STUDY OF DOUBLE-BETA DECAY OF 100MO WITH LIQUID-ARGON IONIZATION-CHAMBER (FIRST RESULTS)

The experiment being discussed is located at the Gran Sasso Undergroun d Laboratory (3500 mwe). The full weight of Mo-100 under investigation is 138.7 g. The first measurement run was performed (measurement time 202 h). As a result, limits on half-lives for the 0 nu and 0 nu chi(0 ) decays of Mo-100 were obtained at a 90% C. L. (greater than 0.8 x 10 (21) yr and 0.8 x 10(20) yr, respectively). Some extra events in the e nergy region 1.6-2.6 MeV lead, in the conservative approach, only to t he limit of greater than 0.5 x 10(19) yr (at a 90% C. L.) on the 2 nu beta beta decay of Mo-100. As a by-product, the limit on the content o f radioactive Ar-42 natural Ar was obtained to be less than 5 x 10(-21 ) g/g. We plan to improve the background conditions of the experiment and to increase the sensitivity to (1-5) x 10(23) yr for the 0 nu deca y mode and to (2-5) x 10(22) yr for the 0 nu chi(0) decay mode in one year of measurement. In addition, we hope to investigate the 2 nu deca y mode for Mo-100 to a high precision.