NuDoubt++: Search for Double Beta Plus Decays with a Hybrid Opaque Scintillation Detector

Double beta plus decay modes, such as 𝛽+𝛽+, 𝛽+𝐸𝐶, and 2𝐸𝐶, represent rare second-order weak nuclear processes involving the conversion of two protons into two neutrons. Their detection is challenging due to extremely low decay probabilities, complex experimental signatures, and the low natural abundance of suitable isotopes. Studying these decays can offer valuable insights into nuclear structure and fundamental symmetries. The decay rate is influenced by nuclear matrix elements (NMEs) and phase space factors (PSFs) - both essential for interpreting measured decay rates and constraining theoretical models of nuclear transitions. In this work, we present NuDoubt++, a new detector concept designed to search for double beta plus decays with unprecedented sensitivity. It combines hybrid and opaque scintillation technologies for the first time, along with advanced light readout techniques, to enhance detection of positron-based signatures. Preliminary sensitivity estimates suggest that a 1-tonne-week exposure with NuDoubt++ could approach the parameter space relevant for 2𝜈𝛽+𝛽+ and 2𝜈𝛽+𝐸𝐶 decays in the candidate isotope 78Kr. Furthermore, the design offers improved background rejection and energy resolution, positioning NuDoubt++ to probe 0𝜈𝛽+𝛽+ decay with sensitivities surpassing current experimental limits by several orders of magnitude.