Zone refining of ultra-high purity sodium iodide for low-background detectors

There has been a growing interest in ultra-high purity, low-background NaI(Tl) crystals for dark matter direct searches. Past research indicates that zone refining is an efficient and scalable way to purify NaI. In particular, K and Rb -- two elements with radioisotopes that can cause scintillation backgrounds -- can be efficiently removed by zone refining. However, zone refining has never been demonstrated for ultra-high purity NaI which became commercially available recently. In this article, we show that many common metallic impurities can be efficiently removed via zone refining. A numerical model for predicting the final impurity distribution was developed and used to fit the ICP-MS measurement data to determine the segregation coefficient and the initial concentration. Under this scheme, the segregation coefficient for K is estimated to be 0.57, indicating that zone refining is still effective in removing K from ultra-high purity NaI. As zone refining tends to move the impurities to one end, elements with concentrations too low to be measured directly in the unprocessed powder can potentially be detected in the end due to the enrichment. We also present an analysis technique to estimate the initial concentrations of impurities with partial data, which effectively enhances the sensitivity of the spectrometer. Using this technique, the initial concentration of $^{85}$Rb is estimated to be between 5 ppt and 14 ppt at 90% CL, at least 14 times lower than the detection limit of ICP-MS and 7 times lower than the current most stringent limit set by the DAMA collaboration by direct counting of radioactive $^{87}$Rb. These results imply that zone refining is a key technique in developing next-generation, NaI-based crystal scintillators for dark matter direct detection.