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Calibrating the Ruler for the Invisible: How Technetium Decay Tests Nuclear Models ✎

Original: "Measurement of the $$^{99}$$Tc $$β$$ spectrum with Silicon Drift Detectors"
· Andrea Nava, Andrea Del Contrasto, Leonardo Bernardini, Matteo Biassoni, Tommaso Bradanini, Chiara Brofferio, Marco Carminati, Silvia Capelli, Francesco Cappuzzello, Manuela Cavallaro, Massimiliano Clemenza, Giovanni De Gregorio, Giulio Gagliardi, Giorgio Grosso, Nunzio Itaco, Irene Nutini, Alessia Pozzi, Nicola Zorzi
arXiv:2609.05640v1 · 2026-09-04 · arXiv non-exclusive · independent analysis · 2 min
Nuclear Experiment Nuclear Theory
A silicon detector the size of a fingernail measured how much weaker particles behave inside the atomic nucleus, providing new benchmarks for the search for the rarest decay.

A ruler for measuring the invisible must be calibrated against something tangible. In particle physics, nuclear models serve as such a ruler, and the decay of technetium-99 provides a tangible benchmark. This artificial isotope lives for 211 thousand years and decays by emitting an electron. Its decay is considered "forbidden": the particle struggles to leave the nucleus due to the intricate dance of angular momenta, and this difficulty makes the process exquisitely sensitive to theoretical details.

Technetium-99 is produced in reactors as a byproduct of uranium fission. A tiny drop of solution dried on a silicon wafer provided a source with an activity of just 10 becquerels, roughly one decay every ten seconds.

The experiment was like listening for a faint whisper in a noisy room. A silicon drift detector a millimeter thick caught electrons from the decay, while a lutetium crystal nearby emitted light at strictly defined energies. Coincidences between signals from the crystal and the detector acted as a tuning fork: they allowed calibrating the energy scale during the measurement itself, without subtracting a calibration source from the data. Six days in a vacuum chamber at minus ten degrees Celsius yielded 180 thousand events.

Comparison with theory revealed a discrepancy. Calculations within the realistic shell model describe the spectral shape reasonably well, but they predict weaker suppression of weak currents than the data show. A Bayesian fit, which simultaneously accounted for the spectral shape and the known half-life, gave suppression factors of 0.40 for the axial current and 0.47 for the vector current. This means that particles inside the nucleus behave noticeably weaker than free ones.

Previously, such measurements were done with cryogenic calorimeters, where the source is embedded in the detector and the background must be modeled. Here, the background was measured directly by simply replacing the substrate with a clean one. This reduces systematic errors.

The obtained numbers are not just abstract coefficients. They directly impact plans to search for neutrinoless double beta decay in molybdenum-100, one of the main candidates for future ton-scale experiments. If models overestimate the interaction strength, then sensitivity predictions for such setups will need revision. A ruler calibrated with technetium helps avoid being fooled in measurements of the invisible.

Technetium-99 is an artificial isotope with a half-life of 211 thousand years. It is produced in significant quantities as a fission product of uranium in nuclear reactors and is one of the main long-lived radionuclides in spent nuclear fuel. Its beta decay leads to ruthenium-99, which is stable.

M(E_{\mathrm{out}}) = \int_0^Q P(E_{\mathrm{out}}|E_{\mathrm{in}}) \cdot \epsilon(E_{\mathrm{in}}) \cdot T(E_{\mathrm{in}}) \, dE_{\mathrm{in}} + B(E_{\mathrm{out}})
The measured spectrum equals the convolution of the theoretical spectrum with the detector response function, multiplied by the detection efficiency, plus background.
\log L(p) = -\frac{1}{2} \left[ \sum_{i=1}^{N_{\mathrm{bins}}} \left( \frac{D_i - M(p)_i}{\sigma_i^D} \right)^2 + \left( \frac{T_{1/2}^D - T_{1/2}^M(p)}{\sigma_{T_{1/2}}^D} \right)^2 \right]
Sum of penalties for mismatch of spectral shape and half-life to experimental data.
q_{g_A} = \frac{g_A}{g_A^{\mathrm{ref}}}, \quad q_{g_V} = \frac{g_V}{g_V^{\mathrm{ref}}}
Quenching factors show how many times the effective coupling constants in the nucleus are smaller than the free values (g_A^ref = 1.2723, g_V^ref = 1).
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beta decay