Limits for Lepton-Conserving and Lepton-Nonconserving Double Beta Decay inCa48

Abstract

A search has been made for double beta decay in ${\mathrm{Ca}}^{48}$ using a new technique which involves the use of ${\mathrm{CaF}}_2$ crystals as scintillation detectors. These crystals, enriched in either ${\mathrm{Ca}}^{48}$ or ${\mathrm{Ca}}^{40}$, were used to detect ionizing particles originating within the crystals. The advantages of this technique are (1) a $4\pi$ geometry for a source without self-absorption and (2) good resolution which is independent of the relative angular and energy distribution of the two electrons. A lower limit of ${2\times 10}^{20}$ yr is placed on the lepton-nonconserving double beta decay without neutrinos. The determination of a lower limit for the Dirac-type double beta decay (with emission of two neutrinos) is complicated by impurities in the ${\mathrm{Ca}}^{48}$ used. By resorting to the usual coincidence type of search for double beta decay, which makes the effect of the impurities negligible, we established a lower limit of ${5\times 10}^{18}$ yr for this process. This is comparable with the lower limit of the theoretical estimates.