Two-neutrino double beta decay of ¹⁰⁰Mo with ELEGANTS-IV

The normal double beta decay with two neutrinos (2 nu beta beta ) has been studied for ¹⁰⁰Mo in order to study experimentally the 2 nu beta beta nuclear matrix element (M_{2 nu} ). A low background spectrometer, ELEGANTS-IV, which consists of eleven Si(Li) beta -detectors and surrounding 4 pi -NaI(Tl) gamma -detectors, has been used for the present experiment. ^natMo and 94% enriched ¹⁰⁰Mo source-disks have been measured simultaneously for 8006 hours. A lower limit for the 2 nu half-life was obtained as 7.9(6.2)*10¹⁸ years at the 68(90)% confidence level. The present experiment suggests a finite 2 nu half-life of the order of 10¹⁹ years. However, it is necessary to estimate the contribution from contamination in the ¹⁰⁰Mo disks by a Monte Carlo calculation.