  Aims. Our goal is to characterize the intermediate age, detached shell carbon
star U Antliae morphologically and physically in order to study the mass-loss
evolution after a possible thermal pulse. Methods. High spatial resolution ALMA
observations of unprecedented quality in thermal CO lines allow us to derive
first critical spatial and temporal scales and constrain modeling efforts to
estimate mass-loss rates for both the present day as well as the ejection
period of the detached shell. Results. The detached shell is remarkably thin,
overall spherically symmetric, and shows a barely resolved filamentary
substructure possibly caused by instabilities in the interaction zone of winds
with different outflow velocities. The expansion age of the detached shell is
of the order of 2700 years and its overall width indicates a high
expansion-velocity and high mass-loss period of only a few hundred years at an
average mass-loss rate of $\approx$ 10$^{-5}$ $M_\odot$ yr$^{-1}$. The
post-high-mass-loss-rate-epoch evolution of U Ant shows a significant decline
to a substantially lower gas expansion velocity and a mass-loss rate amounting
to 4$\times$10$^{-8}$ $M_\odot$ yr$^{-1}$, at present being consistent with
evolutionary changes as predicted for the period between thermal pulses.
