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Register a new userSelective decay from a candidate of the $sigma$-bond linear-chain state in $^{14}$C
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A cluster-transfer experiment $^9$Be($^9$Be,$^{14}$C$^*rightarrowalpha$+$^{10}$Be)$alpha$ was carried out using an incident beam energy of 45 MeV. This reaction channel has a large $Q$-value that favors populating the high-lying states in $^{14}$C and separating various reaction channels. A number of resonant states are reconstructed from the forward emitting $^{10}$Be + $alpha$ fragments with respect to three sets of well discriminated final states in $^{10}$Be, most of which agree with the previous observations. A state at 22.5(1) MeV in $^{14}$C is found to decay predominantly into the states around 6 MeV in $^{10}$Be daughter nucleus, in line with the unique property of the predicted band head of the $sigma$-bond linear-chain molecular states. A new state at 23.5(1) MeV is identified which decays strongly into the first excited state of $^{10}$Be.
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A cluster-transfer experiment of $^9rm{Be}(^9rm{Be},^{14}rm{C}rightarrowalpha+^{10}rm{Be})alpha$ at an incident energy of 45 MeV was carried out in order to investigate the molecular structure in high-lying resonant states in $^{14}$C. This reaction is of extremely large $Q$-value, making it an excellent case to select the reaction mechanism and the final states in outgoing nuclei. The high-lying resonances in $^{14}$C are reconstructed for three sets of well discriminated final states in $^{10}$Be. The results confirm the previous decay measurements with clearly improved decay-channel selections and show also a new state at 23.5(1) MeV. The resonant states at 22.4(3) and 24.0(3) MeV decay primarily into the typical molecular states at about 6 MeV in $^{10}$Be, indicating a well developed cluster structure in these high-lying states in $^{14}$C. Further measurements of more states of this kind are suggested.
The linear-chain states of $^{14}$C are theoretically investigated by using the antisymmetrized molecular dynamics. The calculated excitation energies and the $alpha$ decay widths of the linear-chain states were compared with the observed data reported by the recent experiments. The properties of the positive-parity linear-chain states reasonably agree with the observation, that convinces us of the linear-chain formation in the positive-parity states. On the other hand, in the negative-parity states, it is found that the linear-chain configuration is fragmented into many states and do not form a single rotational band. As a further evidence of the linear-chain formation, we focus on the $alpha$ decay pattern. It is shown that the linear-chain states decay to the excited states of daughter nucleus $^{10}{rm Be}$ as well as to the ground state, while other cluster states dominantly decay into the ground state. Hence, we regard that this characteristic decay pattern is a strong signature of the linear-chain formation.
An inelastic excitation and cluster-decay experiment $rm {^2H}(^{16}C,~{^{4}He}+{^{12}Be}~or~{^{6}He}+{^{10}Be}){^2H}$ was carried out to investigate the linear-chain clustering structure in neutron-rich $rm {^{16}C}$. For the first time, decay-paths from the $rm {^{16}C}$ resonances to various states of the final nuclei were determined, thanks to the well-resolved $Q$-value spectra obtained from the three-fold coincident measurement. The close-threshold resonance at 16.5 MeV is assigned as the ${J^pi}={0^+}$ band head of the predicted positive-parity linear-chain molecular band with ${(3/2_pi^-)^2}{(1/2_sigma^-)^2}$ configuration, according to the associated angular correlation and decay analysis. Other members of this band were found at 17.3, 19.4, and 21.6 MeV based on their selective decay properties, being consistent with the theoretical predictions. Another intriguing high-lying state was observed at 27.2 MeV which decays almost exclusively to $rm {^{6}He}+{^{10}Be{(sim6~ MeV)}}$ final channel, corresponding well to another predicted linear-chain structure with the pure $sigma$-bond configuration.
Unbound states of $^{10}$C nuclei produced as quasi-projectiles in $^{12}$C+$^{24}$Mg collisions at E/A = 53 and 95 MeV are studied with the Indra detector array. Multi-particle correlation function analyses provide experimental evidence of sequential de-excitation mechanisms through the production of intermediate $^{9}$B, $^{6}$Be and $^{8}$Be unbound nuclei. The relative contributions of different decay sequences to the total decay width of the explored states is estimated semi-quantitatively. The obtained results show that heavy-ion collisions can be used as a tool to access spectroscopic information about exotic nuclei.
We have observed an excited state in the neutron-rich semi-magic nucleus O-23 for the first time. No such states have been found in previous searches using gamma-ray spectroscopy. The observation of a resonance in n-fragment coincidence measurements confirms the speculation in the literature that the lowest excited state is neutron unbound and establishes positive evidence for a 2.8(1) MeV excitation energy of the first excited state in O-23. The non-observation of a predicted second excited state is explained assuming selectivity of inner-shell knockout reactions.
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