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I-131 is an important artificial radionuclide and an important detection object in nuclear facilities. The current mainstream method is to use a uniformly distributed Ba-133 source instead of an iodine source to calibrate the I-131 detection instrument. However, in the process of value transmission, the distribution of the I-131 calibration standard source directly affects the setting value of the standard source and affects the calibration of the testing instrument. The plane source is used to determine the detection efficiency at different depths, and the depth efficiency curve is established. The fitting result is η(x)=0.007 655e-0.020 6x. Therefore, the activity efficiency equation of I-131 was established through mathematical analysis, and the positive and negative measurement results were used to solve it to determine its activity distribution. The distribution of the three I-131 sources is uniform distribution, exponential distribution and plane distribution. The deviation of exponential distribution source is between the deviation of the positive and negative measurement results of the uniform distribution source，which does not exceed 1%，and the deviation of the positive and negative measurement results of the planar distribution source，which exceeds 50%. By judging the quick solution of the source distribution, it is easy to prepare the calibration standard source and establish the on-site calibration process.

In organic materials, triplet excitons can be directly generated from the ground state through spinforbidden excitations. These excitations can be realized by efficiently mixing "bright" singlet states with strong transition dipole moments (TDMs) and "dark" triplet states through spin-orbit coupling (SOC). The primary challenge for small-molecule organic materials is achieving this mixture within single molecules. However, most of the previously demonstrated spin-forbidden excitations were generally extremely weak. In this study, we explore a different mechanism where the TDM and SOC originate from neighboring molecules. Specifically, we selected 8-hydroxyquinoline aluminum (Alq3) as a fluorescence molecule to provide the TDM and a phosphorescent Ir complex (Irppy2acac) to provide the SOC. We doped Alq3 into the crystal of Irppy2acac to generate strong intermolecular interactions, which couple SOC and TDM. Under this material design, spin-forbidden excitation was activated. The photoexcitation range was significantly extended and new excitation peaks were observed. Correspondingly, room-temperature long-wavelength phosphorescence induced by the activation of the spin-forbidden excitation was also observed. Excited-state species of the Alq3-Irppy2acac supramolecular system were identified and details of the intermolecular coupling characteristics of SOC and TDM were investigated using a theoretical model. The proposed photoexcitation pathway sufficiently extends the material's absorption range at longer wavelengths due to the low-energy triplet excited states and has a potential for use in solar cell applications. [ABSTRACT FROM AUTHOR]

The instability of blue emitters is one of the shortcomings of organic light-emitting diodes (OLEDs) in industrial applications. This instability is intrinsically associated with the basic transitions and reactions in the excited states. In this work, using the framework of Fermi's golden rule and DFT/TDDFT, the mechanisms of the transitions and reactions of a typical boron based multi-resonance thermally activated delayed fluorescence emitter involving the excited states were investigated. A dynamic stability mechanism describing recycling between the dissociation of the molecular structure in the T1 state and restoration in the S0 state dominated by steric effects was discovered. Applying knowledge of this mechanism, a small modification was made to the molecular structure, and the stability was increased without degrading other luminescence properties such as the luminescence color, FWHM, reverse intersystem crossing, fluorescence quantum yield, and internal quantum yield. [ABSTRACT FROM AUTHOR]