  We compute the isospin susceptibility in an effective O($n$) scalar field
theory (in $d=4$ dimensions), to third order in chiral perturbation theory
($\chi$PT) in the delta--regime using the quantum mechanical rotator picture.
This is done in the presence of an additional coupling, involving a parameter
$\eta$, describing the effect of a small explicit symmetry breaking term (quark
mass). For the chiral limit $\eta=0$ we demonstrate consistency with our
previous $\chi$PT computations of the finite-volume mass gap and isospin
susceptibility. For the massive case by computing the leading mass effect in
the susceptibility using $\chi$PT with dimensional regularization, we determine
the $\chi$PT expansion for $\eta$ to third order. The behavior of the shape
coefficients for long tube geometry obtained here might be of broader interest.
The susceptibility calculated from the rotator approximation differs from the
$\chi$PT result in terms vanishing like $1/\ell$ for $\ell=L_t/L_s\to\infty$.
We show that this deviation can be described by a correction to the rotator
spectrum proportional to the square of the quadratic Casimir invariant.
