The nucleus 229^{229}Th has the lowest known nuclear excited state, an isomer at about 8.356 eV whose decay emits a vacuum-ultraviolet (VUV) photon once internal conversion is suppressed inside a wide-band-gap solid. Tkalya, Borisyuk, Domashenko and Lebedinskii (arXiv:2210.02512, 2022) proposed a "nuclear light source": 229^{229}Th implanted into a 10-20 nm wide-gap dielectric film in a metal-insulator-semiconductor stack, in which an electric current passed through the film excites nuclei to the isomer by inelastic scattering of conduction electrons, and the subsequent radiative decays yield roughly 10510^5 photons/s per ampere per nanogram of 229^{229}Th. The proposal itself identified what remains unresolved: implantation- and radiation-induced defect states that could reopen nonradiative decay channels, the maximum thorium loading that keeps the band gap above the isomer energy, near-interface quenching, and the missing experimental confirmation of the predicted fluxes and Purcell-modified lifetimes. Later literature (2023-2026) settled two underpinnings - the transition energy and the radiative width - and demonstrated laser excitation of the nucleus in crystals, but the current-driven device has never been built or tested, and the defect-state and band-gap landscape of implanted thin films versus thorium loading remains unmapped. The open problem is the experimental demonstration, or a quantitative experimental exclusion, of the current-driven thin-film 229^{229}Th VUV light source, where an exclusion need not establish the material conditions under which it can operate.

Background

The nucleus 229^{229}Th possesses the lowest known nuclear excited state: an isomer (a long-lived excited nuclear state) at an energy now established by direct laser spectroscopy at approximately 8.356 eV, corresponding to a photon wavelength of about 148.4 nm in the vacuum ultraviolet (VUV). Because this energy is so low, the isomer's decay in a bare atom or ion is dominated by internal conversion (IC), a channel in which the nuclear de-excitation energy is transferred to an atomic electron, which is ejected, instead of being emitted as a photon. Theory dating back to the 1990s showed that in a dielectric whose band gap Δ\Delta (the energy interval between the valence and conduction bands containing no electronic states) exceeds the isomer energy EisE_{\mathrm{is}}, IC is energetically forbidden, so an embedded 229m^{229m}Th nucleus must decay radiatively; this suppression is the basis of the whole solid-state 229^{229}Th program. The nucleus can also be excited through its electronic environment: by an electron-bridge process (a second-order process in which the nuclear transition and an electronic shell transition exchange energy) or by nuclear excitation via inelastic electron scattering (NEIES), in which a slow free electron transfers kinetic energy to the nucleus through the electromagnetic interaction. Tkalya calculated (Phys. Rev. Lett. 124, 242501 (2020)) that electrons of about 10 eV excite the isomer with cross sections in the range 10−2510^{-25}-10−26 cm210^{-26}\ \mathrm{cm}^2 for the neutral atom and several thorium ions - enormous by nuclear standards - and, crucially, without any need to tune to a known transition energy.

On this basis, Tkalya, Borisyuk, Domashenko and Lebedinskii proposed a "nuclear light source": 229^{229}Th implanted into a thin (10-20 nm) wide-band-gap dielectric layer, e.g. amorphous SiO2_2 thermally grown on silicon, or MgF2_2, sandwiched between a doped semiconductor and a roughly 10 nm metal (Pd or Au) contact thin enough to transmit 150 nm photons. Passing a current through the film makes conduction electrons excite the nuclei by NEIES; the excited nuclei then decay by VUV photon emission, with the radiative rate enhanced by the refractive index of the host (a factor of order n3n^3) and modified by Purcell-type effects (the change of spontaneous emission near boundaries and lossy media, which also open nonradiative channels through the imaginary parts of the metal dielectric constants). The predicted luminosity is about 10510^5 photons/s per ampere of current per nanogram of 229^{229}Th, with alpha-particle-induced backgrounds estimated about two orders of magnitude lower, giving 10410^4-10510^5 detected counts/s at the stated collection efficiency. The proposal, whose two stated purposes were to excite the isomer and to measure its then-unknown transition energy, explicitly identified the unresolved issues that decide feasibility: implantation damage, thorium-related impurity bands and long-term alpha-decay damage can introduce electronic states inside the band gap that reopen IC and electron-bridge decay; the band gap shrinks with thorium concentration (as established for ThO2_2 in SiO2_2 and for Th-doped CaF2_2 and MgF2_2 in the works the proposal cites), so the usable loading is bounded by the condition Δ>Eis\Delta > E_{\mathrm{is}}; surface states at the metal/dielectric interface enhance nonradiative decay for nuclei within a few monolayers; the radiative width, set by the magnetic-dipole reduced transition probability B(M1)B(M1) (quoted in Weisskopf units, a standard nuclear scale), was uncertain by factors of a few; and the predicted photon fluxes and Purcell-modified lifetimes had no experimental confirmation.

Since the proposal appeared, the surrounding field has moved rapidly. The radiative decay of the isomer in wide-gap crystals was observed, with a half-life of 670(102) s670(102)\ \mathrm{s} in MgF2_2 and wavelength 148.71(42) nm148.71(42)\ \mathrm{nm} (Kraemer et al., Nature 617, 706 (2023)); the radiative lifetime in CaF2_2 was later measured as 618(9) s618(9)\ \mathrm{s} (Schaden et al., 2025), which settles the B(M1)B(M1)/radiative-width question at the percent level in-crystal. Direct laser excitation of the nucleus was demonstrated (Tiedau et al., Phys. Rev. Lett. 132, 182501 (2024); Elwell et al., Phys. Rev. Lett. 133, 013201 (2024)), and the transition frequency was measured against the 87^{87}Sr optical clock with a VUV frequency comb (C. Zhang et al., Nature 633, 63 (2024)), so EisE_{\mathrm{is}} is no longer unknown. On the material side, quantitative theory now confirms the proposal's central risk: whenever defect states lie in the band gap, IC through them, accompanied by multiphonon emission, quenches the isomer much faster than its radiative decay (Morgan et al., Phys. Rev. Lett. 134, 253801 (2025)), and laser spectroscopy of 30-100 nm 229^{229}ThF4_4 thin films found lifetimes of only about 150 s, attributed to exactly such defect-induced quenching (C. Zhang et al., Nature 636, 603 (2024)). What no later work has done is test the electron-current scheme itself, or map the defect-state spectrum and band gap of implanted thin films as a function of thorium loading.

Problem Statement

Determine experimentally whether the proposed current-driven thin-film scheme works as predicted, and establish the material conditions under which it does - that is, convert the 229^{229}Th "nuclear light source" of Tkalya, Borisyuk, Domashenko and Lebedinskii (arXiv:2210.02512) into a validated laboratory source of vacuum-ultraviolet photons from the 229m^{229m}Th isomeric decay, or a quantitative experimental exclusion of its predicted performance. The question has three linked components that jointly define feasibility:

(1) Current-driven excitation and light emission. In a fabricated metal-insulator(-semiconductor) stack with 229^{229}Th implanted in a 10-20 nm wide-band-gap dielectric film (e.g. SiO2_2, MgF2_2, CaF2_2, LiCaAlF6_6), does passing an electric current populate the isomer via inelastic scattering of conduction electrons at the predicted rate, producing a spectrally resolvable line at the nuclear transition wavelength (about 148.4 nm) with the predicted yield (order 10510^5 photons/s per ampere per nanogram of 229^{229}Th, before collection losses) and Purcell-modified lifetime? The measurement must separate the nuclear signal from electron-induced luminescence of the substrate and contacts and from alpha-induced backgrounds, for example by current-on/current-off sequences and thorium-free (or 232^{232}Th) control samples.

(2) Defect-state control versus thorium loading. For the candidate hosts under realistic implantation conditions, what are the density, energy distribution and spatial location of electronic states introduced into the band gap (implantation damage, thorium-related impurity bands, alpha-decay damage), and what is the maximum 229^{229}Th concentration that preserves Δ>Eis\Delta > E_{\mathrm{is}} so that the radiative channel dominates? This requires systematic experimental mapping of the gap-state spectrum and band gap as a function of thorium concentration for the thin-film hosts.

(3) Near-interface losses and device viability. How large are the nonradiative losses for nuclei within a few monolayers of the metal or semiconductor interface, can implantation depth profiles be engineered to avoid them, and can contacts that are simultaneously conductive and VUV-transparent at a thickness of order 10 nm survive ampere-scale currents without adding defect states or background?

An answer may take the form of a working, characterized device, which must resolve all three components, or a quantitative experimental upper bound that excludes the predicted yield of component (1); such an exclusion settles the feasibility question negatively and does not require components (2) and (3). The single scientific objective uniting the components is the validated feasibility, or exclusion, of current-driven isomer excitation and radiative VUV emission in an implanted thin dielectric film.

References

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