| After successfully measuring the 913 in the Daya Bay neutrino experiment,only the phase angle remains unknown among the six parameters describing the neutrino oscillation.The Jiangmen Neutrino Experiment(JUNO)takes the accurate measurement of mass hierarchy as the main physical target,which is a necessary condition for measuring the phase angle.The JUNO experiment will be built in Jiangmen City,Guangdong Province.The Yangjiang and the Taishan Nuclear Power Plants are used as neutrino sources.The center detector is a large spherical detector with a diameter of 35.4 meters.It will be filled into 20,000 tons of liquid scintillator and equipped with 18,000 new 20-inch MCP-PMT photomultiplier tubes on the surface of the sphere.In order to achieve the strict index of the 3%energy resolution set by the JUNO,extremely high requirements have been put forward for detection performance of the new photomultiplier tubes and optical transparency of the liquid scintillators.Linear alkylbenzene(LAB)has been selected by JUNO as the organic solvent for liquid scintillator due to its high light yield,good optical performance,stable chemical structure,and being friendly to environment and human body.After doped with a certain concentration of PPO as fluorescent and bis-MSB as wavelength shifter,it becomes the liquid scintillator.To match the large sphere size of JUNO center detector,the LAB is required to have an unprecedented attenuation length of 30 meters.However,due to the limitations of industrial fabrication processes and manufacturing engineering technology,commercial LABs produced through industrialization are far from meeting this requirement.We have been working with LAB's domestic manufacturer,the Nanjing Alkylbenzene Plant of Jinling Petrochemical Corporation.Through nearly 10 years of efforts,we have gradually increased the attenuation length of the industrially produced LAB from about 10 meters for the Daya Bay neutrino experiment to the current length of 25 meters.For achieving the ultimate goal of 30-meters attenuation length of LAB,on the one hand,we need to continuously explore and improve the new special production processes of Linear alkylbenzene,and carry out a large number of physical and chemical properties measurements and in-depth understanding of LAB samples to obtain high-quality LAB samples with better optical performance;on the other hand,we have to make efforts in the measurement methods,which will further improve the stability and accuracy of the LAB samples' measurements.We have continuously upgraded and reconstructed the attenuation length measurement systems originally built by the laboratory.In the actual measurement experiments,we used the commercially available high-purity purified water as the sample to debug the measurement device,and performed strict calibration to keep the optical attenuation length measurement device stable,reliable,and in good operating condition.At the same time,we tried to install different filters based on the original LED incident light path.We focused on measuring and analyzing the LED spectra before and after the installation of these filters and their actual measurement results.It was shown that after adopting LED incident light with better monochromaticity and better light transmittance,there are obvious changes in the measurement of attenuation length of different LAB samples;we also use this method for various LAB/LS(liquid scintillator)samples and found some new experimental phenomena such as the complexity of impurities components in LAB.For example,this complexity may lead to inconsistencies in the degree of improvement in the measurement of different LAB samples by the filter.In the past few years,we have also made some meaningful explorations in the full waveform data acquisition(DAQ)of the photomultiplier tube output signal.We hope to use the full waveform digital sampling module(V1729)to eventually replace the existing QDC(V965)module for further improvement of the accuracy of data sampling and fitting.We have given the main sampling program of V1729,and pointed out some key issues that have yet to be further solved,laying the foundation for the next practical application.On this basis,we also discussed the use of multi-component Beer-Lambert theorem to estimate the influence of organic impurity concentration in LAB samples,hoping to provide a certain reference for the refined measurement of chemical components of high-quality LAB,preparation of high-quality liquid scintillator samples and some relative aspects in the future.This article describes the above-mentioned new experimental trials we have conducted in detail.We have thoroughly analyzed the latest results of the attenuation length measurements of LAB/LS,and discussed the issues that need to be paid attention during the experimental process and the next steps for improvement. |
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