حالت ِ هویل
Fr.: état de Hoyle
An → excited state in the
→ triple alpha process leading to the production of
the most abundant → isotope of → carbon.
The existence of this state is of extreme astrophysical importance concerning the
→ nucleosynthesis of 12C in stellar
4He + 4He ↔ 8Be,
8Be + 4He ↔ 12C*,
12C*→ 12C + γ.
The process proceeds as follows. First the unstable
→ ground state of 8Be is formed in
the collision of two
→ alpha particles. Since 8Be exists
roughly 7 x 10-17 sec, it must fuse with an alpha particle before breaking up.
However, the probability of three bodies merging simultaneously is extremely
low. Hoyle showed that the 12C nucleus needs an excited state or
resonance at 7.68 MeV to provide for a high reaction probability.
The Hoyle state was soon found at 7.65 MeV with the predicted
→ spin and → parity.
In honor of the British astrophysicist Fred Hoyle (1915-2001), who predicted
this state in 1953 (Hoyle et al. 1953, Physical Review 92, 1095); it
was discovered by W. A. Fowler in 1957; → state.