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

TL;DR

This work develops a rapid, low-background assay for $^{210}$Pb in archaeological Pb by coupling optimized chemical digestion with pulse-shape analysis in a commercial liquid scintillation counter. The method achieves sensitivities on the order of a few $\times 10^{2}$ mBq/kg within about a week for sample masses $<1$ g, and below $100$ mBq/kg after ~40 days, by distinguishing $^{210}$Pb/$^{210}$Bi beta decays from $^{210}$Po alpha decays and verifying secular equilibrium. Key innovations include maximizing the product $m \epsilon$ through an optimized 8/20 cocktail configuration with ~20% HNO$_3$, and calibrating PSA to attain reliable alpha/beta separation across the decay chain. The approach offers rapid radiopurity screening for archaeological Pb and Pb-based shielding in next-generation low-background experiments, with potential extension to other materials used in rare-event physics.

Abstract

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.

Figures (10)

• Figure 1: Decay scheme of the $^{210}$Pb decay chain. For the $^{210}$Pb and $^{210}$Bi only the $\beta^-$ branches are shown, since $\alpha$ decays have far smaller branching ratios.
• Figure 2: Scheme of the Quantulus 1220 detector where the sample site, the sample PMTs, the active muon veto system and the passive lead shielding are shown.
• Figure 3: Detector energy spectrum acquired for the S3_9 calibration sample. The overlayed dashed curves shown are purely qualitative and are intended only to guide the eye. The orange and green curves represent the contributions from the $^{210}$Pb and $^{210}$Bi $\beta$ decays, respectively. The red curve corresponds to the $^{210}$Po $\alpha$ contribution, which appears superimposed on the $^{210}$Bi $\beta$ continuum due to quenching effects. The black solid curve represents the reconstructed total spectrum obtained from the sum of the individual components.
• Figure 4: Effects of different quenching parameters on the detector counting efficiency. In both panels, the 8/20 (5/20) sample set is shown in blue (red). Each sample was spiked with a $^{210}$Pb tracer with a known activity of $5.3 \pm 0.5$ Bq. The error bars include both the statistical uncertainty of the measurement and the systematic uncertainty associated with the activity of the $^{210}$Pb standard.
• Figure 5: Photograph of the s2 sample set. From left to right, the HNO$_3$ concentration in the solution increases (from 1% up to 13%), and a progressive deterioration of the samples is observed. The degradation of the most acidic samples becomes visible after approximately 3--4 months from preparation, appearing initially as a slight yellow hue that darkens over time. This behavior indicates that samples prepared with the highest acidity should be discarded after a few months.