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 t
ube are used to produce x-ray pulses with 60 ps fwhm. Fluorescence photons
are detected with a microchannel phototube and the response of the phototub
e and electronics is 45 ps fwhm. Samples are either powders or thin crystal
s painted black on five sides to reduce delayed scattered photons. The intr
insic scintillators CeF3, CdWO4, Bi4Ge3O12, and CsI have rise times less th
an or equal to 30 ps, indicating that electrons are promptly captured to fo
rm 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 sh
own that for practical scintillation detectors, internal reflections in the
crystal can degrade observed rise times by hundreds of ps depending on siz
e, reflector, and index of refraction.