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 bb 2n half-life measurement, reported here, provides a unique test of the nuclear physics involved in the bb matrix element calculation. Enriched 48 Ca sources of two different thicknesses have been exposed in a time projection chamber. We observe a half-life of T 2n 1͞2 ͑4.3 12.4 21.1 ͓stat͔ 6 1.4͓syst͔͒ 3 10 19 yr, consistent with shell model calculations. [S0031-9007(96)01989-8] PACS numbers: 23.40.Hc, 21.10.Tg, 21.60.Cs, 27.40.+z Neutrinoless double beta decay (bb 0n ) is the most sensitive known test for Majorana neutrino mass. The unfolding of a mass value (or limit) from measured decay rates relies on complicated nuclear structure calculations [1]. Comparison of theoretical and experimental rates for the standard bb 2n mode is an important test for the validity of these calculations.Among all realistically measurable candidates for bb decay, 48 Ca ! 48 Ti is unique since it is the only one which can be treated "exactly" in the nuclear shell model by solving the problem of eight nucleons distributed within the fp shell without truncation. Consequently, this decay has been a favored testing ground of nuclear theories [2]. However, until now only a lower limit of the bb 2n decay half-life, T 2n 1͞2 . 3.6 3 10 19 yr, has been determined experimentally [3].The nuclear shell model, constrained by the requirement that it describe well the spectroscopy of the A 48 nuclei, restricts the corresponding half-life also from above, T 2n 1͞2 # 10 20 yr [4]. Therefore the experimental observation of the decay acquires added significance; if it turns out that the shell model cannot predict this theoretically tractable rate, we have to wonder about our ability to describe the nuclear matrix elements in the more complex nuclei. Here we present the result from a new experiment using a time projection chamber (TPC).The search for bb decay of 48 Ca was among the first to be attempted in live-time experiments beginning in the early fifties (for an extensive chronology see [1]). With the largest energy release of all bb candidates, 48 Ca (Q bb 4.271 6 0.004 MeV [5]) has a bb 2n sumenergy spectrum that extends to higher energies than most radioactive backgrounds. Yet calcium has a tendency to harbor chemically similar radio-impurities such as 90 Sr and 226 Ra, which do intrude on a major fraction of the 48 Ca spectral range.When the two b particles are tracked in a TPC they are seen to both carry negative charge, originate from a common point, and have separately measured energies. Although this distinctive visualization eliminates the bulk of unrelated activity, there remain several well-known mechanisms for production of negative electron pairs that constitute background for bb decay. The most serious of these are Möller scattering of single b particles and b-g cascades in which a g ray internally converts or Compton scatters. These processes are fed principally by th...

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