A Mineral Oil Monitoring System for the Daya Bay Neutrino Experiment.

摘要

The Daya Bay reactor neutrino experiment aims at measuring the neutrino mixing angle theta13 with a sensitivity of 0.01 or better in sin2 2theta13 at the 90% confidence level, through a measurement of the relative rates and energy spectra of reactor anti-neutrinos at different baselines.;There are eight anti-neutrino detectors (AD) deployed in three experimental sites. The AD has a three-zone cylindrical structure. Two acrylic vessels with diameter of 3 m and 4 m, and height of 3 m and 4 m respectively, are nested inside a 5m-diameter stainless steel vessel (SSV). The inner most volume, confined by the 3m-diameter inner acrylic vessel (IAV), is filled with Gadolinium doped liquid scintillator (GdLS), which acts as the neutrino target. The medium volume between the IAV and the 4m-outer acrylic vessel (OAV), is filled with normal liquid scintillator (LS) to capture gamma particles emitted from the target. The outer most volume is filled with transparent mineral oil (MO) which shields radiations from the steel or the 192 photo-multipliers (PMT) from entering the target. Since all the PMTs are mounted near the stainless steel wall of the SSV in the MO, the photons emitted by the liquid scintillator have to travel through MO before being detected by the PMT. It is crucial to monitor the optical properties of the MO for achieving the sensitivity of 0.01 in sin2 2theta13.;We have designed and developed an automatic system for monitoring the light attenuation in the MO. The system utilizes a high power LED to send light pulses into the AD through a 50 m optical fiber. With the stepping motor driven monochromator, we can select several wavelengths in one monitoring run. There is a corner cube retro-reflector at the bottom of the AD, which reflects the light back to the top of the AD, thus doubling the light path in the mineral oil to around 8 m. The reflected light is received by the 2" PMT of the MO monitoring system and digitized by a ash ADC. By comparing the reflected and reference signals of the LED pulses, we can monitor the attenuation in the MO.;I will discuss the detailed design and performance of this MO monitoring system and data taken in the AD calibration runs.