电子跃迁致核激发（NEET）是激发态原子退激的基本机制。近十多年NEET研究成为热点，其中189Os的NEET过程因存在简单清晰的表征标识和潜在的应用前景而最受关注。由于其发生几率PNEET很小（～10-10），目前x射线源通量尚显不足,而未被证实即测得PNEET值。不过已有实验的PNEET上限与理论预期已十分接近，且本世纪以来光源技术的高速发展势必带来更高通量的x射线源。因此，189Os的NEET证实已指日可待。然而，仅剩的影响其PNEET大小的不确定因素：NEET过程中的分支比B的理论值目前存在较大分歧（～3倍），也没有独立可靠的实验数据。本项目创新性提出用比值法在上海光源升级后的BL13W1线站精确测量B，为首次证实189Os的NEET及准确确定其PNEET打下基础。同时本项目将是上海光源首个核荧光实验，不仅能拓展上海光源的基础物理研究领域，而且能为在上海光源开展此类实验打下基础。

Nuclear excitation by electron transition (NEET) is afundamental mechanism of atomic deexcitation. In the last decades the study of NEET has become a focus of study. Especially 189Os has attracted the most interest in NEET study. Because the NEET process in 189Os has a convenient signature as well as a potential application prospect. Due to the low probability (~10-10), the flux of current monochromatic x-ray is insufficient. Hence the NEET process in 189Os is still not confirmed yet and its probability (PNEET) isn't determined. But the present upper limit of PNEET from previous experiments almost reaches the theoretical expectation. Plus the rapid developments of the light source technics in this century will certainly bring x-ray source with higher flux before long. Therefore, the confirmation of the NEET process in 189Os is in sight. However, the decay branching ratio B during NEET process in 189Os, which is the only uncertain factor affecting the determination of PNEET,is still ambiguous (3 times divergence of value), and no independent reliable data is available. This project originally puts forward a ratio measuring method to precisely measure B on the experimental station BL13W1 in Shanghai Synchrotron Radiation Facility (SSRF), thereby lays a solid foundation for the confirmation of the NEET process in 189Os and the accurate determination of PNEET. Meanwhile, as the first nuclear resonance fluorescence experiment study in SSRF, this project will not only extend the area of fundamental physical research in SSRF, but also provide a basis for similar experiments in SSRF in the future.