Reabsorption-free plastic scintillators based on engineered doped nanocrystals

The detection of high-energy radiation has gained relevant interest thanks to its application in technologic-relevant fields, such as particle physics, astronomy, geology, medical diagnostics, nuclear monitoring and space exploration. In all these areas, the most widely used detectors are scintillating materials that convert the energy deposited by incoming ionizing radiation into visible photons which is finally revealed by coupled photodiodes. Beside typical inorganic materials based on high-Z elements, an alternative class of scintillators which further widens their applicability is plastic scintillators, which exhibit the advantages of a large sizes production, low weight and affordable costs making them particularly adapt for radiation monitors in border and industrial control. Despite these advantages, the low density of plastic materials compared to inorganic crystals limits their interaction with ionizing radiation and typically requires doping these systems with high-Z components, such as organometallic complexes, perovskite or heavy metal chalcogenides nanocrystals (NCs). However, the intrinsic small Stokes shift of these materials represents an issue when used as nanoscintillators in highly dense or large volume detectors because of the strong reabsorption of the scintillation light along its path to the waveguide edges. We aim to takle this issue by developing a new strategy of scintillator consisting in a polymeric scintillating matrix incorporating reabsorption free Cd0.5Zn0.5S/ZnS core/shell NCs doped with manganese ions. Doping NCs with Mn is an established approach to activate efficient luminescence at intragap energy arising from the 4T1 -> 6A1 optical transition, yielding the characteristic Mn-emission at ~ 580-600 nm. Crucially, since such a transition is spin-forbidden, the corresponding optical absorption features negligible oscillation strength, resulting in an apparent Stokes shift between the band-edge absorption of the NCs host and the Mn-related luminescence. In our approach, we further adopted a synergic strategy in which both the plastic matrix waveguide and the NCs interact with incoming ionizing radiation, while the propagating emission is generated by the sole NCs, whose optical properties have been properly engineered to efficiently down-convert matrix emission. The emission efficiency and compatibility of the NCs with the polymer host have been optimized resulting in high optical quality nanocomposites completely transparent in the spectral region of their own emission, with scintillation efficiency comparable to commercial plastic scintillators.