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Register a new userIndependent measurement of the Hoyle state $beta$ feeding from 12B using Gammasphere
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Using an array of high-purity Compton-suppressed germanium detectors, we performed an independent measurement of the $beta$-decay branching ratio from $^{12}mathrm{B}$ to the second-excited (Hoyle) state in $^{12}mathrm{C}$. Our result is $0.64(11)%$, which is a factor $sim 2$ smaller than the previously established literature value, but is in agreement with another recent measurement. This could indicate that the Hoyle state is more clustered than previously believed. The angular correlation of the Hoyle state $gamma$ cascade has also been measured for the first time. It is consistent with theoretical predictions.
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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.
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.
In $beta$-decay studies the determination of the decay probability to the ground state of the daughter nucleus often suffers from large systematic errors. The difficulty of the measurement is related to the absence of associated delayed $gamma$-ray emission. In this work we revisit the $4pigamma-beta$ method proposed by Greenwood and collaborators in the 1990s, which has the potential to overcome some of the experimental difficulties. Our interest is driven by the need to determine accurately the $beta$-intensity distributions of fission products that contribute significantly to the reactor decay heat and to the antineutrinos emitted by reactors. A number of such decays have large ground state branches. The method is relevant for nuclear structure studies as well. Pertinent formulae are revised and extended to the special case of $beta$-delayed neutron emitters, and the robustness of the method is demonstrated with synthetic data. We apply it to a number of measured decays that serve as test cases and discuss the features of the method. Finally, we obtain ground state feeding intensities with reduced uncertainty for four relevant decays that will allow future improvements in antineutrino spectrum and decay heat calculations using the summation method.
The fragmentation of quasi-projectiles from the nuclear reaction $^{40}$Ca+$^{12}$C at 25 MeV/nucleon was used to produce excited states candidates to $alpha$-particle condensation. Complete kinematic characterization of individual decay events, made possible by a high-granularity 4$pi$ charged particle multi-detector, reveals that 7.5$pm$4.0% of the particle decays of the Hoyle state correspond to direct decays in three equal-energy $alpha$-particles.
In the context of the search for triples of relativistic $alpha$-particles in the Hoyle state, the analysis of available data on the dissociation of the nuclei ${}^{12}$C, ${}^{16}$O and ${}^{22}$Ne in the nuclear emulsion was carried out. The Hoyle state is identified by the invariant mass calculated from pair angles of expansion in $alpha$-triples in the approximation of the conservation of the momentum per nucleon of the parent nucleus. The contribution of the Hoyle state to the dissociation of ${}^{12}$C $to$ 3$alpha$ is 11%. In the case of the coherent dissociation of ${}^{16}$O $to$ 4$alpha$ it reaches 22% when the portion of the channel ${}^{16}$O $to$ 2${}^{8}$Be is equal to 5%.
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