Development of 3D tracking detectors in the DCBA experiment for studies of double beta decays

DCBA(Drift Chamber Beta-ray Analyzer) experiment is aiming to search for neutrinoless double beta decay (0νββ). The DCBA detector is a momentum analyzer to measure individual electron energy from double beta decays. Since 0νββ is extremely rare, the detector must be low background and accommodate a lot of decays source. And also the excellent energy resolution is required at the Q-value to distinguish 0νββ from two-neutrino double beta decay (2νββ). The DCBA detector consists of drift chambers, a magnet and cosmic-ray veto counters. A test prototype DCBA-T2 has been constructed and operated with natural Mo source containing 9.6% ¹⁰⁰Mo in order to solve operation problems. Another new prototype called DCBA-T3 is under construction at KEK. DCBA-T3 will have better performance than DCBA-T2 in view points of the energy resolution and the source accommodation volume. The results of the DCBA-T2 test operation and the DCBA-T3 status are described together with the future plan temporarily called Magnetic Tracking Detector (MTD), which is designed based on the experiences of DCBA-T2 and T3.