Lowest-energy broad $\alpha$-cluster resonances in ${}^{19}\mathrm{F}$

There is a deep astrophysical interest in the structure of ${}^{19}\mathrm{F}$ states close to the $\alpha$-decay threshold. The nuclear structure of these states is important for understanding of the development of $\alpha$ clustering in the ${}^{20}\mathrm{Ne}$ region. Emergence of clustered states, and generally states that favor coupling to reaction channels near the corresponding decay thresholds, is currently of special interest in theoretical physics. We specify the parameters of broad low spin states in ${}^{19}\mathrm{F}$ near the $\alpha$-decay threshold and present a theoretical study of these states. The study is limited to $\ell =0$ and 1 relative partial wave resonances in the $\alpha +{}^{15}\mathrm{N}$ interaction close to the $\alpha$-decay threshold in ${}^{19}\mathrm{F}$. Excitation function for ${}^{15}\mathrm{N}(\alpha ,\alpha$) elastic scattering was measured by the TTIK method. These new data together with the old, high-energy resolution data, were analyzed using the $R$ matrix approach. ${}^{19}\mathrm{F}$ nuclear structure was calculated using configuration interaction methods with the recently developed effective interaction Hamiltonian. As a result, in this work we identify a series of $\alpha$ clustering resonances in ${}^{19}\mathrm{F}$ and assess the distribution of the clustering strength, which is of importance to questions of astrophysics and for theoretical understanding of many-body physics and emergence of clustering in loosely bound or unstable nuclei. Progress has been made in theoretical understanding of the origins of clustering and questions for future theoretical and experimental research are identified.

Figure 1

(a) $R$ matrix fit of the excitation functions for the $\alpha +{}^{15}\mathrm{N}$ elastic scattering [22] at $169.1^{\circ }$ in comparison with the fit of the Ref. [3]; (b) the same for ${70}^{\circ }$. An inset highlights the difference between the present fit and the fit with parameters of Ref. [3].

Figure 2

The excitation function for $\alpha +{}^{15}\mathrm{N}$ elastic scattering. Inset: $R$ matrix fit of the excitation function for $\alpha +{}^{15}\mathrm{N}$ elastic scattering in the region of $5/2^{+}$ and $7/2^{+}$ resonances, using convolution with experimental energy resolution of 33 keV.

Figure 3

$R$ matrix fit of the region of $1/2^{+}$ (5.3 MeV) and $3/2^{+}$ (5.5 MeV) levels in ${}^{19}\mathrm{F}$ measured by TTIK method. The level parameters are given in Table 1.