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立即衰变回基态时，就会发生核异构体。40年前，钍-229的核同质异能态被发现，它具有特别不寻常的特性。 这种状态的能量仅比基态高出8电子伏，半衰期为15分钟。 这是已知的能量最低的核跃迁，它可以使用真空紫外区（VUV）的激光激发。 如果这种转变能够被利用，那么它将允许使用激光来操纵核状态，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：自然界的常数真的是常数吗？此外，同质异能态应该允许建造一个核时钟，它可能优于所有当前和计划中的光学时钟，从而改善导航和通信。 在此之前，钍-229的同质异能态是在粒子加速器中利用高能碰撞产生的。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，并精确测量激发能量，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态的跃迁。在晶体的情况下，原子核的任何激发将随后导致原子核荧光，其将被灵敏的光子探测器探测到。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，这将被灵敏的电子检测器检测到。这些信号将被监测，以确定核态的跃迁能量和寿命。一旦过渡被发现，过渡作为一个时钟振荡器使用的潜力将通过比较初步的钍钟架构，以更成熟的原子钟进行探索。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.