AMO technologies of ion trapping and cooling and high resolution spectroscopy will be applied to the manipulation of a nuclear excited state. While typical nuclear excitation energies are in the keV to MeV range, there are several exceptional cases where the excitation energies are much lower. The Thorium-229 isotope, uniquely, has an excited state in the UV optical spectrum. The nuclear transition is likely to be exceptionally sensitive to variation of fundamental constants, due to the interplay of the strong and electroweak interactions inside this nucleus. Additionally, it offers a prospect of an optical clock substantially less sensitive to external electromagnetic field perturbations, including black-body radiation. In preliminary work triply-charged ions of Thorium 232 (232Th3+) have been confined and laser cooled in an rf Paul trap. We will now trap and laser cool 229Th3+. The isomer state search will be assisted by the electron-nuclear coupling (the so-called electron-bridge mechanism), via multi-photon excitation with harmonics of an ultra-fast laser. Once the transition has been found, optical spectroscopy of the electronic states of the isomer will be performed. This should determine the enhancement factor for the search of time variation of fine structure constant alpha in this system, resolving an ongoing theoretical controversy.
This research will have broad impact across several areas of physics and technology. Laser excitation and coherent manipulation of nuclear states would establish a new bridge between atomic and nuclear physics, with the promise for new technologies, particularly in the area of precision metrology. In addition, this research will provide training and expertise for a new generation of scientists working on the boundaries of atomic, optical and nuclear physics.
The thrust of our research program was in applying atomic, molecular, and optical (AMO) technologies of ion trapping and cooling and high resolution spectroscopy to the manipulation of a nuclear excited state. While typical nuclear excitation energies are in the keV to MeV range, there are several exceptional cases where the excitation energies are much lower. The subject of our program was the thorium-229 isotope, which, uniquely, has an excited state in the UV optical spectrum. The existence of a very low-lying isomeric state thorium-229 was first proposed by Kroger and Reich in 1976. In 1994, the energy of this state was indirectly determined to be 3.5(1) eV, via spectroscopy of gamma rays emitted from the alpha decay of uranium-233. This measurement motivated numerous studies relating to the unique prospect of controlling nuclear matter with optical radiation. Several experimental searches for optical emission in this range undertaken in the past ten years were unsuccessful or inconclusive. In 2007, a refined gamma ray spectroscopy measurement employing new detector technology has established the energy splitting to be 7.8(5) eV, likely explaining the failure of the early direct measurements. In addition to searching in the wrong energy region, the searches were challenged by the narrow linewidth of the transition. Depending on the size of the contribution of electron bridge processes, the predicted lifetime of this state may be as long as 10,000 s. The nuclear transition is likely to be exceptionally sensitive to the variation of fundamental constants, due to the interplay of the strong and electroweak interactions inside this nucleus. Notable outcomes of our work include the following: 1) Inspired by the landmark work of E. Peik and C. Tamm, Europhys. Lett. 61, 181 (2003), we proposed a nuclear clock based on the ground electronic level of triply charged thorium-229 offering the prospect of unprecedented accuracy. 2) We achieved trapping, laser cooling, and optical spectroscopy of triply charged thorium-229 in an radio-frequency Paul trap, and demonstrated laser excitation of triply charged thorium-232 ion chains into the 7P-electronic level. The 7P-level is important as it is connected to the isomer nuclear manifold of thorium-229 by the strongest available electron-bridge transition. 3) We investigated charge exchange and chemical reactions of triply charged thorium. We have measured the reaction rates of trapped, buffer gas cooled triply charged thorium-232 and various gases and have analyzed the reaction products using trapped ion mass spectrometry techniques. Our results showed that reactions of triply charged thorium with carbon dioxide, methane, and oxygen all occur near the classical Langevin rate, while reaction rates with argon, hydrogen, and nitrogen were found to be orders of magnitude lower. The broader impacts included applications across several areas of physics and technology. In particular, laser excitation and coherent manipulation of nuclear states could establish a new bridge between atomic and nuclear physics, with the promise of new levels of accuracy for frequency metrology. The research program made contributions to integrating undergraduate students into the cutting-edge research effort, and to graduate student training in technologically important scientific areas.
