A measurement of the double beta decay half-life of 48Ca

48 Ca is the lightest of the many double beta decay nuclei, and the only one simple enough to be treated exactly in the shell model without truncation. Thus a ββ 2 v measurement of this isotope provides a unique test of the nuclear physics involved in ββ matrix element calculations. Enriched 48 Ca sources of two different thicknesses have been exposed in a time projection chamber, and yield a preliminary T 1/2 2 v = (5.5 −1.5 +3.5 ) × 10 19 y, in agreement with shell model calculations.