Double Beta Decay of {sup {bold 48}}Ca

{sup 48}Ca, the lightest experimentally accessible double beta decay candidate, is the only one simple enough to be treated exactly in the nuclear shell model. Thus the {beta}{beta}{sub 2{nu}} half-life measurement, reported here, provides a unique test of the nuclear physics involved in the {beta}{beta} matrix element calculation. Enriched {sup 48}Ca sources of two different thicknesses have been exposed in a time projection chamber. We observe a half-life of T{sub 1/2}{sup 2{nu}}=(4.3{sub {minus}1.1}{sup +2.4}[stat]{plus_minus}1.4[syst]){times}10{sup 19} yr, consistent with shell model calculations. {copyright} {ital 1996 The American Physical Society.}