Intermediate-structure fusion resonances observed in the radiative capture of ${}^{12}C$ by ${}^{12}C$

We have investigated the

{}^{12}C

(

{}^{12}C

γ

)

{}^{24}{Mg}

reaction, populating low-lying states of

{}^{24}{Mg}

, with

{}^{12}C

{}^{12}C

c.m. energies between 5 and 11 MeV. High energy

γ

rays were detected in a large volume NaI spectrometer which had sufficient resolution to distinguish clearly the

γ_{0}

γ_{1}

, and

γ_{2, 3}

transitions. Several resonances were observed in each of the decay channels. In particular, the

γ_{0}

excitation function has four narrow resonances at

E_{c . m .} = 5.6, 6.0, 6.8, and 8.0

MeV, which are unambiguously assigned

J^{π} = 2^{+}

. The upper three of these resonances are correlated in all three decay channels but are not correlated either with previously identified

2^{+}

structures in the

{}^{12}C

{}^{12}C

system or with structure in the distribution of ground-state

E 2

strength in

{}^{24}{Mg}

. Furthermore, calculations demonstrate that these resonances have large nonstatistical

{}^{12}C

- and photon-partial widths. We argue that the data are suggestive of a new and unusual form of intermediate structure, namely highly clustered

{}^{12}C

{}^{12}C

configurations built on the ground-state band of

{}^{24}{Mg}

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Intermediate-structure fusion resonances observed in the radiative capture ofC12byC12