Energy of the Th229 Nuclear Clock Isomer Determined by Absolute γ -ray Energy Difference

Abstract

The low-lying isomeric state of 229 Th provides unique opportunities for high-resolution laser spectroscopy of the atomic nucleus. We determine the energy of this isomeric state by taking the absolute energy difference between the excitation energy required to populate the 29.2-keV state from the ground-state and the energy emitted in its decay to the isomeric excited state. A transition-edge sensor microcalorimeter was used to measure the absolute energy of the 29.2-keV γray. Together with the cross-band transition energy (29.2 keV→ground) and the branching ratio of the 29.2-keV state measured in a recent study, the isomer energy was determined to be 8.30±0.92 eV. Our result is in agreement with latest measurements based on different experimental techniques, which further confirms that the isomeric state of 229 Th is in the laser-accessible vacuum ultraviolet range.

Ticking Toward a Nuclear Clock

“…A transition of the thorium-229 nucleus is the only known exception. The energy difference between its ground state and its first, metastable excited state, denoted 229m Th, is exceptionally low-with previously reported values between 3.5 and 8.3 eV [2][3][4][5][6][7]. These energies correspond to wavelengths where lasers are, in principle, available.…”

The $$^{229}$$Th isomer: prospects for a nuclear optical clock

“…This requirement poses a strong constraint on the nuclear transitions useful for clock operation as, from the more than 176,000 known nuclear excited states, only 2 exhibit an energy below 100 eV. These are 229m Th, a metastable excited state of the 229 Th nucleus with an excitation energy of only about 8 eV [12,13,321,328,412], making it the nuclear state of lowest known excitation energy, and 235m U, a metastable state of 235 U with an excitation energy of ≈ 76.7 eV [269]. The existence of a potential third nuclear excited state below 100 eV excitation energy, 229m Pa, is still under investigation [1].…”

“…(6) Based on the difference between three γ lines. (7) Based on the difference to the 29.19 keV excitation energy like in [412] [12], (4): [13], (5): [321], (6): [412], (7): [328] transition is thus of multipolarity M1. Based on γ -ray spectroscopy of nuclear states of higher energies, the 229m Th energy was constrained to be below 10 eV in 1990 [283] and an energy value of 3.5 ± 1.0 eV was determined in 1994 [129].…”

“…This allowed for a precision determination of the excitation energy of the 29.19 keV state. Based on this measurement, in combination with precision γ -ray spectroscopy, the isomeric energy was constrained to 8.30 ± 0.92 eV in [412]. Although being of lower precision, this measurement provides an important consistency check.…”

“…With an energy difference of only about 8 eV to the nuclear ground state, 229m Th is the nuclear excited state of lowest know excitation energy, thereby offering the possibility for laser excitation. While an imprecise knowledge of the transition energy has so far hindered the development of a nuclear clock, recently, three new measurements have led to a significant increase in confidence about the isomer's excitation energy [321,328,412]. This new knowledge is likely to lead to a phase transition in the 229m Th-related research, away from previous "high-energy" nuclear-physics dominated research and more towards "low-energy" precision laser spectroscopy.…”