What is the scintillation crystal?
Scintillation is a process converting an ionizing radiation into detectable lights. It means that flashes of lights are produced in the scintillator by passages of particles such as alpha particles, electrons or high-energy photons. There are many different types of scintillation materials such as plastics, glasses, organic liquid or inorganic crystals.
Among them, inorganic scintillation crystals are used as core materials for detection and measurement instruments applied in various fields: non-destructive testing, medical imaging, dosimetry, homeland security, oil exploration or scientific research (particle detector). Each crystal of different types has different characteristics. Depending on detector or experimental goal, the crystals require to have high light outputs or specific elements.
The crystals to be used in the CUP experiments are ultra-pure scintillation crystals with ultra-low internal background. We are focusing on growth of ultra-pure crystals by optimizing growth conditions for each crystals.
Crystal growth method
Crystal growth method depends on characteristics of crystal materials under investigation such as melting point, volatility, solubility in water or organic solvents and so on. In gernal, based on a principle of melt and crystallization of raw materials, there are various crystal growth techniques such as Czochralski, Kyropoulos, Bridgman, and zone melting method.
This method is widely used for crystal growth. A seed for growth touches the melt and a crystallization starts by controlling speeds of rotation and pulling. The rotation and pulling rates are controlled depending on characteristics of raw materials and crystals, device, and growth environment.
Kyropoulos method is used to obtain a bulk crystal. A seed for growth touches the melt and a crystallization initiates by cooling the grower slowly. For this reason, it is required to have careful and continuous temperature controls.
Ref) Seevakan, K. and S. Bharanidharan. “DIFFERENT TYPES OF CRYSTAL GROWTH METHODS.”, Vol. 119, No. 12, 2018, 5743-5758.
Crystal growth at CUP
We have been working on growths of molybdenum-based CaMoO4 and Li2MoO4 using a Czochralski method for the AMoRE experiment and NaI:Tl crystals using an improved Kyropoulos method for the COSINE experiment.
Growth conditions of CaMoO4 and Li2MoO4 scrystals have been optimized and an ultra-pure crystal was grown through a double-crystallization process. Based on this performance, we have been studying intensively on growing of Li2100MoO4 crystals using 100Mo as an candidate isotope of neutrino-less double beta decay.
For growing NaI:Tl crystals, we have an R&D and a full size growers based on the Kyropoulos method. We are growing 1 kg NaI:Tl crystals to optimize growth conditions using the R&D grower. We plan to grow about 150 kg of NaI:Tl crystals using the full size grower in near future.