Level densities in F-20: Experimental, shell model, and weak coupling results

Shell-model calculations for $^{20}\mathrm{F}$ in an ${(\mathrm{sd})}^{4}$-closed $^{16}\mathrm{O}$ core basis predict significantly fewer than the observed number of levels below any given excitation energy. Calculations in a ${({p}_{\frac{1}{2}}\ensuremath{-}{s}_{\frac{1}{2}}\ensuremath{-}{d}_{\frac{5}{2}})}^{8}$-closed $^{12}\mathrm{C}$ core basis predict more levels than the observed numbers. A simple weak-coupling extrapolation gives quite good agreement up to at least 7 MeV (75 levels).