A nuclear reaction in which a heavy atomic nucleus such as uranium or thorium splits into two or more atomic nuclei with similar mass numbers. It was discovered in 1938 by O. Hahn and F. Strassmann. During nuclear fission, a very large amount of energy is released. A typical example of nuclear fission is when the uranium isotope uranium 235 captures a thermal neutron, releasing two or three neutrons and splitting into two atomic nuclei with mass numbers of 96 and 140. The energy released as the kinetic energy of the two atomic nuclei reaches about 200 MeV. When another uranium 235 captures the released neutron, nuclear fission occurs, causing a chain reaction. An atomic bomb causes this chain reaction to occur all at once, while an atomic reactor causes it to proceed in a controlled manner. Many of the atomic nuclei produced by nuclear fission are radioactive, and become stable nuclei after two to five consecutive beta decays. In addition to thermal neutrons, nuclear fission can occur in nuclides with atomic numbers of 73 or higher due to low- and high-energy neutrons, deuterium nuclei, alpha particles, and gamma rays. Ternary fission, in which an atomic nucleus splits into three nuclei, occurs at a rate of about 0.3% of normal binary fission, and tetradial fission, in which an atomic nucleus splits into four, occurs at a rate of about 0.03%. Spontaneous nuclear fission not caused by particle irradiation also occurs, but the probability of this occurring is generally extremely small. Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information |
ウランやトリウムなどの重い原子核が,同程度の質量数をもつ2個以上の原子核に分裂する核反応。 1938年 O.ハーンと F.シュトラスマンが発見した。核分裂においては,非常に大きなエネルギーが放出される。代表的な核分裂は,ウランの同位体ウラン 235が熱中性子を捕獲して2個ないし3個の中性子を放出するとともに質量数 96と 140を中心とする2個の原子核に分裂する現象で,このとき2個の原子核の運動エネルギーとして解放されるエネルギーは約 200MeV に達する。放出中性子を他のウラン 235が捕獲すると核分裂が起り,次々に連鎖反応を引起す。この連鎖反応を一時に起すのが原子爆弾であり,制御しつつ進行させるのが原子炉である。核分裂で生成される原子核の多くは放射性で,β崩壊を2回ないし5回続けて安定な原子核となる。熱中性子のほかに低・高エネルギー中性子,重水素核,α粒子,γ線などにより原子番号 73以上の核種で核分裂が起ることがある。また通常の二体分裂の約 0.3%の割合で3個の原子核に分裂する三体分裂,約 0.03%の割合で4個に分裂する四体分裂が起る。粒子照射などによらない自発核分裂も起るが,その確率は一般にきわめて小さい。
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