在VUV透明晶体中控制$^{229}$Th同核异能态布居
Controlling $^{229}$Th isomeric state population in a VUV transparent crystal
AI总结：
本研究在VUV透明$^{229}$Th掺杂CaF$_2$晶体中，通过共振X射线泵浦实现同核异能态布居，测得半衰期447秒，并发现X射线淬灭效应可将半衰期缩短至少50倍，为固态核钟开发奠定基础。
摘要：
放射性同位素Th-229以其极低能量、长寿命的核第一激发态而闻名。该同核异能态可通过VUV激光激发，其基态跃迁已被提议作为超高精度核钟的参考跃迁。此类核钟将具有多种应用，涵盖基础物理研究到实际应用。近期研究通过U-233或Ac-229的随机核衰变布居同核异能态，提取了核跃迁能量和寿命的宝贵约束。然而，为评估（固态）核钟概念的可行性和性能，对$^{229}$Th同核异能态进行时间受控的激发和退激发，以及对辐射衰变的时间分辨监测至关重要。在此，我们报告了在VUV透明的$^{229}$Th掺杂CaF$_2$晶体中，通过共振X射线泵浦和辐射衰变探测实现$^{229}$Th同核异能态的布居。测得衰变半衰期为$447\pm 25$ s，跃迁波长为$148.18 \pm 0.42$ nm，辐射衰变份额与1一致。此外，我们报告了一种新的“X射线淬灭”效应，该效应可按需退激发同核异能态，并有效将半衰期至少缩短50倍。这种受控淬灭可用于显著加速未来核钟方案中的询问周期。我们的结果表明，在晶体环境中可以实现对$^{229}$Th核同核异能态布居的完全控制。特别是，可能导致同核异能态跃迁线宽展宽的非辐射衰变过程可忽略不计，为开发紧凑且稳健的固态核钟铺平了道路。需要进一步研究以揭示X射线淬灭效应的潜在物理机制。
英文摘要：
The radioisotope Th-229 is renowned for its extraordinarily low-energy, long-lived nuclear first-excited state. This isomeric state can be excited by VUV lasers and the transition from the ground state has been proposed as a reference transition for ultra-precise nuclear clocks. Such nuclear clocks will find multiple applications, ranging from fundamental physics studies to practical implementations. Recent investigations extracted valuable constraints on the nuclear transition energy and lifetime, populating the isomer in stochastic nuclear decay of U-233 or Ac-229. However, to assess the feasibility and performance of the (solid-state) nuclear clock concept, time-controlled excitation and depopulation of the $^{229}$Th isomer together with time-resolved monitoring of the radiative decay are imperative. Here we report the population of the $^{229}$Th isomeric state through resonant X-ray pumping and detection of the radiative decay in a VUV transparent $^{229}$Th-doped CaF$_2$ crystal. The decay half-life is measured to $447\pm 25$ s, with a transition wavelength of $148.18 \pm 0.42$ nm and a radiative decay fraction consistent with unity. Furthermore, we report a new ``X-ray quenching'' effect which allows to de-populate the isomer on demand and effectively reduce the half-life by at least a factor 50. Such controlled quenching can be used to significantly speed up the interrogation cycle in future nuclear clock schemes. Our results show that full control over the $^{229}$Th nuclear isomer population can be achieved in a crystal environment. In particular, non-radiative decay processes that might lead to a broadening of the isomer transition linewidth are negligible, paving the way for the development of a compact and robust solid-state nuclear clock. Further studies are needed to reveal the underlying physical mechanism of the X-ray quenching effect.
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