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Register a new userDetermination of the cluster-decay branching ratio from a near-threshold molecular state in $^{10}$Be
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A puzzle has long existed for the $alpha$-cluster content in the near-threshold 7.54 MeV state of $^{10}$Be. A new measurement was conducted to measure the cluster-decay partial width of this state, using the reaction $rm{^9Be}(rm{^9Be}, rm{^{10}Be}^{*} rightarrow alpha + rm{^6He})rm{^8Be}$ at 45 MeV beam energy. Special measures were taken to reduce the strong near-threshold background. The neutron-decay strength was also obtained based on the three-fold coincident measurement. A cluster-decay branching ratio of $(4.04 pm 1.26)times 10^{-4}$ is obtained, resulting in a reasonably large $alpha$-cluster spectroscopic factor. The present work confirms the formation of the $sigma$-bond molecular rotational band headed by the 6.18 MeV state in $^{10}$Be.
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Stellar carbon synthesis occurs exclusively via the $3alpha$ process, in which three $alpha$ particles fuse to form $^{12}$C in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the $alpha$ threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the $E0$ decay branching ratio of the Hoyle state, and the current $10$% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The $E0$ branching ratio was deduced from a series of pair conversion measurements of the $E0$ and $E2$ transitions depopulating the $0^+_2$ Hoyle state and $2^+_1$ state in $^{12}$C, respectively. The excited states were populated by the $^{12}$C$(p,p^prime)$ reaction at 10.5 MeV beam energy, and the pairs were detected with the electron-positron pair spectrometer, Super-e, at the Australian National University. The deduced branching ratio required knowledge of the proton population of the two states, as well as the alignment of the $2^+_1$ state in the reaction. For this purpose, proton scattering and $gamma$-ray angular distribution experiments were also performed. An $E0$ branching ratio of $Gamma^{E0}_{pi}/Gamma=8.2(5)times10^{-6}$ was deduced in the current work, and an adopted value of $Gamma^{E0}_{pi}/Gamma=7.6(4)times10^{-6}$ is recommended based on a weighted average of previous literature values and the new result. The new recommended value for the $E0$ branching ratio is about 14% larger than the previous adopted value of $Gamma^{E0}_{pi}/Gamma=6.7(6)times10^{-6}$, while the uncertainty has been reduced from 9% to 5%.
In an experiment performed at the ISOLDE facility of CERN, the super-allowed beta-decay branching ratio of 10C was determined with a high-precision single-crystal germanium detector. In order to evaluate the contribution of the pile-up of two 511 keV gamma quanta to one of the gamma-ray peaks of interest at 1021.7 keV, data were not only taken with 10C, but also with a 19Ne beam. The final result for the super-allowed decay branch is 1.4638(50)%, in agreement with the average from literature.
Knowledge of the gamma-ray branching ratios of the 7.12-MeV state of 16O is important for the extrapolation of the 12C(a,g)16O cross section to astrophysical energies. Ground state transitions provide most of the 12C(a,g)16O total cross section while cascade transitions have contributions of the order of 10-20%. Determining the 7.12-MeV branching ratio will result in a better extrapolation of the cascade and E2 ground state cross section to low energies. We report here on measurements on the branching ratio of the 7.12-MeV level in 16O.
The 15O(alpha,gamma)19Ne reaction is one of two routes for breakout from the hot CNO cycles into the rp process in accreting neutron stars. Its astrophysical rate depends critically on the decay properties of excited states in 19Ne lying just above the 15O + alpha threshold. We have measured the alpha-decay branching ratios for these states using the p(21Ne,t)19Ne reaction at 43 MeV/u. Combining our measurements with previous determinations of the radiative widths of these states, we conclude that no significant breakout from the hot CNO cycle into the rp process in novae is possible via 15O(alpha,gamma)19Ne, assuming current models accurately represent their temperature and density conditions.
A resonant state at $3.21^{+0.12}_{-0.04}$,MeV, located just above the one-neutron separation threshold, was observed for the first time in $^{12}$Be from the $^{11}$Be,$(d,p)^{12}$Be one-neutron transfer reaction in inverse kinematics. This state is assigned a spin-parity of $0^-$, according to the distorted-wave Born approximation (DWBA) and decay-width analysis. Gamow coupled-channel (GCC) and Gamow shell-model (GSM) calculations show the importance of the continuum-coupling, which dramatically influences the excitation energy and ordering of low-lying states. Various exotic structures associated with cross-shell intruding configurations in $^{12}$Be and in its isotonic nucleus $^{11}$Li are comparably discussed.
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