rms radius of the deuteron

An explanation is given for the nearly linear relationship between the deuteron radius and the triplet scattering length as predicted by potential models of the NN interaction. This is accomplished by showing that the quantity 〈${\mathit{r}}^2$〉/${\mathit{a}}_{\mathit{t}}^2$ can be expanded to high accuracy in terms of the small parameter x=${\mathit{r}}_{\mathit{t}}$/${\mathit{a}}_{\mathit{t}}$. We show that shape dependence only enters at order ${\mathit{x}}^3$ and higher. Possible reasons for the disagreement with the experimental result are discussed.