Speaker
Description
The AMoRE-I experiment is the first stage of the AMoRE (Advanced Molybdenum based Rare process Experiment) project intended to search for neutrinoless double beta (0$\nu$$\beta$$\beta$) decay of $^{100}$Mo with CaMoO$_{4}$ scintillation crystals at milli-Kelvin. The $^{100}$Mo has advantages of the relatively high released energy (Q$_{\beta\beta}$ = 3034 keV) and theoretically predicted decay probability. The calcium molybdate (CaMoO$_{4}$) is the brightest one among molybdate scintillation crystals. The experimental sensitivity to the 0$\nu$$\beta$$\beta$ decay will be increased by an enrichment of $^{100}$Mo. The calcium depleted in $^{48}$Ca is used for the CaMoO$_{4}$ crystal production to decrease background caused by the double beta decay of $^{48}$Ca. $^{48depl}$Ca$^{100}$MoO$_{4}$ crystals grown by the FOMOS-Materials in Russia of weight around 5 kg (~2.5 kg of $^{100}$Mo) will be installed in the low radioactive cryostat at the Yang Yang underground laboratory (Y2L) in Korea and cooled to milli-Kelvin to enhance the detection performance of the AMoRE-I experiment. The Y2L is located at 700 m under the surface which is 2000 m-of-water-equivalent deep underground providing a shield against cosmic rays. A goal of AMoRE is an exploration of the inverted scheme of the neutrino mass hierarchy which corresponds to the half-lives of 10$^{26}$ – 10$^{27}$ years. Therefore, the experiment requires a very low radioactive background environment. We have set upper limits of internal radioactivity levels for $^{228}$Th, $^{226}$Ra, $^{227}$Ac (daughters of $^{232}$Th, $^{238}$U and $^{235}$U), and the total alpha activity of U/Th in the crystal scintillators via a GEANT4 Monte-Carlo simulation. As a preceding experiment of the AMoRE-I, we measured internal radioactive contamination of the $^{48depl}$Ca$^{100}$MoO$_{4}$ crystals by $^{228}$Th, $^{226}$Ra and $^{227}$Ac using a delayed-coincidence analysis. The approach allows us to separate the Bi-Po decay sequences in the chains thanks to the relatively short half-lives of polonium radionuclides in the decay chains (= 164 s, = 1.78 ms, = 145 ms). Two unexpected alpha peaks (at 660 keV and 790 keV in an energy scale calibrated with gamma quanta) observed in one of the crystals are identified by using the SRIM simulation code and compared with the previous result of $^{48depl}$Ca$^{100}$MoO$_{4}$ scintillation crystal radioactivity. We also measured optical transmittances, relative light yields, and energy resolutions of the $^{48depl}$Ca$^{100}$MoO$_{4}$ scintillators to be used for the AMoRE-I experiment.
