Thorium-229 Spectroscopy in an Opaque Material ✛
Context
The nuclear isomeric transition in thorium-229 with an energy of about 8.4 eV is the only known nuclear transition accessible to laser spectroscopy. This makes it a candidate for ultra-precise nuclear clocks and a probe of the local environment in solids. Until now, laser excitation of this transition has been observed only in crystals with wide bandgaps that are transparent to vacuum ultraviolet (VUV) light, such as CaF2, LiSrAlF6, and ThF4. The transparency requirement severely limited the choice of host materials. This work fits into the development of solid-state nuclear clocks: neighboring studies have already explored excitation in wide-bandgap crystals, optimization of interrogation time, and materials with spin-free isotopes. This paper takes a step toward materials with narrow bandgaps, where direct observation of fluorescence is impossible.
Methods
The sample was a thin layer of 229ThO2 about 10 nm thick, deposited by electrolytic deposition on a stainless steel disk. A tunable VUV laser system was used to excite the nuclei, generating pulses with an energy of 6–8 μJ at a wavelength of about 148.4 nm. Conversion electrons emitted from the surface upon internal conversion of the nuclear transition were collected using a combination of electric and magnetic fields onto a microchannel plate (MCP). To separate the useful signal from background photoelectrons, the target was alternately biased with positive and negative voltages, and the detector was gated with a delay. The spectrum was recorded by scanning the laser frequency and counting electrons in a window from 6 to 40 μs after the pulse. Additionally, first-principles calculations of the electronic structure of ThO2 were performed using GW and BSE methods to estimate the internal conversion rate and the isomeric shift.
Results
A resonant peak of conversion electrons was recorded at a frequency of 2020407.5(2)stat(30)sys GHz, consistent with previously measured values in wide-bandgap crystals. The peak width was 12.4(4) GHz and was determined by the laser linewidth. The measured lifetime of the isomeric state under internal conversion is 12.3(3) μs. The theoretical estimate of the lifetime for bulk ThO2 gives about 80 μs, which agrees in order of magnitude with the experiment; the discrepancy is attributed to possible defects and surface effects. Based on the measured lifetime and material parameters, the projected instability of a nuclear clock based on ThO2 with conversion readout is about 2×10−18 over 1 second of averaging.
Implications
This work demonstrates for the first time that thorium-229 nuclear spectroscopy can be performed in materials that are opaque to resonant radiation, by detecting conversion electrons instead of photons. This removes a fundamental limitation on the choice of host material: oxides, nitrides, and other compounds with narrow bandgaps are now accessible. Compared to neighboring works that used fluorescence detection in wide-bandgap crystals, this approach employs a fundamentally different decay channel, internal conversion, which expands the class of systems that can be studied. The method also opens the way to nuclear clocks with an interrogation time reduced by 8 orders of magnitude and potentially simple electrical readout.
Future development
The method could develop in several directions. First, studying internal conversion in various narrow-bandgap materials would allow a detailed investigation of how the local environment affects the nuclear transition, including phonons, electronic structure, and defects. Second, nuclear clocks with current readout could be realized: conversion electrons create an electric current that can be measured directly, simplifying and miniaturizing the device. Third, combined nuclear-phonon excitation could lead to laser nuclear resonant vibrational spectroscopy with sub-meV precision, unattainable with conventional Mössbauer spectroscopy.
Impact
The method will impact fields of precision spectroscopy, time and frequency metrology, and solid-state research. It may find applications in nuclear energy for monitoring the chemical state of thorium compounds.
Next steps
Immediate next steps include optimizing the efficiency of electron extraction, eliminating background photoelectrons, and studying internal conversion in other low-bandgap materials. It is also important to measure the dependence of the conversion rate on temperature and strain to calibrate sensors.
Key open problems
For nuclear physics and solid-state physics, a key unresolved problem remains the quantitative description of the relationship between the local environment of the thorium-229 nucleus and the parameters of the nuclear transition (frequency and internal conversion rate). Existing theories provide only order-of-magnitude estimates, and the discrepancy between calculation and experiment in this work underscores the need for more accurate treatment of defects, surfaces, and spin-orbit effects.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study demonstrates, for the first time, the feasibility of laser spectroscopy of the Th-229 nuclear transition in an opaque material (ThO2) by detecting conversion electrons. The transition frequency and the isomer lifetime were measured, and the potential instability of a clock was estimated.
Where its strength is
This method removes the constraint on the bandgap of the host material, opening access to materials where direct observation of photons is impossible.
Where it could go next
- It may be interesting to compare the obtained instability with estimates for other solid-state systems, for example, in the work on chip-scale nuclear clocks.based on: Nuclear Clock on a Chip: A New Step Toward Precision arXiv:2604.20687
- Closely related is the study of crystal acoustics of the nuclear transition; similar environmental effects might need to be considered when optimizing ThO2.based on: Crystalline Acoustics of a Nuclear Whisper arXiv:2607.08941
- For searches of new physics, such as dark matter, the proposed readout method could be useful in opaque media.based on: Nuclear Clock: The Ticking Nucleus in the Search for Dark Matter arXiv:2606.04997
Worth trying
- Test the effect of film thickness and substrate type on the internal conversion rate.
- Compare the measured lifetime with calculations for various defect configurations.
- Assess the possibility of using other narrow-bandgap materials, such as ThF4.
Why this may matter
Expanding the class of materials for thorium-229 nuclear spectroscopy could accelerate the development of compact solid-state nuclear clocks and new probes of the local environment.
Nearby works in our archive
- Nuclear Clock: The Ticking Nucleus in the Search for Dark Mattersimilarity 0.61
- Thorium Nuclear Clock Ticks with a Whisper of Lightsimilarity 0.58
- Crystalline Acoustics of a Nuclear Whispersimilarity 0.58
- The Nuclear Pendulum That Ticks More Precisely Than Anything on Earthsimilarity 0.55
- How to Speed Up Nuclear Clocks a Thousandfoldsimilarity 0.55
- Nuclear Clock on a Chip: A New Step Toward Precisionsimilarity 0.51
The closest work in the archive sits at 0.61 by meaning — the usual level for our corpus.
The energy of the nuclear transition in thorium-229 is only 8.4 eV, which is roughly the energy of an ultraviolet photon, orders of magnitude lower than typical nuclear energies (MeV). This allows the nucleus to be excited by a laser rather than an accelerator.
The idea of ultra-precise clocks based on a nuclear transition echoes the concept of "atomic clocks" in science fiction, such as in Frank Herbert's novel "Dune," where precise timekeeping plays a key role in navigation. In reality, nuclear clocks could surpass atomic clocks in stability and compactness.
Key numbers
- nuclear transition frequency: 2020407.5 GHz
- lifetime under internal conversion: 12.3 μs
- ThO2 sample thickness: 10 nm
- projected clock instability: 2×10−18 over 1 s
- nuclear transition energy: 8.4 eV