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.