Production of ultra-low radioactivity NaI(Tl) crystals for Dark Matter detectors

Scintillating NaI(Tl) crystals are widely used in a large variety of experimental applications. However, for the use as Dark Matter (DM) detectors, such crystals demand a high level of radio-purity, not achievable by means of standard industrial techniques. One of the main difficulties comes from the presence of potassium that always accompanies sodium in alkali halides. On the other hand, the arguable DM detection by DAMA experiment using NaI(Tl) scintillating crystals requires a reliable verification able to either confirm the existence of DM or rule out the DAMA claim. Ultra-low radioactivity NaI(Tl) crystals, particularly with very low potassium content, are therefore indispensable to overcome the current stalemate in Dark Matter searches. Nonetheless, apart from DAMA-LIBRA experiments, to date, no other experiment has succeeded in building a detector from NaI(Tl) crystals with potassium content of ppb level. This work describes recent results in the preparation of ultra-radio-pure NaI(Tl) crystals using a modified Bridgman method. A double-walled platinum crucible technique has been designed and reliability tests show that 5 ppb of potassium in the NaI(Tl) crystals of 2 and 3 inches in diameter can be achieved starting from NaI powder with potassium content of the order of 10 ppb. The potassium excess is segregated in the tail-side of the as grown ingot where measured potassium concentration is above 20 ppb. The purifying effect of Bridgman growth for larger NaI(Tl) crystals is currently being tested. The work also reports on scintillation parameters of our NaI(Tl) crystals measured in a dedicated setup conceived for naked, hygroscopic crystals. The reproducible and reliable production of ultra-low radioactivity NaI(Tl) crystals reported in this work will hopefully spur the construction of new DM search experiments and, anyway, clarify the controversial DAMA-LIBRA results.