The Milano-Gran Sasso double beta decay experiment: toward a 20-crystal array

TeO ₂ thermal detectors are being used by the Milano group to search for neutrinoless double beta decay of ¹³⁰Te. An upper limit for neutrinoless decay half life of 2.1 × 10 ²² yr at 90% CL obtained with a 334 g TeO ₂ detector has been previously reported. To improve the sensitivity of the experiment an array of twenty 340 g TeO ₂ crystals will be realised in the next future. As a first step toward the realisation of that experiment a 4 crystal detector has been tested in the Gran Sasso refrigerator. Detector performances, data acquisition and analysis are discussed.