Search for the 229Pa Parity Doublet via Internal-Conversion Spectroscopy at FRIB
The radioisotope $^{229}$Pa is predicted to possess a nearly degenerate ground-state parity doublet. Previous $γ$-ray and conversion-electron spectroscopy suggests a splitting of $ΔE = (60\pm 50)~\mathrm{eV}$, although the doublet's existence remains unconfirmed. Such a small splitting would give $^{229}$Pa an exceptionally large enhancement of symmetry-violating nuclear properties. However, the splitting, and with it the sensitivity of $^{229}$Pa to CP-violating physics, remains uncertain by orders of magnitude. We propose an experiment to measure this splitting with sub-eV precision within a few days of measurement time by using delayed-coincidence internal-conversion spectroscopy of $^{229}$Pa ions produced by the electron-capture decay of $^{229}$U$^{+}$ ions stored in a Penning trap at the Facility for Rare Isotope Beams (FRIB). This measurement would provide the key nuclear-structure input needed to establish $^{229}$Pa as a leading candidate for next-generation atomic and molecular electric-dipole-moment (EDM) experiments.