Development of highly radiopure NaI(Tl) scintillator for PICOLON dark matter search project

Abstract

The highly radiopure NaI(Tl) was developed to search for particle candidates of dark matter. The optimized methods were combined to reduce various radioactive impurities. The 40K was effectively reduced by the re-crystallization method. The progenies of the decay chains of uranium and thorium were reduced by appropriate resins. The concentration of natural potassium in NaI(Tl) crystal was reduced down to 20 ppb. Concentrations of alpha-ray emitters were successfully reduced by appropriate selection of resin. The present concentration of thorium series and 226Ra were 1:2 ± 1:4 μBq/kg and 13 ± 4 μBq/kg, respectively. No significant excess in the concentration of 210Pb was obtained, and the upper limit was 5.7 μBq/kg at 90% C. L. The achieved level of radiopurity of NaI(Tl) crystals makes construction of a dark matter detector possible.

Scintillation characteristics of a NaI(Tl) crystal at low-temperature with silicon photomultiplier

“…In particular, a direct detection of dark matter particles using NaI(Tl) crystal detectors is currently being conducted in several experiments [8][9][10][11]. The low-background NaI(Tl) crystals have been developed for a success of the dark matter detection at low energies by reducing internal radioactive contaminants in the crystal [12][13][14][15].…”

Snowmass2021 Cosmic Frontier White Paper: Puzzling Excesses in Dark Matter Searches and How to Resolve Them

“…Currently, two experiments, ANAIS at Canfranc Underground laboratory in Spain and COSINE-100 at Yangyang Underground laboratory in South Korea are running with Thallium-doped NaI detector as the target media and published their physics results [246][247][248][249][250]. Other experiments including COSINE-200, SABRE, PICOLON are actively working on growing ultra-low background NaI(Tl) crystals to improve the experimental sensitivity [251][252][253], while COSINUS uses NaI as scintillating bolometers to achieve event discrimination [254].…”

“…The PICOLON (Pure Inorganic Crystal Observatory for LOw-energy Neutr(al)ino) Collaboration has also focused its R&D efforts on growing ultra-pure NaI crystals. It has successfully used a combination of recrystallization and ion exchange resins to reduce the 40 K, 210 Pb, and 226 Ra backgrounds [253]. The recent result of 210 Pb in the NaI(Tl) is less than 5.7 µBq/kg.…”

The Role of Small Scale Experiments in the Direct Detection of Dark Matter

“…The PICOLON (Pure Inorganic Crystal Observatory for LOw-energy Neutr(al)ino) project, placed at Kamioka, is also working on the development of NaI(Tl) detectors with ultra-high purity following recristallization and using appropriate resins, in order to reduce particularly 210 Pb and 40 K, and satisfactory tests with small crystals have been made [77,78]. A first phase using four modules of NaI(Tl) with a total mass of 23.4 kg could be prepared in 2021 [78]. In a longer term and after completing various steps, the installation inside the KamLAND detector of a large mass (hundreds of kg) of NaI(Tl) is being considered.…”