A thin film of thorium dioxide, just 10 nanometers thick, is deposited on a steel disk. A laser pulse strikes it, but instead of the expected glow, the detector catches a stream of electrons. This is how, for the first time, the nuclear transition of thorium-229 was observed in a material that completely absorbs its own radiation. The invisible nucleus spoke the language of electrons.

The transition in thorium-229 is unique: its energy is so low that the nucleus can be driven by an ordinary laser tuned to a wavelength of about 148 nanometers. But until now, such experiments were only possible in crystals transparent to vacuum ultraviolet light, such as calcium fluoride or thorium fluoride. Transparency was a prerequisite: otherwise, the photon emitted by the nucleus would not escape and could not be detected. This limited the choice of materials and prevented the study of many thorium compounds, such as its oxide ThO2.

The new approach uses a different decay channel of the excited nucleus: internal conversion. Instead of transferring energy to a photon, the nucleus transfers it to one of its own atomic electrons, kicking it out of the shell. The electron escapes the material and can be captured by a detector. This makes the nuclear transition audible even in an opaque medium. The experiment used a microchannel plate that collected electrons using electric and magnetic fields. To filter out background photoelectrons, the target was alternately charged positive and negative, and the detector was switched on with a delay after the laser pulse. By scanning the laser frequency, a resonant peak was observed at 2020407.5 GHz, consistent with measurements in transparent crystals.

The measured lifetime of the isomeric state, 12.3 microseconds, is an order of magnitude shorter than the theoretical estimate for a bulk crystal. The discrepancy is attributed to defects and surface effects. But even this lifetime allows estimating the potential instability of nuclear clocks based on ThO2: about 2×10⁻¹⁸ per second of averaging. This is on par with the best atomic clocks, but in a device that could fit on a chip.

The method of conversion electron spectroscopy lifts a fundamental limitation: now, not only transparent crystals but also oxides, nitrides, and other compounds with narrow bandgaps are suitable for thorium nuclear spectroscopy. This opens the possibility of studying how the local environment of the nucleus, including phonons, electronic structure, and defects, affects the transition frequency and conversion rate. In the long run, it could lead to nuclear clocks with electrical readout: conversion electrons create a current that can be measured directly, without bulky optics.

The thin film on a steel disk becomes a sensor capable of picking up the signal of an invisible beacon: the nuclear transition. A beacon that could previously only be seen in a transparent medium can now be heard in the dark.