  Observations of several Fanaroff-Riley (FR) type I sources reveal outflowing
bipolar bubbles of hot gas surrounded by a weak forward shock. We consider the
possibility that these bubbles were driven by choked relativistic jets which
failed to penetrate the ambient intracluster medium (ICM). Using new results on
choked jets linking the geometry of the forward shock to the jet properties, we
infer robust limits on the radius $R_{ch}$ at which the jet was quenched in 5
well-studied FRI sources, finding typically $R_{ch}\sim 10$ kpc. We further
show that, in order to reach this radius in less than the current age of the
system, the jet must have been tightly collimated, with the jet head subtending
an angle of $\theta_h < 2^\circ$. The ambient pressure is not high enough to
explain this collimation, suggesting that the jet was collimated by interaction
with its own cocoon. Although the choking radius is well-constrained, we find a
degeneracy between the initial jet opening angle before collimation,
$\theta_0$, and the duration of jet activity, $t_b$, with
$(t_b/1\rm{Myr})(\theta_0/5^\circ)^{-2} \sim 0.1$. We speculate that the
working time and/or opening angle of the jet may be important factors
contributing to the FR type I/type II morphology in galaxy clusters, with
short-lived or wide jets being choked to form bipolar bubbles filled with
diffuse radio emission, and longer-lived or narrow jets successfully escaping
the cluster core to produce cocoons with radio hotspots.
