Radiographical imaging with X-rays, gamma-rays, and thermal neutrons (~25 meV) has developed into a crucial tool for medical imaging and security applications over the past century. Fast neutron (> 1 MeV) imaging is a rising technique which benefits from the high penetration power of fast neutrons, ideal for imaging large-scale objects such as construction beams and as-built plane turbines. However, widespread application of fast neutron imaging is hindered by inefficient detection of fast neutrons. The leading material in the field consists of microscale ZnS:Cu embedded in hydrogen-dense polypropylene (PP), with indirect detection of fast neutrons through the detection of recoil protons generated by fast neutrons scattering off hydrogen. However, such detectors exhibit drawbacks such as long-lived afterglows (order of minutes), light scattering at the plastic-phosphor interface, and high gamma-ray absorption and sensitivity. Hence, alternative solutions are needed to improve the performance of fast neutron detectors. Meanwhile, the advent of semiconductor nanocrystals (NCs) has ushered in a golden age for nanoscale emissive materials, with the defect-tolerant halide perovskites receiving significant attention in the past five years.

Here, we demonstrate the efficacy of colloidal perovskite nanocrystals in hydrogen-dense solvents as scintillators for fast neutron imaging. Light yield, spatial resolution, and neutron-vs.-gamma sensitivity of several compositions are compared, including both chalcogenide (CdSe and CuInS₂)-based and perovskite-based NCs (FAPbBr₃, CsPbBr₃, and CsPbBrCl₂:Mn). FAPbBr₃ NCs exhibit the brightest total light output at 19.3% of the commercial ZnS:Cu(PP) standard. Colloidal NCs uniformly showed less sensitivity to gamma radiation than ZnS:Cu, with the ratio of detected neutrons to gamma-rays ranging from 2.2 in CsPbBrCl₂:Mn NCs to 4.1 for CsPbBr₃ NCs, compared to 1.0-1.1 for ZnS:Cu(PP). For example, 79% of the FAPbBr₃ light yield results from neutron-induced radioluminescence and hence the neutron-specific light yield of FAPbBr₃ is 30.4% of that of ZnS:Cu(PP), despite the tenfold higher phosphor load of ZnS:Cu(PP) relative to the perovskite NCs. Metal blocks with sharp edges used to estimate the spatial resolution reveal that the high Stokes shift CsPbBrCl₂:Mn NCs offer the best spatial resolution at ~2.6 mm, while that of FAPbBr₃ NCs is ~5.2 mm due to greater reabsorption and re-emission. Importantly, all NCs showed no evidence for afterglow on the order of seconds. Concentration and thickness-dependent measurements highlight the importance of high concentrations and reducing self-absorption, yielding design principles for perovskite NC-based scintillators to enable effective fast neutron imaging.