A multisection liquid argon ionization detector was developed by the DBA collaboration to study the double beta- decay of $^{ 100}$Mo. The experiment was carried out in the Gran Sasso underground laboratory in Italy. The detector design and main characteristics are described. The $\beta \beta (2\nu)$ decay of $^{ 100}$Mo was observed and its half-life measured: $T_{ 1/2}=[7.2 \pm 0.9(stat) \pm 1.8(syst)] \times 10^{ 18}$ yr. Limits on the 0$\nu $ and 0$\nu \chi ^{ 0}$ modes of the decay were obtained: $T_{ 1/2}> 8.4(4.9) \times 10^{ 21}$ yr and $T_{ 1/2}> 4.1(3.2)\times 10^{ 20}$ yr at 68% (90%) C.L., respectively. In addition the upper limits on the $^{ 42}$Ar content and $^{ 222}$Rn activity in liquid Ar were found to be $4.3 \times 10^{ -21}$ g/g and $1.2 \times 10^{ -3}$ Bq/kg, respectively.
Categories: nucl-ex