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arXiv:1211.5437 · Ukrainian Journal of Physics

Direct Two-Photon Excitation of Isomeric Transition in Thorium-229 Nucleus

V. I. Romanenko, Ye. G. Udovitskaya, Леонід Петрович Яценко, A.V. Romanenko, A.N. Litvinov, Georgy A. Kazakov

Published Dec 1, 2021• 6 citations
arXivHTMLPDF

Abstract

A possibility of the two-photon excitation of an isomeric state in a nucleus of thorium-229 has been discussed. The fluorescence intensity of the excitation is demonstrated to be identical for the irradiation of nuclei with either monochromatic light or polychromatic radiation consisting of a sequence of short lightpulses of the same intensity. The two-photon excitation of Th3+ ion in an electromagnetic trap with a focused laser beam with a wavelength of about 320 nm and power of 100 mW can lead to the absorption saturation, at which the fluorescence emission with the frequency of the transition in a nucleus is maximal. In crystals doped with Th4+ to a concentration of about 1018 cm-3 and irradiated with a laser radiation 10 W in power, the emission of several photons persecond with a wavelength of about 160 nm becomes possible.

DOI
10.15407/ujpe57.11.1119

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A possibility of the two-photon excitation of an isomeric state in a nucleus of thorium-229 has been discussed. The fluorescence intensity of the excitation is demonstrated to be identical for the irradiation of nuclei with either monochromatic light or polychromatic radiation consisting of a sequence of short lightpulses of the same intensity. The two-photon excitation of Th3+ ion in an electromagnetic trap with a focused laser beam with a wavelength of about 320 nm and power of 100 mW can lead to the absorption saturation, at which the fluorescence emission with the frequency of the transition in a nucleus is maximal. In crystals doped with Th4+ to a concentration of about 1018 cm-3 and irradiated with a laser radiation 10 W in power, the emission of several photons persecond with a wavelength of about 160 nm becomes possible.
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A low-energy nuclear transition in the isotope thorium-229 has been excited in thorium-doped crystals with laser light. This opens the perspective towards a highly stable and robust solid-state optical nuclear clock. The required laser radiation at 148 nm wavelength has so far been produced using pulsed laser systems where only a small fraction of the incident photons has been resonant with the narrow nuclear transition. Here we show that the nuclear resonance can be excited with a continuous-wave narrow-bandwidth laser source with a power of less than 1 nW, and that the resonance signal can be detected in absorption rather than in fluorescence. This eliminates the slow nuclear fluorescence decay from the detection process and offers a considerable advantage for clock operation through fast signal acquisition. The VUV laser source is based on three sequential frequency doublings, starting from a diode laser at 1187 nm that is well suited for linewidth narrowing and for frequency comparisons with optical atomic clocks. We use absorption spectroscopy for the quantitative characterization of two different Th-centers in calcium fluoride crystal and measure the isomeric shift between them. One of the centers shows a very small static electric crystal field gradient 0.1 V/A˚2Å^2A˚2, to be compared to gradients in the range of 100 V/A˚2Å^2A˚2 observed earlier. This indicates a center with high symmetry of the ions surrounding the Th nucleus, promising nuclear resonance lines that are nearly independent of the lattice spacing.
Hyperfine-resolved laser excitation and detection of nuclear isomer in trapped 229^{229}229Th3+^{3+}3+ ions
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The impact of coherent light propagation on the excitation and fluorescence of thorium nuclei in a crystal lattice environment is investigated theoretically. We find that in the forward direction, the fluorescence signal exhibits characteristic intensity modulations dominated by a sped-up initial decay signal that is orders of magnitude faster. This feature can be exploited for the optical determination of the isomeric transition energy. In order to obtain a unmistakable signature of the isomeric nuclear fluorescence, we put forward a novel scheme for the direct measurement of the transition energy via electromagnetically modified nuclear forward scattering involving two fields that couple to three nuclear states.
Experiments on one- and two-photon laser excitation of 232{}^{232}232Th+{}^{+}+ ions in a radio-frequency ion trap are reported. As the first excitation step, the strongest resonance line at 402 nm from the (6d27s)J=3/2(6{d}^{2}7s)J=3/2(6d27s)J=3/2 ground state to the (6d7s7p)J=5/2(6d7s7p)J=5/2(6d7s7p)J=5/2 state at 24874 cm${}^{\ensuremath{-}1}$ is driven by radiation from an extended-cavity diode laser. Spontaneous decay of the intermediate state populates a number of low-lying metastable states, thus limiting the excited state population and fluorescence signal obtainable with continuous laser excitation. We study the collisional quenching efficiency of helium, argon, and nitrogen buffer gases, and the effect of repumping laser excitation from the three lowest-lying metastable levels. The experimental results are compared with a four-level rate equation model, which allows us to deduce quenching rates for these buffer gases. Using laser radiation at 399 nm for the second step, we demonstrate two-photon excitation to the state at 49 960 cm${}^{\ensuremath{-}1}$, among the highest-lying classified levels of Th+{}^{+}+. This is of interest as a test case for the search for higher-lying levels in the range above 55 000 cm${}^{\ensuremath{-}1}$ which can resonantly enhance the excitation of the 229{}^{229}229Th+{}^{+}+ nuclear resonance through an inverse two-photon electronic bridge process.
The well-known narrowing of the Mössbaur γ-line with increasing age of the excited energy state ℏω0 of the emitter nucleus (the half-life is T1/2) and broadening of the γ-line with decreasing irradiation time of the absorber nucleus are analyzed. Taking into account the short time of emission of quantum transition energy – much less than T1/2, which can be seen, for example, from the duration of the γ-signal on the scintillator, the analysis leads to the idea of the structure of the γ-quantum (photon) as an electromagnetic wave of frequency ω0 that does not carry energy (0-wave), which is emitted by the nucleus from the moment of formation of the excited state to the quantum transition, with the energy quantum ℏω0 in the form of a short-term singularity on the “tail” of this 0-wave. A possible source of 0-waves can be virtual transitions of the nucleus from the excited state to the ground state and back before the emission of an energy quantum. Detection of 0-waves is possible due to changing the width of the γ‑line with additional irradiation of the absorber nuclei by resonant 0-waves. The possibility of predicting the future decay of an excited state by 0-waves detecting and the prospects of using 0-waves for information transmission are noted. The principle of a 0-wave generator without energy radiation is proposed.