Observation of the $\pi^2\sigma^2$-bond linear-chain molecular structure in $^{16}$C

TL;DR

This study provides experimental evidence for a linear-chain molecular structure in carbon-16, confirming theoretical predictions through resonance measurements and decay pattern analysis.

Contribution

First experimental observation of a $pi^2 sigma^2$-bond linear-chain structure in $^{16}$C, validating theoretical models of exotic nuclear configurations.

Findings

Resonances at 16.5, 17.3, 19.4, and 21.6 MeV assigned as members of a linear-chain band.

Identification of a high-lying state at 27.2 MeV consistent with a pure $sigma$-bond configuration.

Decay patterns match theoretical predictions for linear-chain molecular states.

Abstract

Measurements of the $^2$H($^{16}$C,$^{16}$C$^{*}$$\rightarrow^4$He+$^{12}$Be or $^6$He+$^{10}$Be)$^2$H inelastic excitation and cluster-decay reactions have been carried out at a beam energy of about 23.5 MeV/u. A specially designed detection system, including one multi-layer silicon-strip telescope at around zero degrees, has allowed the high-efficiency three-fold coincident detection and therefore the event-by-event determination of the energy of the unstable nucleus beam. The decay paths from the $^{16}$C resonances to various states of the final $^{10}$Be or $^{12}$Be nucleus are recognized thanks to the well-resolved $Q$-value spectra. The reconstructed resonances at 16.5(1), 17.3(2), 19.4(1) and 21.6(2) MeV are assigned as the $0^+$, $2^+$, $4^+$ and $6^+$ members, respectively, of the positive-parity $(3/2_\pi^-)^2(1/2_\sigma^-)^2$-bond linear-chain molecular band in $^{16}$C, based on the angular correlation analysis for the 16.5 MeV state and the excellent agreement of decay patterns between the measurements and theoretical predictions. Moreover, another intriguing high-lying state was observed at 27.2(1) MeV which decays almost exclusively to the $\sim$6 MeV states of $^{10}$Be, in line with the newly predicted pure $\sigma$-bond linear-chain configuration.