Spark-Chamber Search for Double Beta Decay inCa48

Double beta decay of ${\mathrm{Ca}}^{48}$ has been sought with a detection system that combines two four-gap aluminum-foil spark chambers of 32-mg/${\mathrm{cm}}^2$ thickness with plastic scintillation counters operated in time coincidence. This combined electronic and visual detection system had the advantages of (1) increasing the specificity of the "signature" that can be demanded for double-beta-simulating events and (2) reducing the effective background from $\gamma$ rays at the low-energy end of the two-electron sum spectrum. This considerably increased the sensitivity for the two-neutrino mode of the decay. Approximately 420 000 events were photographed during a 1838.4-h run with a matrix of source elements consisting of a total of 3 g of ${\mathrm{Ca}}^{48}$ and dummy elements (a total of 1 g of ${\mathrm{Ca}}^{40}$). In a scan of these events, no statistically significant difference between the rate of double-beta-like events from the source and that from the dummy was noted either in the energy region in which two-neutrino double beta decay or in the region in which neutrinoless double-beta decay might be expected to occur. A lower limit on the half-life for the two-neutrino mode of the decay was set at 3.2×${10}^{18}$ years; for the neutrinoless mode, at 9×${10}^{18}$ years.