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Register a new userHigh precision probe of the fully sequential decay width of the Hoyle state in $^{12}$C
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The decay path of the Hoyle state in $^{12}$C ($E_x=7.654textrm{MeV}$) has been studied with the $^{14}textrm{N}(textrm{d},alpha_2)^{12}textrm{C}(7.654)$ reaction induced at $10.5textrm{MeV}$. High resolution invariant mass spectroscopy techniques have allowed to unambiguously disentangle direct and sequential decays of the state passing through the ground state of $^{8}$Be. Thanks to the almost total absence of background and the attained resolution, a fully sequential decay contribution to the width of the state has been observed. The direct decay width is negligible, with an upper limit of $0.043%$ ($95%$ C.L.). The precision of this result is about a factor $5$ higher than previous studies. This has significant implications on nuclear structure, as it provides constraints to $3$-$alpha$ cluster model calculations, where higher precision limits are needed.
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Production of $alpha$-particle triples in the Hoyle state (HS) in dissociation of ${}^{12}$C nuclei at 3.65 and 0.42 $A$ GeV in nuclear track emulsion is revealed by the invariant mass approach. Contribution of the HS to the dissociation ${}^{12}$C $to$ 3$alpha$ is (11 $pm$ 3) %. Reanalysis of data on coherent dissociation ${}^{16}$O $to$ 4$alpha$ at 3.65 $A$ GeV is revealed the HS contribution of (22 $pm$ 2) %.
Electron scattering off the first excited 0+ state in 12C (the Hoyle state) has been performed at low momentum transfers at the S-DALINAC. The new data together with a novel model-independent analysis of the world data set covering a wide momentum transfer range result in a highly improved transition charge density from which a pair decay width Gamma_pi = (62.3 +- 2.0) micro-eV of the Hoyle state was extracted reducing the uncertainty of the literature values by more than a factor of three. A precise knowledge of Gamma_pi is mandatory for quantitative studies of some key issues in the modeling of supernovae and of asymptotic giant branch stars, the most likely site of the slow-neutron nucleosynthesis process.
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%.
Production of ensembles of $alpha$-particle triples associated with the Hoyle state (the second excited state of the ${}^{12}$C nucleus) in peripheral dissociation of relativistic ${}^{12}$C nuclei is studied. Stacks of nuclear track emulsion pellicles exposed to ${}^{12}$C with an energy from hundreds MeV to a few GeV per nucleon serve as the material for studies. The Hoyle state decays are reconstructed via measurement of emission angles of $alpha$ particles with the precision sufficient for identification of the unstable ${}^{8}$Be nucleus. The estimate of the contribution of Hoyles state to the ${}^{12}$C $to$ 3$alpha$ dissociation is 10-15%.
The cascading 3.21 MeV and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in $^{12}$C, excited by the $^{12}$C(p,p$^{prime}$) reaction at 10.7 MeV proton energy. From the proton-$gamma$-$gamma$ triple coincidence data, a value of ${Gamma_{rm rad}}/{Gamma}=6.2(6) times 10^{-4}$ was obtained for the radiative branching ratio. Using our results, together with ${Gamma_{pi}^{E0}}/{Gamma}$ from Eriksen et al., Phys. Rev. C 102, 024320 and the currently adopted $Gamma_{pi}(E0)$ values, the radiative width of the Hoyle state is determined as $Gamma_{rm rad}=5.1(6) times 10^{-3}$ eV. This value is about 34% higher than the currently adopted value and will impact on models of stellar evolution and nucleosynthesis.
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