We describe a novel approach to directly measure the energy of the narrow, low-lying isomeric state in 229Th. Since nuclear transitions are far less sensitive to environmental conditions than atomic transitions, we argue that the 229Th optical nuclear transition may be driven inside a host crystal with a high transition Q. This technique might also allow for the construction of a solid-state optical frequency reference that surpasses the short-term stability of current optical clocks, as well as improved limits on the variability of fundamental constants. Based on analysis of the crystal lattice environment, we argue that a precision (short-term stability) of 3×10(-17)<Δf/f<1×10(-15) after 1 s of photon collection may be achieved with a systematic-limited accuracy (long-term stability) of Δf/f∼2×10(-16). Improvement by 10(2)-10(3) of the constraints on the variability of several important fundamental constants also appears possible.
We have recently described a novel method for the construction of a solid-state optical frequency reference based on doping 229 Th into high energy band-gap crystals [1]. Since nuclear transitions are far less sensitive to environmental conditions than atomic transitions, we have argued that the 229 Th optical nuclear transition may be driven inside a host crystal resulting in an optical frequency reference with a short-term stability of 3 × 10 −17 < ∆f /f < 1 × 10 −15 at 1 s and a systematic-limited repeatability of ∆f /f ∼ 2 × 10 −16 . Improvement by 10 2 − 10 3 of the constraints on the variability of several important fundamental constants also appears possible. Here we present the results of the first phase of these experiments. Specifically, we have evaluated several high energy band-gap crystals (Th:NaYF, Th:YLF, Th:LiCAF, Na2ThF6, LiSAF) for their suitability as a crystal host by a combination of electron beam microprobe measurements, Rutherford Backscattering, and synchrotron excitation/fluorescence measurements. These measurements have shown LiCAF to be the most promising host crystal, and using a 232 Th doped LiCAF crystal, we have performed a mock run of the actual experiment that will be used to search for the isomeric transition in 229 Th. This data indicates that a measurement of the transition energy with a signal to noise ratio (SNR) greater than 30:1 can be achieved at the lowest expected fluorescence rate.
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