[ Instrument Network Instrument Development ] X-ray detectors are widely used in security inspection, medical imaging, industrial flaw detection, nuclear power plants and scientific research. The working mode is mainly divided into the following two types: direct conversion of X-rays into direct detection of electrical signals, and indirect detection of converting X-rays into visible light and then converting them into electrical signals by photodetectors. Since X-rays have certain damage to the human body, it is particularly important to find an X-ray detector material with high sensitivity and low detection limit, and conventional X-ray detector materials such as direct detection materials or indirect detection materials have complex preparations. (such as CZT), poor stability (such as a-Se), low sensitivity (such as CsI: Tl) and other shortcomings. Professor Tang Jiang's research group is committed to the development of highly sensitive X-ray conversion materials and high-performance detectors. Recently, two X-ray detection articles were published in Advanced materials, and breakthroughs have been made in both direct detection and indirect detection.
Figure 1. Schematic diagram of a. Rb2CuBr3 crystal structure; b. Quantum yield; cX ray excitation fluorescence; d. Rb2CuBr3 light yield comparison
In the field of indirect detection of scintillators, the light yield of the scintillator is one of the important indicators for determining X-ray conversion efficiency and detecting contrast. Having a large Stokes shift and a high photon yield (PLQY) is a prerequisite for achieving high light yield. The Tangjiang research group used RbBr and CuBr as precursors to synthesize a novel non-lead metal halide scintillator R2CuBr3 with one-dimensional crystal structure by cooling crystallization. The excitation wavelength is 300nm, the emission wavelength is 385nm, the Stokes shift is 85nm (0.91eV), and the self-absorption effect is low. The one-dimensional crystal structure and the easily distorted lattice characteristics make it easy to form self-limiting domains. The exciton (STE) has an exciton binding energy of 758.9 meV, which ensures high radiation recombination efficiency and a photon yield of 98.6%, which provides the necessary conditions for high light yield. The scintillation characterization shows that Rb2CuBr3 and traditional scintillators (CsI:Tl or LYSO) have comparable X-ray absorption capacity, and their emission wavelengths have good matching with silicon photomultiplier tubes (SiPM) or photomultiplier tubes (PMT). Achieve a high light yield of 91056 photons/MeV (as shown in Figure 1).
In the field of direct detection, lead-based perovskite semiconductors have made a series of advances in the field of X-ray detection due to their high X-ray absorption coefficient and high carrier collection efficiency, which proves that the perovskite is huge in the field of X-ray detection. potential. However, in the process of preparing a thick film by the solution method, there are still holes caused by solvent evaporation, and the crystal grain size is small, the crystallinity is low, and the stability of the organic-inorganic hybrid perovskite is poor. One of the bottlenecks in the realization of X-ray imaging using perovskite materials is to realize a quasi-single-crystal, high-sensitivity thick film of perovskite. The quasi-single crystal ensures that the polycrystalline film has the transmission property of single crystal, which is beneficial to improve the collection efficiency of the carrier, the thick film contributes to the sufficient absorption of the X-ray, and the high sensitivity is advantageous for achieving high contrast imaging.
In response to this bottleneck, Prof. Tang Jiang proposed to melt the CsPbBr3 into a liquid state, disperse it on a conductive glass substrate, and then use quartz to form a film by hot pressing. The CsPbBr3 prepared by this method has high crystallinity and uniform orientation, has a thickness of 240 μm, and has crystal grains penetrating up and down. Electrically, the CsPbBr3 thick film has a carrier mobility (38 cm2 V-1 s-1) and a μτ value (1.32 × 10-2 cm2 V-1) comparable to a single crystal. Theory and experiments have also proved that CsPbBr3 is easy to form shallow bromine vacancy defects during thick film preparation, which is beneficial to photoconductive gain. The researchers then prepared a CsPbBr3 quasi-single-crystal thick film into a photoconductive X-ray detector with a sensitivity of 55 684 μC Gyair-1 cm-2@ 5.0 V mm-1 (see Figure 2).
Figure 2. a. CsPbBr3 thick film prepared by hot pressing; b. cross-section SEM; c. photocurrent changes with X-ray dose at different electric field strengths
The researchers believe that the new metal halide scintillators R2CuBr3 and CsPbBr3 direct detectors with the above excellent performance will show great potential in the field of X-ray imaging.
Two papers entitled "Lead-Free Halide Rb2CuBr3 as Sensitive X-Ray Scintillator" (18 September 2019) and "Hot-Pressed CsPbBr3 Quasi-Monocrystalline Film for Sensitive Direct X-ray Detection" (16 September 2019) were published online in Advanced Materials (DOI: 10.1002/adma.201904711; DOI: 10.1002/adma.201904405). Professor Tang Jiang and Associate Professor Niu Guangda are co-authors of the two papers.
(Original title: "Advanced materials" published two research progress of X-ray detectors of Professor Tang Jiang's team)
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