CUP Center for Underground Physics

4K setup

4K test setup at CUP (IBS)

The Center for Underground Physics (CUP) has a series of projects running or being planned for searching dark matter and neutrino-less double beta decay at down-to cryogenic temperatures (~10 mK). Among them, two major experiments are AMoRE (Advanced Mo-based Rare process Experiment) and COSINE-LT (COSINE at Low Temperature searching for dark matter). The AMoRE has been using ¹⁰⁰Mo based CaMoO₄ crystals with a mass of ~6 kg at currently running experiment setup, called AMoRE-Pilot, to understand its cryogenic detectors and background levels from outside and inside of the detectors. The project is preparing an upgraded experiment by increasing total mass of crystals and reducing the background levels. But the final crystal is not decided yet for a relatively huge mass of ~200 kg. For R&D purposes, it is necessary to study different crystals’ properties at room and low temperatures. Most of Mo based crystals have very low or no light yield at room temperature due to its high thermal quenching behavior. But studying crystals’ properties at mK temperatures has challenges such as time (several days to weeks to prepare one crystal sample into a detector) and a rather complicated test setup. Also the COSINE-LT is being planned to run its experiment in low temperatures to reduce thermal noises in electronics and to have more light yields. From other experimental results at low temperatures for a NaI(Tl) crystal, light quenching also observed to occur in different temperatures. In order to optimize the quenching temperature for the NaI(Tl) crystal, it is necessary to study the crystal’s properties at low temperatures.

With these aims, the CUP has established a 4K test setup. For cooling down a crystal sample, liquid Helium is used as a coolant in a compressor. For studying luminescence properties, several excitation sources are available such as 266 nm UV Laser, 280 nm UV LED, 260 nm UV LED. Moreover, scintillation characterization of the crystals can be studied with several radioactive sources. A few pictorial views of the setup are shown in Fig. 1. A specification of the CUP 4K setup is also summarized in Table 1.

- Figure 1. 4 K test setup at CUP, Cryostat (left) and DAQ (right).

- Table 1. Summary of the 4 K test setup specification at the CUP.

Here are brief explanations on scintillation and quenching which were mentioned in the above. Scintillators convert a fraction of energy lost by moving charged particles in the form of photons. Conversion of incident radiation energy to photons involves many subsequent processes which may reduce the scintillator’s efficiency. Quenching refers to non-radiative energy transfer process, which decreases light yield of the material. Among several quenching mechanism, main is thermal quenching. Thermal quenching is a reduction in efficiency of scintillation as temperature increases due to a competition of non-radiative relaxation process with radiative process.

We have some results with this experimental setup for CaMoO₄ crystals which are grown by CUP and NIIC (Nikolaev Institute of Inorganic Chemistry SBRAS, Russia) and the results are presented in Figs. 2 and 3. From the results, we concluded that it works quite consistently.

- Figure 2. Luminescence spectra measurements of a CaMoO₄ (NIIC) crystal in a temperature range of 7 - 300 K with a 260nm LED (left). Normalized peak intensities of a CaMoO₄ crystal measured with two different energy LED sources. (right).

- Figure 3. Peak intensities of a CaMoO₄ (CUP) crystal measured with a 280 nm LED source in different days to see the stability of the measurements in the setup.

We also have the same low temperature setup at Kyungpook National University (KNU) (Prof. HongJoo Kim, a collaborator member of the CUP), but it can reach down to only 10 K. Recently we have published several results based on crystals characterized in that setup [1,2,3,4].

Reference
1. Indra Raj Pandey, H.J. Kim, Y.D. Kim, J Cryst Growth. 480, 62 (2017).
2. Indra Raj Pandey, Sujita Karki, H. J. Kim, Y. D. Kim, M. H. Lee, and N. V. Ivannikova, IEEE Trans. Nucl. Sci. 65, 2125 (2018).
3. Indra Raj Pandey, H.J. Kim, H.S. Lee, Y.D. Kim, M.H. Lee, V.D. Grigorieva, V.N. Shlegel, Eur. Phys. J C 78, 973 (2018)
4. J. K. Son, Indra Raj Pandey, H. J. Kim, Y. D. Kim and M. H. Lee, IEEE Trans. Nucl. Sci. 65, 2120 (2018)