Abstract
The reaction $^{11}B+p$ has been used to populate the $(J^{\pi},T)=(2^+,1)$ state at an excitation energy of 16.11 MeV in $^{12}$C, and the breakup of the state into three $\alpha$ particles has been studied in complete kinematics. A two-step breakup model which includes interference effects is found to provide the most accurate description of the experimental data. The branching ratio to the ground state of $^8$Be is determined to be 5.1(5)% in agreement with previous findings, but more precise by a factor of two, while the decay to the first-excited state in $^8$Be is found to be dominated by $d$-wave emission.
Highlights
The breakup of the excited 12C nucleus into three αparticles has been studied since the days of Lord Rutherford, motivated by a desire to understand the breakup mechanism and gain new insights into the nuclear structure [1]
This led to the suggestion that the breakup of the 16.11 MeV state proceeds directly to the 3α final state [3], but it was later shown that the breakup can be described within a more sophisticated sequential model, which takes into account the interference due to Bose symmetry in the 3α final state [4,5,6,7,8]
Using the β decay of 12N as a means to populate the 12.71 MeV state, the breakup was measured in complete kinematics for the first time and was shown to be in quantitative agreement with a sequential model based on the R-matrix formalism [10]
Summary
The breakup of the excited 12C nucleus into three αparticles has been studied since the days of Lord Rutherford, motivated by a desire to understand the breakup mechanism and gain new insights into the nuclear structure [1]. The sequential model was successfully applied to describe the breakup of several states in 12C [2], but it failed in the case of the (Jπ, T ) = (2+, 1) state at an excitation energy of 16.11 MeV. This led to the suggestion that the breakup of the 16.11 MeV state proceeds directly to the 3α final state [3], but it was later shown that the breakup can be described within a more sophisticated sequential model, which takes into account the interference due to Bose symmetry in the 3α final state [4,5,6,7,8]. In the same experiment the breakup of the (0+, 0) state at 7.65 MeV was shown to be primarily sequential [12]; see refs. [13,14,15]
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