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 $\beta\beta(2\nu)$ 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. 
A. Balysh, A. De Silva, V. I. Lebedev, K. Lou, M. K. Moe, M. A.   Nelson, A. Piepke, A. Pronskiy, M. A. Vient, and P. Vogel