Application of zone refining to the development of NaI(Tl) detectors for SABRE North

TL;DR

This study demonstrates the scalable purification of NaI powder for SABRE North via zone refining, supported by a physics-informed mathematical model and extensive experimental runs. By coupling zone-refining dynamics with normal freezing, the authors extract contaminant-specific distribution coefficients $k$ at ppb levels, notably obtaining $k_{\rm K}=0.63^{+0.03}_{-0.09}$, which translates to roughly an 80% reduction of initial potassium contamination. The integrated analysis across commissioning, production, and crystal growth indicates that zone refining, together with optimized handling and potential chemical pre-purification, can meet or approach the target background of $\lesssim1$ dru in the 1–6 keV ROI, potentially bringing background levels in SABRE North to the levels expected for an active veto setup. The work also provides a framework for predicting background contributions and guiding purification strategies for ultra-high-purity NaI(Tl) crystals in rare-event searches.

Abstract

The SABRE North experiment is developing ultra-high radiopurity NaI(Tl) detectors to investigate dark matter. To achieve this, SABRE North utilizes the technique called zone refining for NaI powder purification. This work details the mathematical model developed to describe the purification process. By comparing this model to the results of the commissioning and production runs conducted prior to crystal growth, the distribution coefficients were determined for various impurities, contained in the powder at the parts-per-billion (ppb) level. Furthermore, the synthesis of data from both zone refining and normal freezing is discussed. These findings can be used to predict the SABRE North detectors background level in the energy region-of-interest for dark matter search and to optimize the production of ultra-high purity crystals through multiple purification strategies.

Figures (15)

• Figure 1: Working principle of zone refining. The blue zones are solid while the red zone is liquid. The dots show the contaminant concentration in the different zones. $C_L$ and $C_S$ are the concentrations in the liquid and solid, respectively. The molten zone is moving from left to right.
• Figure 2: Distribution of impurities for $k=0.5$ and $L/w=5$ as function of position $x$ normalized to ingot length $w$ for 1, 5, 10, 15, 20 passes. The effect of including a cone tip with $X_C/w=0.3$ are shown (dashed lines)
• Figure 3: Like Fig. \ref{['fig:distribution_k05_short_ampoule']} but with $L/w=10$ and for 1, 10, 20, 25 and 40 passes (notice the different scale on y-axes).
• Figure 4: Average impurity concentration as function of number of passes for $L/w=5$ and $10$ and for $X_{\rm cut}/L=0.6$, $0.7$ and $0.8$.
• Figure 5: Average impurity concentration as function of length of the ingot for $X_{\rm cut}/L=0.6$, $0.7$ and $0.8$ and for 24 passes.