Low-dose real-time X-ray imaging with nontoxic double perovskite scintillators

The study addresses the need for low-dose, high-stability, high-spatial-resolution X-ray imaging. Conventional scintillators (e.g., CsI:Tl and LuAG:Ce) require expensive, time-consuming synthesis, challenging device processability. Lead halide perovskites offer facile fabrication, fast response, and good spatial resolution but suffer from relatively low light yield, lead toxicity, and instability, limiting applications in low-dose, real-time, and robust imaging. Recent lead-free emitters (double perovskites, copper- and bismuth-based metal halides) have emerged as promising alternatives; however, reported materials like Rb₂CuBr₃ and Cs₂NaTbCl₃ exhibit long decay times and afterglow or mismatched emission wavelengths, impeding high-contrast and CT imaging. The research proposes nontoxic double-perovskite Cs₂Ag₀.₆Na₀.₄In₁₋ₓBiₓCl₆ scintillators, tuning Bi³⁺ content to enhance X-ray absorption and radiative recombination, aiming to achieve high light yield, fast decay, negligible self-absorption, and stable low-dose static and dynamic X-ray imaging.

The paper situates its work within several threads: (1) Conventional scintillators such as CsI:Tl and LuAG:Ce are effective but have costly, time-intensive synthesis impacting processability. (2) Lead halide perovskites for X-ray detection offer advantages (facile fabrication, fast response, good spatial resolution) yet are constrained by low light yield, self-absorption, lead toxicity, and instability. (3) Lead-free alternatives including double perovskites and Cu- or Bi-based metal halides have been developed; some (Rb₂CuBr₃, Cs₂NaTbCl₃) show high light yields but suffer from long decay and strong afterglow, limiting high-contrast and CT applications; Rb₂CuBr₃ also emits in the blue, mismatched to common camera sensitivity. These gaps motivate exploration of nontoxic double perovskite scintillators with improved light yield, decay dynamics, and emission matching.

- Material synthesis: Prepared a series of lead-free double-perovskite Cs₂Ag₀.₆Na₀.₄In₁₋ₓBiₓCl₆ single crystals (x varied) by partial substitution of In³⁺ with Bi³⁺. Na⁺ alloying in Cs₂AgInCl₆ (x = 0.2–0.8) optimized at x = 0.4 based on PLQY. - Structural and compositional characterization: Powder X-ray diffraction (PXRD) to confirm double-perovskite Fm-3m structure; monitored peak shifts with Bi content due to larger Bi³⁺ ionic radius. SEM-EDS on multiple random spots verified elemental ratios matching design. - Optical characterization: Measured steady-state absorption and photoluminescence excitation (PLE); calculated Urbach energy from absorption tails indicating reduced band-tail states with Bi³⁺; determined bandgaps via Tauc plots showing monotonic decrease with Bi incorporation. Photoluminescence (PL) spectra and photoluminescence quantum yield (PLQY) measured across Bi contents, observing intensity evolution and emission redshift with Bi³⁺. - Time-resolved PL: Time-correlated single-photon counting (TCSPC) using a femtosecond laser (400 nm, <300 fs, 1 MHz). TRPL analysis revealed biexponential decay in undoped sample (~1 ns fast, 2.8 μs slow STE emission). Used a 700–800 nm filter to isolate STE band; observed disappearance of fast component with Bi doping, attributing it to defect trapping suppressed by Bi³⁺ passivation. Increased Bi³⁺ shortened slow component lifetime to nanoseconds, indicating improved radiative recombination via breaking parity-forbidden transitions. - Radioluminescence (RL) and light yield: Pressed powders into compact wafers; mounted on integrating sphere window at fixed distance from X-ray source (50 kV, dose rate 189 µGy_air s⁻¹). Recorded RL spectra with fibre-coupled spectrometer across Bi contents. Verified linear RL response versus X-ray dose rate. Quantified light yield by benchmarking against a commercial LuAG:Ce (22,000 ± 4000 photons/MeV). Matched absorbed X-ray energy by calculating attenuation efficiency vs thickness at 22 keV (dominant tube energy) and fabricating wafers: Cs₂Ag₀.₆Na₀.₄In₀.₈₅Bi₀.₁₅Cl₆ (0.4 mm) and LuAG:Ce (0.11 mm). Compared RL outputs under identical conditions to derive absolute light yield. - Afterglow measurement: Assessed decay of luminance post X-ray excitation; compared with CsI:Tl and literature Rb₂CuBr₃. - Imaging system and performance: Built a custom optical X-ray imaging system. Fabricated Cs₂Ag₀.₆Na₀.₄In₀.₈₅Bi₀.₁₅Cl₆ wafers with thicknesses 0.1, 0.2, 0.4, and 0.6 mm. Imaged a standard resolution test-pattern plate; quantified spatial resolution via modulation transfer function (MTF) using slanted-edge analysis. Demonstrated static imaging at ~1 µGy_air dose and dynamic finger-bending imaging without ghosting at 47.2 µGy_air s⁻¹. Evaluated stability by thermal treatment at 85 °C for 50 h followed by 50 h X-ray irradiation in ambient air, monitoring RL intensity and image quality.

- Achieved a maximum scintillation light yield of 39,000 ± 7000 photons/MeV for Cs₂Ag₀.₆Na₀.₄In₀.₈₅Bi₀.₁₅Cl₆, exceeding colloidal CsPbBr₃ (21,000 photons/MeV) and comparable to CsI:Tl. - Large Stokes shift from STE emission yields negligible self-absorption; PLQY peaked at 90% for 2% Bi, while optimal scintillator LY occurred at 15% Bi due to combined effects of hot-electron energy transfer S and radiative efficiency Q. - TRPL showed Bi³⁺ incorporation suppresses ultrafast defect trapping and shortens slow decay to nanoseconds, indicating accelerated radiative recombination (breaking parity-forbidden transitions). - Linear RL response versus X-ray dose rate confirmed suitability for contrast imaging. - Strong X-ray absorption efficiency in ~36–60 keV range (relevant to medical radiography). - Afterglow: luminance decayed to 0.1% at ~16 μs, outperforming CsI:Tl (1.5% at 3 ms) and Rb₂CuBr₃ (2.72% at 20 ms), enabling real-time imaging and CT. - Imaging: Static X-ray imaging at extremely low dose (~1 µGy_air). Dynamic finger-bending imaging without ghosting at 47.2 µGy_air s⁻¹. - Spatial resolution (MTF at 0.2) depended on wafer thickness: 4.3 lp/mm (0.1 mm), 3.2 lp/mm (0.2 mm), 2.3 lp/mm (0.4 mm), 1.4 lp/mm (0.6 mm). The 0.1 mm wafer performance is comparable to Se direct imagers (4.8 lp/mm at MTF 0.2). - Stability: After 85 °C for 50 h and subsequent 50 h X-ray irradiation in air, RL intensity and image quality remained almost unchanged.

The results demonstrate that nontoxic double-perovskite Cs₂Ag₀.₆Na₀.₄In₁₋ₓBiₓCl₆ scintillators overcome key limitations of lead-based perovskites and certain lead-free halides by combining high light yield, fast decay, negligible self-absorption, and stability. Bi³⁺ serves dual roles: enhancing X-ray absorption due to its high atomic number and modifying the band structure to break parity-forbidden transitions, thus increasing radiative recombination rates and reducing defect-related trapping. The optimal Bi content (15%) maximizes the product of hot-electron energy transfer (S) and radiative efficiency (Q), aligning with the observed peak in scintillation light yield rather than PLQY alone. Low afterglow ensures high signal-to-noise and minimal lag, crucial for dynamic imaging and CT. The imaging demonstrations at ultra-low dose and low dose-rate validate practical performance, while the spatial-resolution dependence on thickness highlights trade-offs between absorption (signal) and optical crosstalk. Collectively, these findings address the research goal of enabling low-dose, real-time X-ray imaging with a nontoxic, robust scintillator material system.

The study introduces a family of nontoxic double-perovskite scintillators, Cs₂Ag₀.₆Na₀.₄In₁₋ₓBiₓCl₆, and identifies an optimal composition (x = 0.15) that achieves high light yield (39,000 ± 7000 photons/MeV), fast decay with minimal afterglow, and negligible self-absorption due to STE-mediated large Stokes shift. The materials exhibit stable performance after prolonged thermal and X-ray exposure and enable high-quality static imaging at ~1 µGy_air and dynamic imaging at 47.2 µGy_air s⁻¹ without ghosting. These results demonstrate a competitive, lead-free scintillator platform for low-dose, real-time X-ray imaging. Future work could focus on engineering wafer thickness and optical coupling (e.g., closer coupling to CMOS panels) to further enhance spatial resolution without compromising dose, and on compositional tuning to optimize the balance of hot-electron energy transfer and radiative efficiency.

- Spatial resolution decreases with increasing scintillator thickness due to optical crosstalk; thinner wafers improve resolution but require higher dose rates to maintain signal for real-time imaging. - While close attachment to CMOS can reduce crosstalk and boost resolution, it limits field of view to the chip size. - Stability tests were conducted for 50 h at 85 °C followed by 50 h X-ray irradiation; longer-term or higher-dose lifetime and radiation-hardness beyond these conditions were not reported.