Role of rare-earth ions in scintillating materials

Scintillators are materials able to efficiently convert the energy deposited by high energy photons or ionizing particles into a great number of photons in the visible or near ultraviolet range, that can be detected by conventional photodetectors. They are extensively used for various applications, like for example medical diagnostics, high energy physics experiments, security, or industrial controls. The investigations on scintillators started soon after the discovery of X-rays, and faced a notable improvement especially in the last decades with the advent of several novel materials [1]. Among inorganic materials, fluorides, chlorides, bromides, iodides, and oxides single crystals are being developed. In several cases rare earth ions (RE) are present, either as lattice constituents and, especially, as luminescent activators. RE-doped scintillating glasses and, more recently, optical ceramics are also investigated. Indeed, RE ions present electronic configurations that permit a rich variety of radiative transitions and make them useful for different optical applications, including scintillation. For the purpose of scintillation, the allowed 5d-4f radiative transition is particularly useful, due to its high quantum efficiency and fast decay in the 10-7-10-8 s time scale. For this reason, Ce3+ , Pr3+ or Eu2+, featuring 5d-4f transitions in the visible or near ultraviolet in most host materials, are frequently considered. Scintillation is a complex process involving i) the absorption of the primary radiation beam and the production of a great number of free carriers; ii) the diffusion of free carriers and their transport to the luminescent centers, like RE ions; and finally, iii) the radiative recombination of electron-hole pairs giving rise to photon emission (Fig. 1). Good ionizing radiation absorption, radiation resistance, efficient and fast response are crucial properties to be considered for the engineering of a novel scintillator material. This lecture will first describe the scintillation phenomenon and the parameters that characterize scintillator material performances; the role of RE ions is then introduced, providing several examples in which RE are present, like lutetium/yttrium silicate single crystals [2], silicate glasses [3], complex garnet optical ceramics [4]; the particular case of PbWO4, the material used in the Large Hadron Collider at CERN for the discovery of Higgs Boson and in which RE ions were used as optically inactive dopants, is also presented [5]. A look to future scintillator needs is finally presented, especially concerning positron emission tomography medical diagnostics apparatuses in which a time response faster than 1 ns will be required; possible solutions involving RE doped materials, or nanocomposites made by luminescent nanoparticles embedded in a polymeric material [6] are outlined. [1] C. Dujardin et al., “Needs, trends and advances in inorganic scintillators”, IEEE TNS 65, 1977 (2018). [2] S. Blahuta et al., “Evidences and consequences of Ce in LYSO:Ce,Ca and LYSO:Ce,Mg Single Crystals for Medical Imaging Applications”, IEEE TNS 60, 3134 (2013). [3] F. Cova et al., “Dual Cherenkov and scintillation response to high-energy electrons of rare-earth doped silica fibers” Phys. Rev. Appl., 11, 024036 (2019). [4] S. Liu et al., “Towards bright and fast Lu3Al5O12:Ce,Mg optical ceramics scintillators”, Adv. Opt. Mater 4, 731 (2016). [5] S. Baccaro et al., "Influence of La3+ - doping on radiation hardness characteristics of PbWO4", Phys. Stat. Sol. 160, R5 (1997). [6] M. Gandini et al., “ Efficient, Fast and Reabsorption-free Perovskite-based Sensitized Plastic Scintillators", Nature Nanotechnology 15, 462 (2020).