Lead-210: A contaminant in particle detectors for dark matter studies
Michaela Froehlich1, Silke Merchel1,2,3, Zuzana Slavkovská1, Ferdos Dastgiri1, Leslie Keith Fifield1, Michael Hotchkis4, Dominik Koll1, Stefan Pavetich1, Stephen Tims1, Anton Wallner1,3
1Research School of Physics, The Australian National University, Canberra, Australia, 2Faculty of Physics, University of Vienna, Vienna, Austria, 3Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany, 4Australian Nuclear Science and Technology Organisation, Sydney, Australia
The DAMA/LIBRA (DArk Matter/Large sodium Iodide Block for RAre processes) is a very low background NaI(Tl) detector array that has been running for two decades in the Gran Sasso underground laboratory in Italy. It gives a robust annual modulation signal in the 2 to 6 keV region that may be due to dark matter . In order to verify this result with higher sensitivity, the SABRE (Sodium iodide with Active Background REjection) experiment  is being developed. Radioimpurities such as ⁴⁰K, ²³⁸U, ²¹⁰Pb and ²³²Th, either intrinsic to the detector material or surface contamination, provide a fundamental limit to the sensitivity of SABRE. Therefore, it is crucial to characterise this background for improved identification of any additional signal above it.
Here, we focus on ²¹⁰Pb (half-life of 22.2 years) as its beta decay to ²¹⁰Bi contributes to the low-energy “dark matter” spectra . Lead-210 measurements are usually performed using alpha -, beta – or gamma counting depending on the sample size and concentration . However, in recent years, the interest and therefore developments to measure ²¹⁰Pb using accelerator mass spectrometry (AMS) has increased , .
From a chemical point of view, we need to optimise the Pb extraction of ~1 mg of stable Pb carrier through precipitations and ion exchange chromatography using about a kilogram of NaI. This is not trivial and methods using two different resins, i.e., 1×8 anion exchange resin and Sr® resin, have been tested. It is also essential that the stable Pb carrier and any material and chemical product in use should contain as little ²¹⁰Pb as possible. Hence, several materials have been investigated including a piece from a 16th century roof and radiation shielding blocks as a source of Pb carrier. Furthermore, we studied PbO and PbF₂ samples to identify the optimal negative-ion beam and the suitability of using either Fe₂O₃ or NaF as bulk material for the AMS target to reduce the stable Pb content. AMS measurements related to this work have been made using the 14UD pelletron accelerator at the Australian National University and the 1 MV VEGA accelerator at the Australian Nuclear Science and Technology Organisation.
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Michaela Froehlich has a PhD in chemistry and significant experience in radiochemistry.
She worked ever since in a multidisciplinary field and specialised on chemical isolation of rare isotopes from environmental matrices measured by Accelerator Mass Spectrometry (AMS). She joined the AMS Group at the Australian National University in 2012. Michaela is also part of the Metrology Team in collaboration with ANSTO to develop the chemical separation techniques for the characterisation of high-purity detector material and low background measurements, e.g. lead in NaI. She is the elected secretary of the South Pacific Environmental Radioactivity Association.