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Register a new userPair decay width of the Hoyle state and carbon production in stars
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Peter von Neumann-Cosel
Publication date
2010
fields
Physics
and research's language is
English
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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.
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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.
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.
We use a sequential $R$-matrix model to describe the breakup of the Hoyle state into three $alpha$ particles via the ground state of $^8mathrm{Be}$. It is shown that even in a sequential picture, features resembling a direct breakup branch appear in the phase-space distribution of the $alpha$ particles. We construct a toy model to describe the Coulomb interaction in the three-body final state and its effects on the decay spectrum are investigated. The framework is also used to predict the phase-space distribution of the $alpha$ particles emitted in a direct breakup of the Hoyle state and the possibility of interference between a direct and sequential branch is discussed. Our numerical results are compared to the current upper limit on the direct decay branch determined in recent experiments.
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%.
Background: The structure of the Hoyle state, a highly $alpha$-clustered state at 7.65 MeV in $^{12}mathrm{C}$, has long been the subject of debate. Understanding if the system comprises of three weakly-interacting $alpha$-particles in the 0s orbital, known as an $alpha$-condensate state, is possible by studying the decay branches of the Hoyle state. Purpose: The direct decay of the Hoyle state into three $alpha$-particles, rather than through the $^{8}mathrm{Be}$ ground state, can be identified by studying the energy partition of the 3 $alpha$-particles arising from the decay. This paper provides details on the break-up mechanism of the Hoyle stating using a new experimental technique. Method: By using beta-delayed charged-particle spectroscopy of $^{12}mathrm{N}$ using the TexAT (Texas Active Target) TPC, a high-sensitivity measurement of the direct 3 $alpha$ decay ratio can be performed without contributions from pile-up events. Results: A Bayesian approach to understanding the contribution of the direct components via a likelihood function shows that the direct component is $<0.043%$ at the 95% confidence level (C.L.). This value is in agreement with several other studies and here we can demonstrate that a small non-sequential component with a decay fraction of about $10^{-4}$ is most likely. Conclusion: The measurement of the non-sequential component of the Hoyle state decay is performed in an almost medium-free reaction for the first time. The derived upper-limit is in agreement with previous studies and demonstrates sensitivity to the absolute branching ratio. Further experimental studies would need to be combined with robust microscopic theoretical understanding of the decay dynamics to provide additional insight into the idea of the Hoyle state as an $alpha$-condensate.
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