The detection of ionizing radiation is an important topic owing to its significant scientific and real-life technological implications. Many detectors exploit a scintillation counter, which records the light pulses produced upon interaction of radiation with a luminescent material, i.e. the scintillator. An important example of their use is the time-of-flight based positron emission tomography (TOF-PET), an imaging technique used in oncology. It uses a coincidence time resolution (CTR) method based on the detection at different times of two back-to-back gamma photons emitted by positrons annihilation in tissues. This time difference is proportional to the difference of photons path length, and therefore contains information on the annihilation position. This latter is blurred by a time measurement uncertainty, which is assessed on a CTR ~200-300ps in commercial devices resulting in an effective spatial resolution of few centimetres. Forefront research aims to achieve a 10ps CTR, which would enable an effective resolution at the millimeter scale without the instrumentation required to manage reconstruction algorithms that increases acquisition time and signal noise (Fig.1). The sensitivity would be improved by one order of magnitude and cost-effective scanners for broad use would be fabricated owing to a simpler architecture. Realizing new scintillators is a decisive synthetic challenge towards this development, since common bulk and nanostructured inorganic crystals, organic chromophores and plastics do not present the synthetic versatility required to full control the system CTR. To partially fulfil this issue, the concept of the scintillating heterostructure has been recently introduced [1]. In this case, the scintillator is realized by coupling different materials, whose properties are synergistically exploited (Fig.1). The simplest heterostructure consists in a dense material, which provides the interaction with the ionizing radiation and a slow luminescence (>200ns) with the energetic resolution required to select the PET events, coupled to a low density, fast scintillator (<20ns) that sets the CTR. The fast emission is activated by a fraction of the excitation energy shared with the dense component. SHERPA’s aim is to realize a scintillating heterostructure employing a polymeric nanocomposite embedding metal-organic framework (MOF) nanocrystals, as fast emitter, coupled to an industrially processable dense crystalline scintillator. To further advance towards technological transfer, a polymeric ultra-porous layer will be incorporated between the two materials to simultaneously achieve mechanical integrity and optical isolation of the fast emission component. Thanks to the excellent time response of the MOF nanocomposites recently demonstrated [2], this architecture will optimize the outcoupling of the fast scintillation thus realizing a prototype device with a CTR<50ps that will be tested in operative conditions. |