombardment. The isotope uranium-235 is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, can be broken apart by thermal neutrons. The isotope uranium-238 is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile. ==Uranium-232== Uranium 232 (, , U-232) is an isotope of uranium. It has a half-life of 68.9 years and is a side product in the thorium cycle. It has been cited as an obstacle to nuclear proliferation using 233U as the fissile material, because the intense gamma radiation of 232U's decay products makes the 233U contaminated with it more difficult to handle. Production of 233U (through the neutron irradiation of 232Th) invariably produces small amounts of 232U as an impurity, because of parasitic (n,2n) reactions on uranium-233 itself, or on protactinium-233: :232Th (n,γ) 233Th (β−) 233Pa (β−) 233U (n,2n) 232U :232Th (n,γ) 233Th (β−) 233Pa (n,2n) 232Pa (β−) 232U The decay chain of 232U quickly yields strong gamma radiation emitters: :232U (α, 68.9 years) :228Th (α, 1.9 year) :224Ra (α, 3.6 day, 0.24 MeV) (at this point, the decay chain is identical to that of 232Th) :220Rn (α, 55 s, 0.54 MeV) :216Po (α, 0.15 s) :212Pb (β−, 10.64 h) :212Bi (α, 61 m, 0.78 MeV) :208Tl (β−, 3 m, 2.6 MeV) (35.94% branching ratio) :208Pb (stable) This makes manual handling in a glove box with only light shielding (as commonly done with plutonium) too hazardous, (except possibly in a short period immediately following chemical separation of the uranium from thorium-228, radium-224, radon-220, and polonium) and instead requiring remote manipulation for fuel fabrication. Unusually for an isotope with even mass number, 232U has a significant neutron absorption cross section for fission (thermal neutrons , resonance integral ) as well as for neutron capture (thermal , resonance integral ). 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 uranium」の詳細全文を読む スポンサード リンク
|