Optimal $\beta$-beam at the CERN-SPS

A $\beta$-beam with maximum $\gamma =150$ (for $\text{helio}$ ions) or $\gamma =250$ (for $\text{neon}$) could be achieved at the CERN-SPS. We study the sensitivity to ${\theta }_{13}$ and $\delta$ of such a beam as function of $\gamma$, optimizing with the baseline constrained to CERN-Frejus (130 km), and also with simultaneous variation of the baseline. These results are compared to the {\it standard} scenario previously considered, with lower $\gamma =60\mathrm{/}100$, and also with a higher $\gamma \sim 350$ option that requires a more powerful accelerator. Although higher $\gamma$ is better, loss of sensitivity to $\theta _{13}$and$\delta$ismostpronouncedfor$\gamma$ below 100.