  $^{48}$Ca, the lightest double beta decay candidate, is the only one simple
enough to be treated exactly in the nuclear shell model. Thus, the
$\beta\beta(2\nu)$ half-life measurement, reported here, provides a unique test
of the nuclear physics involved in the $\beta\beta$ matrix element calculation.
Enriched $^{48}$Ca sources of two different thicknesses have been exposed in a
time projection chamber, and yield T$_{1/2}^{2\nu} = (4.3^{+2.4}_{-1.1} [{\rm
stat.}] \pm 1.4 [{\rm syst.}]) \times 10^{19}$ years, compatible with the shell
model calculations.
