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 eve nts caused by Bi-214 which exist in and around the source films by det ecting alpha-rays of Po-214 decays. The chamber has been installed ins ide of ELEGANT V to search for the neutrino less double beta decays (O nu beta beta) of Mo-100. The delayed signal of alpha-ray with 164 mu s half-life from Po-214, which is the daughter nucleus of Bi-214, is c learly seen in the time spectrum of the field wire of this chamber. Th e amounts of (1.50 +/- 0.32) X 10(-3) Bq of Bi-214 in the source film and less than 4.9 x 10(-2) Bq/m(3) in the operation gas were estimated by analyzing the background data, respectively. Since the total 179 g of enriched Mo-100 source films are set inside of the drift chamber s ystem, the Rn-222 gas around the source films are reduced to less than 10(-2) as compared with previous set up. The study in the region of T -1/2(0 nu)(Mo-100) similar to 10(23)yr is expected with ELEGANT V afte r installation of this drift chamber.