First search for resonant enhanced neutrinoless double electron capture in 152Gd and other rare decays in natural Gd isotopes.

A first search for rare decays of gadolinium isotopes was performed with an ultra-low background high-purity germanium detector at Gran Sasso Underground Laboratory (Italy). A 198 g Gd 2 O 3 powder sample was measured for 63.8 d with a total Gd exposure of 12.6 kg × d. 152 Gd is the most promising isotope for resonant enhanced neutrinoless double electron capture which could significantly increase the decay rate over other neutrinoless double beta decay processes. The half-life for this decay was constrained to > 4.2 × 10 12 year (90% credibility). This limit is still orders of magnitude away from theoretical predictions but it is the first established limit on the transition paving the way for future experiments. In addition, other rare alpha and double beta decay modes were investigated in 152 Gd, 154 Gd, and 160 Gd with half-life limits in the range of 10 17 - 20 year. [ABSTRACT FROM AUTHOR]