Moreover, the attenuation efficiency (Supplementary Fig. ) of BTD-FL, BTD-FLBr, and BTD-HeBr was calculated to be 18.7 and 69.3 and 75.1% at a thickness of 10 mm, respectively. In a further set of experiments, we explored the RL behaviors of the HLCT emitters in the solid state (Fig. ). Under the same X-ray irradiation (dose rate, 2.023 mGy s -1 ), BTD-FL and BTD-FLBr display weak X-ray emission, which is not obviously observable by the naked eye owing to their weak X-ray absorption or low PLQY. In contrast, BTD-HeBr exhibits clearly brighter yellow X-ray luminescence with a narrow FWHM of 56 nm under a continuous X-ray source, which is considerably narrower than that of commercial inorganic scintillators such as CsI:Tl, PbWO4, YAlO3:Ce, and Bi4Ge3O12. This narrow FWHM can be attributed to the LE character of the HLCT state. Therefore, the much higher radioluminescence intensity of BTD-HeBr reveals the through-space heavy atom strategy are highly successful for designing bright X-ray scintillators. A comparison with standard scintillators shows that BTD-HeBr exhibited high relative light yield of ~42,400 photons MeV -1 (dose rate, 2.023 mGy s -1 ) (Supplementary Fig. ). The high relative light yield of BTD-HeBr is mainly due to their high X-ray absorption cross-section, unit exciton utilization efficiency and low non-radiative decay. In addition, BTD-HeBr films exhibits good photostability (Fig. ). Specifically, when BTD-HeBr was exposed to a high dose rate of X-ray (2.023 mGy s -1 ) for a continuous 10 h, the RL intensity remains at around 98% of the initial value, which is comparable with that of commercial plastic scintillators .