Development of a thin drift chamber for the double beta decay experiment

A thin drift chamber for the double beta decay spectrometer ELEGANT V was developed. This drift chamber is designed to reject background events caused by 214Bi which exist in and around the source films by detecting α-rays of 214Po decays. The chamber has been installed inside of ELEGANT V to search for the neutrino less double beta decays (0νββ) of 100Mo. The delayed signal of α-ray with 164 μs half-life from 214Po, which is the daughter nucleus of 214Bi, is clearly seen in the time spectrum of the field wire of this chamber. The amounts of (1.50 ± 0.32) × 10−3 Bq of 214Bi in the source film and less than 4.9 × 10−2Bq/m3 in the operation gas were estimated by analyzing the background data, respectively. Since the total 179 g of enriched 100Mo source films are set inside of the drift chamber system, the 222Rn gas around the source films are reduced to less than 10−2 as compared with previous set up. The study in the region of T120ν(100Mo) ∼ 1023yr is expected with ELEGANT V after installation of this drift chamber.