Liquid Argon Ionization Detector for Double Beta Decay Studies

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\mathrm{.}Limitsonthe0$\nu $and0$\nu \chi ^{0}$modesofthedecaywereobtained:$T_{1/2}> 8.4(4.9) \times 10^{21}$yrand$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.