"Radiative capture in the 12C+16O system: structural vs statistical aspects of the decay"
Sandrine Courtin, IPHC and University of Strasbourg
(id #82)
Seminar: YesPoster: No
Invited talk: No
The radiative capture process is the complete fusion of target and projectile to form a
compound nucleus which deexcitates solely by γ-ray emissions. This mechanism is a
powerful tool to investigate the interplay between reaction mechanisms and the structure of
the populated compound nucleus (CN). Such informations result from an overlap not only
between the entrance channel and the CN states but also between these states and lower lying
states. From the connecting γ transitions, information can be obtained on spins, isospins and
deformations of the populated CN states [1]. Heavy ion radiative capture is nonetheless a rare
process compared to dominant fusion-evaporation channels. From the experimental point of
view, measuring the CN at 0° and the coincident γ-rays accurately can be very challenging.
We have investigated the light heavy-ion 12C(16O,γ)28Si resonant reaction at Triumf using the
Dragon spectrometer to identify the 28Si recoiling nuclei at 0° and measure the complete 28Si
decay spectrum in the associated BGO array [2]. Our first data, taken at Ecm= 8.5, 8.8 and 9
MeV revealed a previously unobserved large feeding of intermediate energy doorway states
(around 11 MeV) in 28Si. This decay branch represents 60% of the total γ width. A question
arises about these doorway states: are they the result of structural or statistical aspects of the
decay? For instance, we have observed in this system an important flux to 28Si deformed
states such as the octupole 3- and members of the prolate band of this oblate ground state
nucleus. A discussion will be presented about this selective feeding: this could highlight the
long debated link between resonances in the 12C+16O collisions and molecular 12C-16O
configurations in 28Si which is a key nucleus for the occurrence of clusters in the sd shell.
Moreover these results have large consequences on the cross-section of the capture process
itself.
The very recent focus of our experimental programme is to explore 12C+16O resonances below
the Coulomb barrier (CB) where the reduction of the phase space may reveal new structural
features of the resonance decay. We have thus performed recently a new Dragon
12C(16O,γ)28Si capture experiment at resonant energies Ecm= 6.6 MeV and 7.2 MeV, below CB
(ECB~ 7.8 MeV) well below CB to deepen our understanding of the statistical vs structural
part of the decay. First results will be presented, which could be important for our
understanding of fusion not only at CB but also at lower energies, down to the astrophysics
region where unexpected resonances are observed close to the Gamow energy in the similar
12 C+12C system [3].
[1] A.M. Sandorfi, Treatise on Heavy-ion Science, Vol. 2, 53 (1983) and references therein.
[2] S. Courtin et al., J. Phys.: Conf. Ser. 111, 012005 (2008).
[3] T. Spillane et al., Phys. Rev. Lett. 98 122501 (2007) and F. Strieder, private
communication.
compound nucleus which deexcitates solely by γ-ray emissions. This mechanism is a
powerful tool to investigate the interplay between reaction mechanisms and the structure of
the populated compound nucleus (CN). Such informations result from an overlap not only
between the entrance channel and the CN states but also between these states and lower lying
states. From the connecting γ transitions, information can be obtained on spins, isospins and
deformations of the populated CN states [1]. Heavy ion radiative capture is nonetheless a rare
process compared to dominant fusion-evaporation channels. From the experimental point of
view, measuring the CN at 0° and the coincident γ-rays accurately can be very challenging.
We have investigated the light heavy-ion 12C(16O,γ)28Si resonant reaction at Triumf using the
Dragon spectrometer to identify the 28Si recoiling nuclei at 0° and measure the complete 28Si
decay spectrum in the associated BGO array [2]. Our first data, taken at Ecm= 8.5, 8.8 and 9
MeV revealed a previously unobserved large feeding of intermediate energy doorway states
(around 11 MeV) in 28Si. This decay branch represents 60% of the total γ width. A question
arises about these doorway states: are they the result of structural or statistical aspects of the
decay? For instance, we have observed in this system an important flux to 28Si deformed
states such as the octupole 3- and members of the prolate band of this oblate ground state
nucleus. A discussion will be presented about this selective feeding: this could highlight the
long debated link between resonances in the 12C+16O collisions and molecular 12C-16O
configurations in 28Si which is a key nucleus for the occurrence of clusters in the sd shell.
Moreover these results have large consequences on the cross-section of the capture process
itself.
The very recent focus of our experimental programme is to explore 12C+16O resonances below
the Coulomb barrier (CB) where the reduction of the phase space may reveal new structural
features of the resonance decay. We have thus performed recently a new Dragon
12C(16O,γ)28Si capture experiment at resonant energies Ecm= 6.6 MeV and 7.2 MeV, below CB
(ECB~ 7.8 MeV) well below CB to deepen our understanding of the statistical vs structural
part of the decay. First results will be presented, which could be important for our
understanding of fusion not only at CB but also at lower energies, down to the astrophysics
region where unexpected resonances are observed close to the Gamow energy in the similar
12 C+12C system [3].
[1] A.M. Sandorfi, Treatise on Heavy-ion Science, Vol. 2, 53 (1983) and references therein.
[2] S. Courtin et al., J. Phys.: Conf. Ser. 111, 012005 (2008).
[3] T. Spillane et al., Phys. Rev. Lett. 98 122501 (2007) and F. Strieder, private
communication.
