Performance of the drift chamber beta-ray momentum analyzer for double beta decay experiments

One of the most important problems in neutrino physics is whether neutrinos are Majorana particles or Dirac ones. If a neutrino has a Majorana nature, neutrinoless double beta decay (0 nu 2 beta) certainly takes place. On the other hand, there is a theory in which neutrinoless quadruple beta decay (0 nu 4 beta) is possible, allowing for lepton number violation by four units, even if a neutrino has a Dirac nature. Since the half-lives of both 0 nu 2 beta and 0 nu 4 beta are theoretically expected to be very long, detectors have to have an excellent capability to eliminate backgrounds. We have been developing a series of electron momentum analyzers called the drift chamber beta-ray analyzer (DCBA) at KEK for double beta decay experiments. DCBA consists of drift chambers detecting charged particle tracks, and a superconducting solenoid serving a uniform magnetic field. Since the momentum acceptance is in the region of 0.5-3.5 MeV/c, it is easy to eliminate background particles like alpha particles, protons, and muons, which have much higher momenta because of their large masses. The particle identification property and the 3D position determination capability are powerful tools to search for 0 nu 2 beta and 0 nu 4 beta events. In this article, we describe the performance of DCBA as well as the details of the detector construction processes.