Towards Real-Time Aquatic Monitoring of Strontium-90: Performance Evaluation of CaF₂(Eu) and ZnSe(Al,O) Scintillators

A compact, in situ beta-spectroscopy approach for real-time monitoring of Strontium-90 (Sr-90) in contaminated groundwater has been investigated. Two inorganic scintillators, CaF₂(Eu) and ZnSe(Al,O), were coupled to silicon photomultipliers (SiPMs) and evaluated experimentally using custom front-end electronics. This was also modelled with Monte Carlo simulations using the Geant4 toolkit. Although simulations correctly predicted ZnSe(Al,O) has an advantage due to its higher light yield and optical transport, experimental measurements additionally revealed practical limitations of the readout electronics which were not captured in the simulation model. ZnSe(Al,O) showed excellent agreement with the simulated detector response (R² ≈ 0.86; χ2/NDF ≈ 27). It also attains a higher relative detection efficiency (∼61.5%), yielding faithful capture of the composite Sr-90/Y-90 spectrum with only minor suppression at the extreme high-energy tail. CaF₂(Eu) exhibits a deficit at low-mid energies and an apparent enhancement in the high-energy tail. This is consistent with threshold and photon-statistics losses and leads to poorer agreement with simulation (χ2/NDF ≈ 179) and lower overall efficiency (∼22.7%). These findings identify ZnSe(Al,O) as the stronger candidate for an underwater, in situ Sr-90 beta-spectroscopy system and motivate targeted optimisation of SiPM coupling and crystal-edge reflectivity in future designs.