Double beta decay of $^{48}$Ca

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