New limits on $2 \varepsilon$, $\varepsilon \beta^{+}$ and $2 \beta^{+}$ decay of ¹³⁶Ce and ¹³⁸Ce with deeply purified cerium sample

Abstract.

A search for double electron capture ($2\varepsilon$), electron capture with positron emission ($\varepsilon\beta^{+}$), and double positron emission ($2\beta^{+}$) in ¹³⁶Ce and ¹³⁸Ce was realized with a 465 cm³ ultra-low background HP Ge $\gamma$ spectrometer over 2299 h at the Gran Sasso underground laboratory. A 627 g sample of cerium oxide deeply purified by liquid-liquid extraction method was used as a source of $\gamma$ quanta expected in double $\beta$ decay of the cerium isotopes. New improved half-life limits were set on different modes and channels of double $\beta$ decay of ¹³⁶Ce and ¹³⁸Ce at the level of $T_{1/2} > 10^{17} - 10^{18}$ yr.