Measurements of the intrinsic rise times of common inorganic scintillators

The intrinsic rise times of a number of common inorganic scintillators are determined using ultrafast measurements of luminescence following pulsed x-ray excitation. A Ti-sapphire mode-locked laser and a light-excited x-ray tube are used to produce x-ray pulses with 60 ps fwhm. Fluorescence photons are detected with a microchannel phototube and the response of the phototube and electronics is 45 ps fwhm. Samples are either powders or thin crystals painted black on five sides to reduce delayed scattered photons. The intrinsic scintillators CeF3, CdWO4, Bi4Ge3O12, and CsI have rise times less than or equal to 30 ps, indicating that electrons are promptly captured to form the excited states. The activated scintillators CaFa(2):Eu, ZnO:Ga, and Lu2SiO5:Ce have rise times less than or equal to 40 ps, indicating that the luminescent centers are excited by rapid sequential hole capture- electron capture. The activated scintillators CsI:TI and YAlO3:Ce have slower rise times due to processes that delay the formation of excited states. It is shown that for practical scintillation detectors, internal reflections in the crystal can degrade observed rise times by hundreds of ps depending on size, reflector, and index of refraction.