A novel X-ray scintillator constructed based on stable luminescent free radicals

Classification of scintillators

Scintillation materials can be roughly divided into the following three categories:

Inorganic scintillators: including alkali metal halide crystals (such as NaI (Tl), CsI (Tl), etc., where Tl is the activator), other inorganic crystals (such as CdWO4, BGO, etc.), vitreous organic scintillators: organic crystals (such as anthracene, astragalus, etc.), organic liquids, plastic scintillators.



Gas scintillators: such as argon, xenon, etc.

The properties of scintillators

The ideal scintillator used for radiation detection should have the following properties:

1. High luminous efficiency: able to convert the kinetic energy of incident charged particles into scintillation photons as much as possible

2. Good linearity: The energy lost by incident charged particles maintains a linear relationship with the number of scintillation photons generated over a large range

3. The emission spectrum and absorption spectrum do not overlap: The scintillation medium is transparent to its own emitted light and there is no self absorption

4. Short decay time of luminescence: The duration of the incident particle's flash is short, and the detector responds quickly

5. Other properties: good processing performance, suitable refractive index, readily available and non-toxic raw materials, low cost, etc
Generally speaking, inorganic scintillators have high photon yield and good linearity, but their luminescence decay time is relatively long; Organic scintillators have a short decay time for luminescence, but a lower photon yield.
X-ray scintillators have a wide range of applications in medical imaging, X-ray non-destructive testing, electron microscopy, and high-energy particle detectors. High performance scintillators are the core components of nuclear medicine imaging technologies such as computed tomography (CT) and positron emission tomography (PET) imaging. The development of X-ray scintillators with high X-ray excitation luminescence efficiency is the focus of researchers' attention. Recently, Professor Zang Shuangquan's team from Tianjian Advanced Biomedical Laboratory collaborated with Academician Tang Benzhong's team from The Chinese University of Hong Kong (Shenzhen) to introduce stable luminescent free radicals from open shell emitters into the design of high-performance X-ray scintillators for the first time. The unique dual line emission characteristics can significantly improve the exciton utilization rate during radiative luminescence, providing a new strategy for the construction of high-performance X-ray scintillators.


For classical closed shell X-ray scintillators with paired electrons, radiative luminescence arises from the radiative transition of excitons from singlet/triplet states to the ground state. However, due to the spin forbidden transition of triplet excitons, their utilization is usually limited. For an open shell luminescent material, both the ground state (D0) and the first excited state (D1) of the molecule are double states, and there is no spin forbidden transition. The luminescence generated by the transition between double states is called double state luminescence. Compared with traditional closed shell luminescent materials, the theoretical exciton utilization rate of the dual state emission characteristic is 100%, which is expected to fundamentally avoid the problem of spin forbidden transitions during radiative luminescence.
Based on the above radiation, this work introduces stable luminescent free radicals into the design of X-ray scintillators, and prepares stable luminescent free radical scintillators containing Au (I) complex structures. Due to its inherent heavy atomic composition, this free radical scintillator exhibits good absorption potential for X-rays and emits bright luminescence under X-ray excitation, with intensity comparable to commercial scintillators. The lowest detection limit can reach 105 nGy s-1. Far below the detection limit of 5.5 μ Gy s-1 required for medical scintigraphy examination. This free radical scintillator has excellent X-ray photostability, high thermal stability, and good processability. The flexible scintillator screen prepared by combining it with PDMS polymer successfully achieved X-ray imaging of different actual objects with a resolution of up to 20 LP mm-1.