A rapid low-background assay of $^{210}$Pb in archaeological lead

In this work, we present a fast and highly efficient method for the measurement of $^{210}$Pb in metallic archaeological lead using the commercial low-background liquid scintillation counter Wallac Quantulus 1220 installed at the University of Milano-Bicocca (Italy). By combining an optimized chemical preparation with pulse-shape analysis (PSA), the technique achieves sensitivities at the level of a few $10^2$ mBq/kg within one week of measurement, using sample masses below 1 g. The method enables the simultaneous identification of the $β$ decays of $^{210}$Pb and $^{210}$Bi and the $α$ decay of $^{210}$Po, allowing a direct verification of secular equilibrium within the decay chain. With extended acquisition times, detection limits below 100 mBq/kg are reached after approximately 40 days. This approach provides a rapid, accessible, and reliable tool for the radiopurity screening of lead, and is well suited for quality control and R&D activities in next-generation low-background and rare-event physics experiments. Moreover, the method has the potential to be extended to other materials relevant for low-background applications.

💡 Research Summary

The paper presents a rapid, highly sensitive method for quantifying the activity of ^210Pb in metallic archaeological lead using a commercial low‑background liquid scintillation counter (LSC), the Wallac Quantulus 1220, installed at the University of Milano‑Bicocca. The authors combine an optimized chemical digestion protocol with pulse‑shape analysis (PSA) to achieve detection limits of a few hundred mBq kg⁻¹ within a single week of measurement, using less than 1 g of lead per sample.

Archaeological lead is a valuable source of ultra‑pure material for rare‑event physics because it has been shielded from cosmogenic activation for centuries, and its ^210Pb content is the dominant intrinsic background when lead is used as shielding or as an active target. Conventional techniques—direct γ‑spectroscopy of the 46 keV photon, β‑counting of ^210Bi after chemical separation, or α‑counting of ^210Po—each suffer from low efficiency, extensive sample preparation, or the need to assume secular equilibrium. The new approach overcomes these limitations by measuring β‑decays of both ^210Pb (63 keV endpoint) and its daughter ^210Bi (1.16 MeV endpoint) together with the α‑decay of ^210Po (5.3 MeV) in a single LSC run.

Sample preparation involves dissolving up to 0.8 g of lead in 4 M HNO₃, then mixing the acidic solution with liquid scintillator in an 8 ml : 12 ml (sample : scintillator) ratio (the “8/20” configuration). This ratio maximizes the product of sample mass and detection efficiency (m·ε ≈ 0.57) while keeping quenching manageable. The Quantulus 1220 operates in a coincidence mode between two photomultiplier tubes (PMTs) and includes an active muon veto, dramatically reducing random background. Energy calibration is performed with standards that mimic the chemical composition of the lead solutions, because quenching depends strongly on acidity and LS‑to‑sample ratio.

Pulse‑shape discrimination exploits the fact that α particles generate a larger fraction of slow scintillation light compared with β particles, despite a lower total light yield (≈ 1/10). By adjusting PSA parameters, the authors achieve clean separation of the ^210Po α‑peak from the β continuum, allowing simultaneous quantification of all three radionuclides. This enables a direct test of secular equilibrium: if equilibrium holds, the activities of ^210Pb and ^210Po must be equal, and the measured α/β ratio provides a consistency check for any contamination introduced during refining (e.g., ^222Rn ingress).

The detection limit (DL) is derived from the Currie formula: DL = kα σs / (m ε T BR), with kα = 2.365 for a one‑sided 95 % confidence level. Using a 0.8 g sample, a 7‑day counting period, and the optimized 8/20 configuration, the authors report DL values around 200–300 mBq kg⁻¹ for ^210Pb. Extending the counting time to ~40 days pushes the DL below 100 mBq kg⁻¹. These sensitivities surpass those of high‑purity germanium γ‑spectroscopy (≈ 1 Bq kg⁻¹) and bolometric α‑spectroscopy (≈ 10 mBq kg⁻¹), while requiring far less material and measurement time.

The method was applied to several low‑background lead samples, confirming that the archaeological lead used for the CUORE shielding and for the RES‑NOVA PbWO₄ crystals exhibits ^210Pb activities well below the few hundred mBq kg⁻¹ level, and that secular equilibrium is maintained within experimental uncertainties. The authors argue that this rapid assay is ideally suited for quality‑control monitoring during lead purification (e.g., recasting, zone refining) where ^222Rn‑derived ^210Pb re‑contamination can occur.

Beyond lead, the authors suggest that the same LSC‑PSA strategy could be adapted to other materials of interest for low‑background experiments, such as copper or tungsten, provided appropriate chemical dissolution and scintillator compatibility are achieved.

In summary, the paper delivers a practical, low‑cost, and fast analytical tool that combines commercial liquid scintillation counting with tailored chemistry and pulse‑shape discrimination to measure ^210Pb, ^210Bi, and ^210Po simultaneously in sub‑gram lead samples. This capability fills a critical gap in the radiopurity screening workflow for next‑generation rare‑event searches, enabling rapid feedback during material processing and ensuring that shielding and detector components meet the stringent background requirements of future neutrino, dark‑matter, and neutrinoless double‑beta decay experiments.