Fast Neutron Imaging with Semiconductor Nanocrystal Scintillators

Abstract

Fast neutrons offer high penetration capabilities for both light and dense materials due to their comparatively low interaction cross sections, making them ideal for the imaging of large-scale objects such as large fossils or as-built plane turbines, for which X-rays or thermal neutrons do not provide sufficient penetration. However, inefficient fast neutron detection limits widespread application of this technique. Traditional phosphors such as ZnS:Cu embedded in plastics are utilized as scintillators in recoil proton detectors for fast neutron imaging. However, these scintillation plates exhibit significant light scattering due to the plastic−phosphor interface along with long-lived afterglow (on the order of minutes), and therefore alternative solutions are needed to increase the availability of this technique. Here, we utilize colloidal nanocrystals (NCs) in hydrogen-dense solvents for fast neutron imaging through the detection of recoil protons generated by neutron scattering, demonstrating the efficacy of nanomaterials as scintillators in this detection scheme. The light yield, spatial resolution, and neutron-vs-gamma sensitivity of several chalcogenide (CdSe and CuInS 2 )-based and perovskite halide-based NCs are determined, with only a short-lived afterglow (below the order of seconds) observed for all of these NCs. FAPbBr 3 NCs exhibit the brightest total light output at 19.3% of the commercial ZnS:Cu(PP) standard, while CsPbBrCl 2 :Mn NCs offer the best spatial resolution at ∼2.6 mm. Colloidal NCs showed significantly lower gamma sensitivity than ZnS:Cu; for example, 79% of the FAPbBr 3 light yield results from neutron-induced radioluminescence and hence the neutron-specific light yield of FAPbBr 3 is 30.4% of that of ZnS:Cu(PP). Concentration and thickness-dependent measurements highlight the importance of increasing concentrations and reducing self-absorption, yielding design principles to optimize and foster an era of NC-based scintillators for fast neutron imaging.

Luminescent Lead Halide Ionic Liquids for High-Spatial-Resolution Fast Neutron Imaging

“…The total light yield is reported as a percentage of the total light yield of the reference ZnS:Cu(PP) scintillator under the same conditions, and we find that the light yields of the IL and 1/5-IL are 7.6% and 4.0% of the reference, respectively ( Figure 3 b). The light yields of IL and 1/5-IL are competitive with semiconductor NC colloids (FAPbBr 3 , CdSe/CdS NCs and NPLs, CsPbBrCl 2 :Mn, CsPbBr 3 ) as scintillators, which we introduced previously, 3 despite higher PLQYs of these NCs (up to 90%). This is likely the result of the large Stokes shift of the IL sample, which limits self-absorption losses that plague many of the NC scintillators.…”

“…Lead(II) halide ILs combine attractive features of prospective scintillators for fast neutron imaging: 35 − 37 intimately mixed highly emissive molecular phosphors and long organic chains to provide sufficient stopping power for fast neutrons. 3 Furthermore, the transparency of these interdissolved materials eliminates the phosphor–plastic interface, which scatters light in the commercial ZnS:Cu(PP) phosphor screens, while their short-lived emission lifetimes (450 ns, Figure S2 ) also stand in stark contrast to the minutes-long afterglow effects observed in ZnS:Cu(PP) screens. 2 Most importantly, the large Stokes shifts of these emitters minimize self-absorption losses in light yield and eliminates photon recycling (reabsorption and emission) that would reduce spatial resolution.…”

“… (a) Radiograph of IL under fast neutron irradiation (300 s exposure) as compared with nanocrystal scintillators 3 and a commercial ZnS:Cu(PP) screen, used here as a reference. (b) Total light yield comparison of an ionic liquid with leading fast neutron NC scintillators, 3 given as a percentage of the light yield obtained for the reference ZnS:Cu(PP) scintillator screen under identical conditions (300 s exposure, full concentration, 10-mm-thick cuvettes).…”

“… (a) Radiograph of IL under fast neutron irradiation (300 s exposure) as compared with nanocrystal scintillators 3 and a commercial ZnS:Cu(PP) screen, used here as a reference. (b) Total light yield comparison of an ionic liquid with leading fast neutron NC scintillators, 3 given as a percentage of the light yield obtained for the reference ZnS:Cu(PP) scintillator screen under identical conditions (300 s exposure, full concentration, 10-mm-thick cuvettes). (c) Light yield of concentrated and dilute IL vs vertical pixel position from the radiograph in panel (a), showing the light outcoupling at the cuvette edge and meniscus, as well as the imperfect mixing of the diluted sample leading to a higher PbCl 2 concentration at the bottom of the sample.…”

“…This indicates that the charge conversion efficiency (CCE), or the proportion of ionized charge carriers that are sufficiently close to emissive centers to induce light output, 3 is reduced at lower concentration because the ionized electron cloud generated by the recoil proton interacting with fewer luminescent lead(II) centers. This trend was previously observed also for NC colloids as scintillators, 3 which also found that higher concentrations are beneficial for enhanced light yield and thus showed that the ideal concentration for unity CCE is not achieved ( Figure 3 d). 3 In the present case, which is a 10-fold increase in the mass loading (25 mass %) relative to the NC colloids (e.g., 2.1 mass % for FAPbBr 3 NCs) 3 and is on par with the ideal loading observed for ZnS:Cu(PP), this may indicate that there is a proportion of lead(II) metal centers that are nonemissive and act as parasitic absorbers of charge carriers, degrading the PLQY and lowering the observed CCE.…”