Cesium lead halide nanocrystals and their nanocomposites have attracted a lot of attention in the field of radiation detection thanks to their excellent luminescent properties, especially their potential for fast timing. However, most research on their nanocomposites focuses on low loadings (around 1 wt%) that is insufficient for detecting high-energy X-rays or γ-rays. Only a few studies are exploring materials with higher loading but with limited success in terms of transparency of the final material. In our work, we present nanocomposites of cesium lead halide nanocrystals with loading up to 40 wt%. We employ innovative surface functionalization to
enhance nanocrystal dispersion within the matrix, thereby improving the final material transparency. We show this in detail using confocal photoluminescence microscopy and we then characterise the radioluminescent properties of nanocomposites of cesium lead
bromide and cesium lead bromochloride nanocrystals with varying chloride content. To asses their performance as radiation detectors, we measure their timing capabilities under X-rays. Our findings reveal a significant improvement in time resolution under X-rays compared to previously published values for cesium lead bromide polymer nanocomposites. When combined with increased stopping power of high nanocrystal content, this advancement holds great promise for practical applications, e.g. in time-of-flight positron emission tomography and computed tomography or high energy physics.