Investigation of the ${}^{14}\mathrm{C}+\alpha$ molecular configuration in ${}^{18}\mathrm{O}$ by means of transfer and sequential decay reaction

A multinucleon transfer and cluster-decay experiment, namely ${}^9\mathrm{Be}({}^{13}\mathrm{C},{}^{18}\mathrm{O}^{*}\to {}^{14}\mathrm{C}+\alpha )\alpha$, was performed at a beam energy of 65 MeV. Resonant states in ${}^{18}\mathrm{O}$ from 7 to 19 MeV, including some newly observed ones, are reconstructed with high resolution, based on the coincident detection of various combinations of the final fragments. The $\alpha$-decay branching ratios for 14 states are extracted from both the invariant-mass and the missing-mass measurements. Angular correlation analysis was conducted for the 10.3-MeV ($4^{+}$) state. The present work supports the existence of the positive-parity rotational band associated with the ${}^{14}\mathrm{C}+\alpha$ molecular structure in ${}^{18}\mathrm{O}$, but the negative-parity band members were not identified.

Figure 1

Schematic view of the experimental setup.

Figure 2

Relationship between energy and angle (Lab) for the recoil $\alpha$-particles (the black solid line) and for the decay $\alpha$-particles originated from excited ${}^{18}\mathrm{O}$ (18 MeV) (the color band).

Figure 3

$Q$-value spectra obtained from the present measurement, for events with detected ${}^{14}\mathrm{C}+\alpha$ (the dotted line) or $\alpha +\alpha$ (the solid line) fragments.

Figure 4

${}^{18}\mathrm{O}^{*}$ excitation-energy spectra from (a) events with detected ${}^{14}\mathrm{C}+\alpha$ and (b) events with detected $\alpha +\alpha$. Peaks in each spectrum are fitted by a number of Gaussian functions (red dotted lines) plus a continuous background (the blue smooth line).

Figure 5

(a) Excitation-energy spectrum of ${}^{18}\mathrm{O}$ obtained from the present experiment. States below 10 MeV are reconstructed from events with detected ${}^{14}\mathrm{C}+\alpha$ fragments from the same telescope (L0 or R0), while states between 10 and 19 MeV are from events with detected two-$\alpha$ (and deduced ${}^{14}\mathrm{C}$) fragments from L0 $+$ R0 telescopes. These resonances are compared with the previous results: (b) from Ref. [28] and (c) from Ref. [26].

Figure 6

(a) The invariant-mass spectrum reconstructed from ${}^{18}\mathrm{O}^{*}\to {}^{14}\mathrm{C}+\alpha$ decay [using events with detected 2-$\alpha$ (by L0 $+$ R0) and deduced ${}^{14}\mathrm{C}]$, in comparison to (b) the corresponding missing-mass spectrum using events with detected recoil $\alpha$-particles. Each spectrum is fitted by a number of Gaussian-function peaks (red dashed lines) plus a smooth background (the blue solid line). For a better view of the peak, the structure built on the large background as exhibited in the inset of panel (b), the inclusive missing-mass spectrum is plotted with the background subtracted. In both cases, the black dot-dashed lines show the simulated experimental detection efficiency.

Figure 7

(a) Excitation-energy spectrum for ${}^{18}\mathrm{O}$, reconstructed from events with one $\alpha$-particle detected by the L0/R0 telescope while another is detected by the R2/L2 telescope. (b) Dalitz-type two-dimensional excitation-energy spectrum for reconstructed ${}^8\mathrm{Be}$ (from 2-$\alpha$) versus ${}^{18}\mathrm{O}$ (from $\alpha {+}^{14}{\mathrm{C}}_{\mathrm{deduced}}$).

Figure 8

The angular correlation for the 10.3-MeV state, compared with the Legendre polynomials of order 4 (the red dotted line). A uniformly distributed background is assumed for the uncorrelated component (the long dot-dashed line). All the theoretical angular distributions are corrected for the detection efficiency. The corresponding reduced ${\overline{\chi }}^2$ is also indicated in this plot.