When a scintillator is struck by high-energy ionising radiation such as X-rays, it absorbs the energy and reemits some of it as low energy visible light. Scintillators are widely used for X-ray imaging screens in applications including airport security scanners and medical radiography, and many are made from ceramic or perovskite materials, which are often fabricated under harsh conditions and suffer from poor stability over time when exposed to light and air.
“Organic-based scintillators have inherent advantages, such as low toxicity, high mechanical flexibility, low cost and straightforward large-scale production,” said Jian-Xin Wang at KAUST (King Abdullah University of Science and Technology), who worked on the project under the supervision of Omar Mohammed and colleagues. “However, balancing the X-ray absorption capability, exciton utilisation efficiency and photoluminescence quantum yield of organic scintillators has proven challenging.”
According to KAUST, organic scintillator materials have so far been hampered by the small range of X-ray frequencies that they can naturally absorb. Wang and colleagues realised that X-ray absorption should increase as the atomic number of the incorporated elements increases and the team hypothesised that the addition of heavy atoms to the scintillator material could resolve this issue as x-ray photons interact efficiently with heavy atoms due to their photoelectric effect.
“We used a simple molecular engineering strategy to design novel organic scintillators,” Wang said in a statement. “We began by introducing chlorine, bromine or iodine to thermally activated delayed fluorescence [TADF] chromophores. We then observed how these heavy atoms altered the efficiency and resolution of the resulting X-ray images.”
TADF chromophores exist in an excited quantum ‘triplet state’ in the form of excitons — bound states of electrons and electron holes that are created when a high-energy X-ray photon is absorbed, “lifting an electron out of its hole,” the team said. The triplet state converts to a singlet state when the chromophores absorb thermal energy. They can then de-excite to the ground state and emit light in a process called delayed fluorescence.
“This means that, due to the minimised singlet-triplet energy gap, TADF chromophores can harness both the singlet and triplet excitons that are generated when they are exposed to X-ray radiation,” said Wang. “This dramatically improves the exciton utilisation efficiency of the scintillator, which in turn provides much higher X-ray spatial imaging resolution and ultralow detection sensitivity,” said Mohammed.
The technique of fabricating screens using scintillators doped with heavy atoms is said to have proved successful so far. One of the team’s scintillators, made using TADF-Br (bromine) chromophores, exceeded the resolution of most reported organic and organometallic scintillation screens.
“These fabricated screens provide a powerful design approach and promising new alternative materials for making X-ray imaging scintillators with outstanding sensitivity, low cost and high stability,” said Mohammed.
The team is manufacturing a portable X-ray sensor with their fabricated screen for high-resolution medical imaging, including dental examinations and health checks. Their design could also advance the development of tiny wearable X-ray devices.
The team’s findings have been published in Nature Photonics.
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