Electron capture branching ratio measurements in an ion trap for double beta decay experiments at TITAN

Abstract

Double beta decay (ββ) is a nuclear decay mode expected to appear in at least two varieties, the double-neutrino (2ν) and the zero-neutrino (0ν) mode. The 0νββ-decay is of particular interest as it requires the neutrino to be a Majorana particle. The search for such a decay is presently being carried out or planned in a number of experiments, such as EXO, MAJORANA, GERDA, CUORE, COBRA, NEMO-III and SNO+. The 0ν-decay rate depends on the neutrino mass but, unfortunately, also on a rather complex nuclear matrix element, making the extraction of the mass heavily dependent on the underlying theoretical nuclear model. However, all theoretical models can readily be tested against the 2ν mode, which, unlike its 0ν counterpart, only involves simple Gamow–Teller nuclear matrix elements. These elements can be determined experimentally either through charge-exchange reactions or, for the ground-state transition, through the electron capture (EC) or single β-decay of the intermediate odd–odd nucleus. The present program is geared towards the measurement of the EC branching ratios (BR). In most cases, these ratios are poorly known or not known at all, because EC is usually suppressed by several orders of magnitude compared to the β-decay counterpart due to energy considerations. Traditional methods for measuring these ratios have so far suffered from overwhelming background generated by these high-energy electrons. Recently, a unique background-free method for measuring EC branching ratios was proposed using the TITAN ion trap at the TRIUMF ISAC (Isotope Separator and ACcelerator) radioactive beam facility. The measurements will make use of the EBIT (Electron Beam Ion Trap) operating in Penning mode where electrons from the β−-decay will be confined by the magnetic field. K-shell X-rays from EC will be detected by seven X-ray detectors located around the trap, thus providing orders of magnitude background suppression and thus ideal low-BR measurement environment.