Characterization of the proposed $4\text{−}\alpha$ cluster state candidate in ${}^{16}\mathrm{O}$

The ${}^{16}\mathrm{O}(\alpha ,{\alpha }^{\prime })$ reaction was studied at ${\theta }_{\mathrm{lab}}=0^{\circ }$ at an incident energy of $E_{\mathrm{lab}}=200$ MeV using the K600 magnetic spectrometer at iThemba LABS. Proton decay and $\alpha$ decay from the natural parity states were observed in a large-acceptance silicon strip detector array at backward angles. The coincident charged-particle measurements were used to characterize the decay channels of the $0_6^{+}$ state in ${}^{16}\mathrm{O}$ located at $E_x=15.097\left(5\right)$ MeV. This state is identified by several theoretical cluster calculations to be a good candidate for the $4\text{−}\alpha$ cluster state. The results of this work suggest the presence of a previously unidentified resonance at $E_x\approx 15$ MeV that does not exhibit a $0^{+}$ character. This unresolved resonance may have contaminated previous observations of the $0_6^{+}$ state.

Figure 1

(a) The background-subtracted inclusive excitation spectrum from the ${\mathrm{Li}}_2{\mathrm{CO}}_3$ target, with fitted $R$-matrix Voigt line shapes that are superimposed upon the background of fitted lithium resonances (filled in orange). (b) The excitation energy region of interest highlighting the $0_6^{+}$ resonance (dashed blue line) and the neighboring $J^{\pi }=2^{+}$ resonance (dotted green line). (c) The raw (i), background-subtracted (ii), and instrumental background inclusive excitation (iii) spectra from the ${\mathrm{Li}}_2{\mathrm{CO}}_3$ target. Spectrum (iv) is the background-subtracted inclusive excitation spectrum from the ${}^{12}\mathrm{C}$ target, normalized to spectrum (ii) through the 9.641(5) MeV resonance in ${}^{12}\mathrm{C}$. The linear fit of spectrum (iii), used for background subtraction, is displayed as a dashed red line. (d) The coincident matrix of silicon energy versus the excitation energy of the recoil nucleus for decay particles detected within the angular range: ${156}^{\circ }<{\theta }_{\mathrm{lab}}<{163}^{\circ }$ (two silicon strips within the array). The ${\alpha }_0$-, ${\alpha }_1$-, and $p_{\text{0–3}}$-decay lines from ${}^{16}\mathrm{O}$ are indicated. The proton punch-through structure from the $p_0$ decay is labeled $p_{\mathrm{pt}}$. The lithium breakup and the *$p_0$-decay line from ${}^{12}\mathrm{C}$ are indicated. A display color threshold of $>1$ is imposed.

Figure 2

(a) The coincident matrix of silicon energy versus the excitation energy of the recoil nucleus, highlighting the decay modes observed at the excitation energy region of interest at $E_x\approx 15.1$ MeV. The excitation energy projections of the ${\alpha }_0$-, $p_0$-, and ${\alpha }_1$-decay lines are displayed in panels (b), (c), and (d), respectively. The resonance energies, extracted with single-channel $R$-matrix fits, are indicated.

Figure 3

Angular correlations of charged-particle decays from ${}^{16}\mathrm{O}$ in the laboratory frame relative to the beam axis: (a) ${\alpha }_0$ decay from the 11.521(9) MeV $J^{\pi }=2^{+}$ resonance, (b) ${\alpha }_0$ decay from the 12.049(8) MeV $J^{\pi }=0^{+}$ resonance, (c) $p_0$ decay from the 14.929(8) MeV $J^{\pi }=2^{+}$ resonance, (d) ${\alpha }_1$ decay observed at 15.046(8) MeV, and (e) ${\alpha }_0$ decay observed at 15.090(7) MeV. *The ${\alpha }_0$ decays from $J^{\pi }=0^{+}$ and $2^{+}$ resonances, corresponding to $l=0\hslash$ and $2\hslash$, respectively, exhibit the same angular correlations. Data points affected by the electronic thresholds of the CAKE array are omitted.