Precise determination of relative and absolute ββ-decay rates of ¹²⁸Te and ¹³⁰Te

Abstract

Double beta decay of ¹²⁸Te has been confirmed and the ratio of half-lives for ββ decay of ¹³⁰Te and ¹²⁸Te has been precisely determined as T_½¹³⁰/T_½¹²⁸=(3.52±0.11)×10^-4 by ion-counting mass spectrometry of Xe in ancient Te ores, using techniques that reduce interferences due to trapped Xe. We have also detected excesses of ¹²⁶Xe originating in high energy reactions of cosmic ray muons and their secondaries on Te; such reactions make minor contributions to the measured ¹²⁸Xe excesses in the Te ores. The Xe measurements, combined with common Pb dating of the ores, yield a ¹³⁰Te half-life of (2.7±0.1)×10²¹ yr and thus a ¹²⁸Te half-life of (7.7±0.4)×10²⁴ yr, the longest radioactive decay lifetime measured to date. These results give limits on the effective Majorana mass of the neutrino (<1.1-1.5 eV) and right-handed currents (||<η>||<5.3×10^-8) comparable to the best obtained from direct neutrinoless ββ-decay searches. They also imply new limits on unconventional Majorons not constrained by measurements of the Z⁰ decay width.