Radioactive Study On The Process Of Liquid Scintillation Purified By Al₂O₃

The main scientific target of JUNO(Jiangmen Underground Neutrino Observatory) is the quality order of reactor neutrinos and the oscillation of solar neutrinos. JUNO intends to use 20000-ton liquid scintillator as its detector. Since the counts of neutrino is rare, the liquid scintillation must be purified to achieve the requirements of neutrino detection. The preliminary design of purification system including two process: primary purification system and on-line purification system. The liquid scintillation purified by Al2O3 adsorption, water extraction, distillation and nitrogen stripping to improve its optical performance and reduce radioactive background. The Institute of High Energy Physics, Chinese academy of sciences found that Al2O3 adsorption can remove chemical impurities to improve optical performance of liquid scintillation. But Al2O3 has radionuclide, it may bring the problem of radioactive contamination of low background liquid scintillation. So it is must to be studied the content of radionuclides in Al2O3 and the radioactive change of liquid scintillation in the process of Al2O3 purification.The radioactive concentration of two kinds of Al2O3 samples had been measured, which is found that the radioactivity of Zibo Al2O3 is more than 104 Bq/kg and Guoyao Al2O3 is more than 61 Bq/kg. Comprehensive analysis of the results of liquid scintillator spectrometer, the anti-Compton gamma spectrometer, and LS time fit measurement, it can be inferred that there are 238 U, 234 U, 234 Pa, 234 Th, 235 U, 231 Th in the Al2O3 sample. 238 U, 234 Th, 234 Pa and 234 U reached equilibrium, the concentration of thorium series is low. The results of LS filtration experiment with Al2O3 column shows that contamination of liquid scintillation was reduced. And further reduction of contamination of LS was obtained from the similar filtration experiment in nitrogen environment instead of air. It was illustrated that the contamination of liquid scintillation may come from the Radon in air. Under the nitrogen environment, there was no significant difference in radioactivity between filtered LS and unfiltered LS in the range of standard deviation(0.01~0.1 cps/kg). In other words, the radioactive contamination of LS from Al2O3 is below 0.1 cps/kg and Al2O3 filtration has no significant radioactive purification effect. It suggests that the whole purification system must be isolated air.To achieve the desired background of liquid scintillation, the content of U and Th must be reduced to 10-15g/g which calculated about is 1×10-8 Bq/kg. The detection sensitivity of LS spectrometer can't reach the background level of purified liquid scintillation. If the study wants to distinguish the radioactive effects on liquid scintillation by Al2O3 filtration, it is need to research on large scale purification equipment and measure big volume liquid scintillation. So this paper tests have been able to give references about purification system of JUNO.