Investigation of the Low-Lying Nuclear Isomeric Transition in the A=229 Isotope of Thorium

当原子核中的质子或中子被激发但不会立即衰变回基态时，就会出现核异构体。四十年前，在钍-229中发现了一个核同分异构体，它具有特别不寻常的特征。这个态的能量只比基态高8电子伏半衰期是15分钟。这是已知的能量最低的核跃迁，它可以用真空紫外区（VUV）的激光激发。如果这种转变可以被利用，那么它将允许使用激光操纵核国家，这是以前从未做过的。最终目标是利用这种状态的特征来研究现代科学中最引人注目的问题之一：自然界的常数实际上是常数吗？此外，这种同分异构体状态将允许建造一种核时钟，其性能可能超过所有现有的和计划中的光学时钟，从而改善导航和通信。在此之前，钍-229的异构体状态已经在粒子加速器中使用高能碰撞产生。该奖项将使加州大学洛杉矶分校的一个研究小组能够使用台式激光器创建异构体，并精确测量激发能，将我们对跃迁能量的了解提高4到5个数量级。研究小组将使用定制的VUV激光系统，在掺杂钍-229的晶体和含有a = 229同位素的氧化钍样品中激发从基态到异构体态的转变。在晶体的情况下，核的任何激发将随后导致核荧光，这将被灵敏的光子探测器检测到。在金属的情况下，原子核的任何激发都将随后导致内部转换电子的发射，这将被灵敏的电子探测器探测到。这些信号将被监测，以确定核状态的跃迁能量和寿命。一旦发现跃迁，将通过比较初步的钍时钟架构和更成熟的原子钟来探索跃迁作为时钟振荡器的潜力。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A nuclear isomer occurs when protons or neutrons in an atomic nucleus become excited but do not immediately decay back to their ground state. Forty years ago, a nuclear isomeric state was discovered in Thorium-229 which has especially unusual characteristics. The energy of this state is only 8 electron volts above the ground state and it has a half-life of 15 minutes. This is the lowest energy nuclear transition known and it can potentially be excited using lasers in the vacuum ultraviolet region (VUV). If the transition can be harnessed, then it would allow for the manipulation of a nuclear state using lasers, which has never been done before. The ultimate goal is to use the characteristics of this state to investigate one of the most compelling questions in modern science: Are the constants of nature actually constants? Further, the isomeric state should allow the construction of a nuclear clock which may outperform all current and planned optical clocks, thereby improving navigation and communication. Previously, the Thorium-229 isomeric state has been produced using high energy collisions in a particle accelerator. This award will enable a research team from UCLA to create the isomer using table-top lasers, and to precisely measure the excitation energy, improving our knowledge of the transition energy by 4 to 5 orders of magnitude. The research team will use a custom VUV laser system to excite the transition from the ground state to the isomeric state in both a crystal doped with thorium-229 and a sample of thorium oxide containing the A = 229 isotope. In the case of the crystal, any excitation of the nucleus will subsequently lead to nuclear fluorescence which will be detected by sensitive photon detectors. In the case of the metal, any excitation of the nucleus will subsequently lead to emission of an internal conversion electron which will be detected by a sensitive electron detector. These signals will be monitored to determine the transition energy and lifetime of the nuclear state. Once the transition is found, the potential of the transition for use as a clock oscillator will be explored by comparing a preliminary thorium clock architecture to more established atomic clocks.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.