Trace Element Characterisation in the SABRE and CYGNUS WIMP Search Experiments

Abstract

This thesis reports on various experimental developments for the SABRE South and CYGNUS experiments, with a focus on the characterisation of trace elements so as to achieve the high levels of detector purity required to attain world-leading sensitivity for Weakly Interacting Massive Particle (WIMP) direct detection.

Intrinsic impurities in NaI(Tl) crystals for the SABRE South experiment were measured, including $^{40}$K, $^{238}$U/$^{232}$Th decay products, $^{210}$Pb, and the Tl dopant concentration profile. An external company, ALS, was used to make ICP-MS measurements of thallium and to assess the feasibility of low level potassium measurements. Tip and tail cut-offs of an ultra-pure crystal (NaI-35) produced by RMD were used, requiring the development of extensive chemical decomposition techniques to produce ICP-MS samples. The results confirmed a long suspected inhomogeneity in the concentration of thallium throughout the length of the crystal, and lower than expected average concentration, which may have implications for the crystal light yield and possibly the quenching factor. For $^{39}$K, the lowest detection limit at ALS was 10~ppb, and it was determined that cleaner laboratories would be required for the sample preparation.

The measurement of $^{210}$Pb in NaI(Tl) using Accelerator Mass Spectrometry (AMS) requires a lead carrier with low intrinsic $^{210}$Pb levels. An old Roman lead sample from the Laboratori Nazionali del Gran Sasso (LNGS) was chemically treated using column chromatography and acid transformation to create an AMS sample, which was measured at the ANSTO Vega accelerator. The $^{210}$Pb content was two orders of magnitude lower than required for use as a carrier with NaI(Tl), proving viability for a future AMS measurement. The ability of AMS to measure $^{210}$Pb at such levels is potentially transformational for future dark matter detectors.

Direct background counting of $^{238}$U and $^{232}$Th decay products in NaI-35 was performed underground at LNGS. Coincidence tagging of alpha and beta decays (slow and fast bismuth-polonium chains) as well as triple alpha decays were used to characterise the $^{226}$Ra and $^{228}$Ra radioisotopes that are believed to be in secular equilibrium with their daughters. The measured time difference distributions were fitted to exponential decay functions with a constant background using a log-likelihood minimisation. The $^{226}$Ra activity was determined to be 2.80$\pm$$^{0.36}_{0.41}$~$\mu$Bq/kg, with the best fitted half-life of 142~$\mu$s agreeing within 1$\sigma$ of the literature half-life of $^{214}$Po, providing extra confidence in the deduced activity. The bismuth-polonium and triple alpha decays below $^{228}$Ra, yielded results of 1.58$\pm$0.69~$\mu$Bq/kg and 2.91$\pm$$^{0.35}_{0.47}$~$\mu$Bq/kg, respectively, for its activity.

A prototype TPC in an approximately 20~L chamber was constructed and coupled to a Residual Gas Analyser (RGA). This system was used to measure increases in electronegative impurities in pure CF$_4$ gas, as well as their effect on Gas Electron Multiplier (GEM) gain. As much as a 19$\%$ drop in gain was observed when there was a 6$\%$ rise in electronegative impurity concentration.

Additional work for SABRE South and CYGNUS is also described, including the development of a trigger system for SABRE South and a gas system for CYGNUS. The latter is capable of supplying gas mixtures at controlled pressures and has been designed with expansion capabilities for radon and other impurity filtration, paving the way for future large scale gaseous TPCs.