Zone Refining of Ultrahigh-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 Rb -- two elements with radioisotopes can cause scintillation backgrounds be efficiently removed by refining. However, never demonstrated purity NaI which became commercially available recently. this article, we show many common metallic impurities via A numerical model predicting the final impurity distribution was developed used fit ICP-MS measurement data determine segregation coefficient initial concentration. Under scheme, estimated 0.57, indicating still effective removing from As tends move one end, concentrations too low measured directly unprocessed powder potentially detected end due enrichment. We also present analysis technique estimate of partial data, effectively enhances sensitivity spectrometer. Using technique, concentration $^{85}$Rb between 5 ppt 14 at 90% CL, least times lower than detection limit 7 current most stringent set DAMA collaboration counting radioactive $^{87}$Rb. These results imply key developing next-generation, NaI-based crystal scintillators detection.