Fall 2008

Progress with Daya Bay

Daya Bay Tank drawing

The Daya Bay experiment is being constructed by a collaboration of scientists from the United States, the Republic of China, and Europe. The experiment will measure how anti-neutrinos change from one flavor to another (electron neutrinos into muon or tau neutrinos). Three underground experimental halls are being constructed in a mountain near a nuclear power plant complex that supplies power to Hong Kong and Shenzhen. Each hall will have a water pool with instrumentation to detect and cancel cosmic background radiation from the sun, atmosphere, and surrounding rock. Eight anti-neutrino detectors (large tank with transparent vessels filled with special liquids and instrumentation) will be installed into these pools. The number measured in the two halls nearest to the reactors will be compared to the number of neutrinos measured in the hall about 2km away to determine how many neutrinos are “disappearing” or changing. This measurement will allow the collaboration to determine the last unknown neutrino mixing angle θ13 and make progress towards understanding the predominance of matter in the Universe.

PSL and the UW team have primary responsibility for the design and fabrication of the anti-neutrino detectors (AD). The UW team led by Assistant Professor Karsten Heeger coordinates the activities of engineers and scientists at several universities and collaborates closely with Berkeley, Brookhaven National Laboratory and our Chinese colleagues at the Institute of High Energy Physics in Beijing. The unique engineering resources of PSL allow the UW to make the single largest university contribution to this project.

Daya Bay Tank Leak Test

PSL has a broad range of project responsibilities. Key to the AD detectors is the construction of eight pairs of thin-walled, nested acrylic vessels that allow the detection of the faint light generated in the anti-neutrino interaction with the detector liquids. Lee Greenler is responsible for design and analysis of the thin-walled vessel. A prototype vessel was fabricated this past year at Reynolds Polymer of Colorado. It has proved to be a very challenging job. Late in October Lee was there for a dimensional and leak check of the prototype. It was accurately built and when the lid is tightened down it sealed perfectly. A contract has been placed for the fabrication of the eight production tanks; the first two are being built now. The 3m acrylic vessel to nest inside the 4m vessel is being built in Taiwan to the PSL design. The 3m inner vessel will be filled with the “target,” a special Gadolinium doped liquid scintillator. The space between the 4m and 3m will be filled with undoped liquid scintillator, and the outer space will be filled with mineral oil.

To meet the delicate structural constraints of this nested vessel system the liquids must be introduced into the various detector volumes at the right rate so they are always at about the same level on both sides of the thin walls. Dan Wenman has been working with Tom Wise on designing a filling system that carefully controls this process and will measure the target mass to an unprecedented precision of <0.1%.

There are eight “dry boxes” on each AD to allow passing cables carrying high voltage and signal to the PMTs inside the tank. Our shop has made over 1600 feedthrough plugs and shipped them to Hamamatsu for attachment to the PMT cables. Interface rings for mounting the dry boxes to the tanks were also manufactured at PSL and shipped to China to be welded into the outer stainless steel vessel. Jeff Cherwinka has been working on the dry box design and visited the plant as part of an international team to survey the progress and coordinate interfaces. Dan Wahl has been working on the cables and connectors.

Drafter Amy Pagac maintains a 3D Solidworks model of the AD including parts designed at PSL, LBL, CalTech, and IHEP. This serves as the reference model for the project. Currently she is doing layouts of cable runs for the near and far halls where the detectors will be located. Amy is also working with Dan Wenman on creating parts and layouts for the pumping station which will be used for filling the detector vessels with their respective liquids.

With his extensive experience from IceCube, Jeff Cherwinka has served as the deputy manager of the AD system and is leading the planning for its assembly and installation.

Civil construction of the tunnels and the surface facilities is progressing. The surface assembly building will be done by the end of 2008 so that the prototype 4m vessel can be shipped there and an assembly test can be performed with the first 5m stainless steel tank and a prototype 3m acrylic vessel. The assembly and filling of the first pair of detectors is scheduled for completion in late 2009.

 

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