$^{210}$Pb measurements at the Andr\'e E. Lalonde AMS Laboratory for the radioassay of materials used in rare event search detectors

Abstract

Naturally occurring radionuclide $^{210}$Pb ($T_{1/2}$=22.2 y) is an important source of background in rare event searches, such as neutrinoless double-$\beta$ decay and dark matter direct detection experiments. When a sample mass of hundreds of grams is available, $\gamma$-counting measurements can be performed. However, there are other cases where only grams of sample can be used. For these cases, better sensitivities are required. In this paper, in collaboration with the Astroparticle Physics group at Carleton University, the capabilities of the A.E. Lalonde AMS Laboratory at the University of Ottawa for $^{210}$Pb measurements are discussed. PbF$_{2}$ and PbO targets were used, selecting in the low energy sector, respectively, (PbF$_{3}$)$^{-}$ or (PbO$_{2}$)$^{-}$ ions. For fluoride targets, the blank $^{210}$Pb/$^{206}$Pb ratio was in the 10$^{-14}$ to 10$^{-13}$ range, but current output was lower and less stable. For oxide targets, current output showed better stability, despite a significant difference in current output for commercial PbO and processed samples, and background studies suggested a background not much higher than that of the fluoride targets. Both target materials showed, therefore, good performance for $^{210}$Pb AMS assay. Measurements of Kapton films, an ultra-thin polymer material, where masses available are typically just several grams, were performed. 90% C.L. upper limits for the $^{210}$Pb specific activity in the range of 0.74-2.8 Bq/kg were established for several Kapton HN films.