Lead-210: a contaminant in particle detectors for dark matter studies
Loading...
Date
2021-11-17
Journal Title
Journal ISSN
Volume Title
Publisher
Australian Nuclear Science and Technology Organisation
Abstract
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 [1].
In order to verify this result with higher sensitivity, the SABRE (Sodium iodide with Active Background
REjection) experiment [2] is being developed. Radioimpurities such as ⁴ ⁰ K, ²³⁸ U, ²¹⁰ Pb 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 ²¹⁰ Pb (half-life of 22.2 years) as its beta decay to ²¹⁰ Bi contributes to the low-energy
“dark matter” spectra [3]. Lead-210 measurements are usually performed using alpha -, beta - or gamma
counting depending on the sample size and concentration [4]. However, in recent years, the interest and
therefore developments to measure ²¹⁰ Pb using accelerator mass spectrometry (AMS) has increased [5], [6].
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., 1x8 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 ²¹⁰ Pb
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 Fe₂ O₃ 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.
Description
Keywords
Lead 210, Nonluminous matter, Particles, Detection, Laboratories, Italy, keV range 01-10, Sodium iodides, Reflection, Mass spectroscopy, ANSTO
Citation
Froehlich, M., Merchel, S., Slavkovská, Z., Dastgir, F., Fifield, L. K., Hotchkis, M., Koll. D., Pavetich, S., Tims, S., & Wallner, A. (2021). Lead-210: a contaminant in particle detectors for dark matter studies. Paper presented to the 15th International Conference on Accelerator Mass Spectrometry. ANSTO Sydney, Australia. November 15th – 19th, 2021. (pp. 254). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf