The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks^1,2,3. This nuclear clock will be a unique tool for precise tests of fundamental physics^4,5,6,7,8,9. Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older¹⁰, the proof of existence has been delivered only recently by observing the isomer’s electron conversion decay¹¹. The isomer’s excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured^{12,13,14,15,16}. In spite of recent progress, the isomer’s radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (^229mTh). By performing vacuum-ultraviolet spectroscopy of ^229mTh incorporated into large-bandgap CaF₂ and MgF₂ crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements^14,15,16 and the uncertainty is decreased by a factor of seven. The half-life of ^229mTh embedded in MgF₂ is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus.

放射性核素钍 229 的异构体具有极低的激发能量，可以直接用激光操纵核态。它是下一代光学时钟^1,2,3中使用的主要候选材料之一。该核钟将成为精确测试基础物理的独特工具^4,5,6,7,8,9 。虽然这种异常核态存在的间接实验证据要早得多¹⁰ ，但直到最近才通过观察异构体的电子转换衰变¹¹来证明其存在。测量了异构体的激发能、核自旋和电磁矩、电子转换寿命和异构体的精细能量^{12,13,14,15,16} 。尽管最近取得了进展，但异构体的辐射衰变（核钟发展的关键因素）仍未被观察到。在这里，我们报告了在钍 229 ( ^229m Th) 中检测到这种低能异构体的辐射衰变。通过在 CERN 的 ISOLDE 设施中对纳入大带隙 CaF ₂和 MgF ₂晶体的^229m Th 进行真空紫外光谱，测量到 8.338(24) eV 的光子，与最近的测量结果^14,15,16和不确定性一致减少了七分之一。嵌入MgF ₂中的^229m Th的半衰期被测定为670(102) s。大带隙晶体中辐射衰变的观察对未来核钟的设计具有重要影响，并且能量不确定性的改善简化了对原子核直接激光激发的研究。