Compound nucleus

Nuclear reactions include direct processes in which an incident particle collides with a nucleon of a target nucleus once to proceed with the reaction, multi-step processes in which two or more collisions occur, and, in the extreme case, a process in which an incident particle combines with a target nucleus to form a single nucleus, which then decays and emits particles. The nucleus formed in this intermediate state is called a compound nucleus, and a reaction that occurs via a compound nucleus is called a compound nuclear reaction.

This reaction was discovered by E. Fermi of Italy in 1934, and was later explained by N. H. D. Bohr of Denmark invented the concept of "compound nuclei."

In the region of relatively low excitation energies of compound nuclei, resonances appear corresponding to discrete energy levels. If the resonance width is Γ, then it is ħ/Γ (ħ=h/2π, h is the Planck constant).
is the lifetime of a compound nucleus, which is generally very long compared to the collision time. Therefore, in nuclear reaction theory, the process of compound nuclei formation and the process of their decay by emitting particles are treated as independent processes. If the excitation of nuclei by incident particles is likened to forced vibration, and the emission of particles to the damping of vibration, compound nuclear reactions are classically analogized by a forced vibration system with damping, and can be treated as being of the same form as the dispersion phenomenon of light. The Breit-Wigner dispersion formula is a quantum mechanical calculation of the cross section of a reaction based on this idea.

When the excitation energy of a compound nucleus becomes high, the energy levels become so wide that they overlap and no resonant levels appear, and the density of states becomes so large that it can be considered a statistical thermodynamic condensed phase, and particle emission can be considered a phenomenon similar to the evaporation of a liquid drop.

[Mitsuo Muraoka]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

原子核反応には、入射粒子が標的核の核子と1回衝突して反応が進む直接過程、2回以上衝突する多段階過程、さらにその極限の場合として入射粒子が標的核と一体となって一つの原子核を形成し、それが崩壊して粒子を放出する過程がある。この中間状態にできる原子核を複合核とよび、複合核を経由しておこる反応を複合核反応という。

　1934年イタリアのE・フェルミがみいだした反応で、デンマークのN・H・D・ボーアが「複合核」を考え出して説明した。

　複合核の比較的低い励起エネルギーの領域においては離散的なエネルギー準位に対応して共鳴が現れる。共鳴幅をΓとすると
　　ħ/Γ （ħ=h/2π,hはプランク定数）
が複合核の寿命で、一般に衝突時間に比べて非常に長い。そのため核反応の理論では、複合核のできる過程と、粒子を放出して崩壊する過程を独立なものとして取り扱う。入射粒子による核の励起を振動の強制に、そして粒子の放出を振動の減衰になぞらえると、複合核反応は古典的には減衰を伴う強制振動系によって類似され、光の分散現象と同じ形式のものとして扱える。この考えに従って量子力学的に反応の断面積を計算したものにブライト‐ウィグナーの分散公式がある。

　複合核の励起エネルギーが高くなるとエネルギー準位幅が大きくなり、互いに重なり合って共鳴準位は現れなくなる。そして状態密度がきわめて大きくなり、統計熱力学的な凝集相とみなすことが可能で、粒子放出は液滴の蒸発に似た現象と考えられる。

［村岡光男］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例