  Time-of-Flight Positron Emission Tomography is a medical imaging technique,
based on the detection of two back-to-back {\gamma}-photons generated from
radiotracers injected in the body. Its limit is the ability of employed
scintillation detectors to discriminate in time the arrival of {\gamma}-pairs,
i.e. the coincidence time resolution (CTR). A CTR < 50 ps that would enable
fast imaging with ultralow radiotracer dose. Monolithic materials do not have
simultaneously the required high light output and fast emission
characteristics, thus the concept of scintillating heterostructure is proposed,
where the device is made of a dense scintillator coupled to a fast-emitting
light material. Here we present a composite polymeric scintillator, whose
density has been increased upon addition of hafnium oxide nanoparticles. This
enhanced by +300% its scintillation yield, surpassing commercial plastic
scintillators. The nanocomposite is coupled to bismuth germanate oxide (BGO)
realizing a multilayer scintillator. We observed the energy sharing between its
components, which activate the nanocomposite fast emission enabling a net CTR
improvement of 25% with respect to monolithic BGO. These results demonstrate
that a controlled loading with dense nanomaterials is an excellent strategy to
enhance the performance of polymeric scintillators for their use in advanced
radiation detection and imaging technologies.
