Status of the Daya Bay Reactor Neutrino Experiment

Meng Wang, for the Daya Bay Collaboration

School of Physics, Shandong University, Shanda Nanlu 27, Jinan, 250100, China

Abstract

The Daya Bay reactor neutrino experiment is aimed at a measurement of the neutrino mixing angle θ13 with asensitivity of 0.01 in sin2 2θ13 at the 90% confidence level. The experiment design takes two short baselines andone long baseline positioned with identical detectors to measure the relative rates and energy spectra of electronantineutrinos. Civil and detector construction is ongoing. Data taking will begin in Autumn 2012 and last for threeyears.

Keywords: Daya Bay, neutrino mixing, θ13

1. Introduction

The Daya Bay experiment will perform a precisionmeasurement of the last unknown neutrino mixing angleθ13 using reactor antineutrinos at Daya Bay, Shenzhen,China. The value of θ13 is vital for future measurementsof CP violation in the lepton sector. A reactor-baseddetermination of this angle will provide important andcomplementary information to that from long-baselineaccelerator-based experiments. With an approach ofmeasuring the relative rates and energy spectra of re-actor antineutrinos, the Daya Bay experiment can deter-mine θ13 with a sensitivity of 0.01 or better in sin2 2θ13at the 90% confidence level.

The Daya Bay nuclear power complex is one of themost prolific sources of electron antineutrinos in theworld. There are two pairs of reactor cores (Daya Bayand Ling Ao, separated by about 1.1 km) generating11.6 GWth of thermal power. A third pair of reactorcores (Ling Ao II) is in commissioning and will be inoperation at the beginning of 2011. By then the totalthermal power will increase to 17.4 GWth.

The location of the power complex is adjacent tomountainous terrain, ideal for building undergroundlaboratories with tunnel access and providing sufficientoverburden to suppress cosmogenic backgrounds.

This paper reports the status of the Daya Bay experi-

ment. Much progress has been made since the talk wasgiven in June. For consistence, however, the proceed-ings summarizes mainly the progress up to June. A fewposters on Daya Bay were presented at this conferenceas well and the corresponding proceedings can be re-ferred to for more information.

2. Experimental design

The detail of the Daya Bay experimental design canbe found elsewhere [1]. A brief description is providedhere.

Table 1: The Daya Bay baselines and overburden of three experimen-tal sites.

Baseline (m) DYB site LA site far siteoverburden 98 112 355DYB cores 363 1347 1985LA cores 857 481 1618LA-II cores 1307 526 1613

The basic layout consists of three underground ex-perimental halls, one far and two near, accessed by hor-izontal tunnels as shown in Figure 1. The baselines andoverburden, listed in Table 1, are optimized for excellentsensitivity in sin2 2θ13. Figure 2 depicts the detector sys-tem at the far site. Modularized identical antineutrino

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detectors (ADs) are deployed, four at the far site andtwo at each near site. ADs are immersed in a water poolwhich, equipped with photomultiplier tubes (PMTs) asa water Cerenkov detector, provides shielding to atten-uate ambient neutrons and gammas. The water pool iscovered with resistive plate chambers (RPCs) to detectcosmic-ray muons. Both the water pool and the RPCdetector constitute a veto system with a combined effi-ciency of better than (99.50 ± 0.25)%.

Daya Baycores

Ling Aocores

Ling Ao IIcores

Daya Baynear

Ling Ao near

Far

LShall

Entrance

Construction tunnel

Waterhall

Figure 1: The Daya Bay experimental layout.

Figure 2: Baseline configuration at the far site. The RPC in green willbe on top of the antineutrino detectors during data taking.

The Daya Bay antineutrino detector module is athree-zone cylindrical detector made of a stainless steelvessel (SSV) and two acrylic vessels (AV), as shown inFigure 3. The inner AV holds 20 ton gadolinium dopedliquid scintillator (Gd-LS) to detect antineutrinos via theinverse β-decay reaction with the target volume definedby the physical dimensions of the central region. Theouter AV is filled with 20 ton undoped liquid scintilla-tor (LS) to catch γ rays escaping from the central re-gion. The SSV, arranged with 192 8-inch PMTs alongthe inner circumferential surface, is filled with mineraloil (40 ton) to separate the PMTs from the LS for sup-pressing natural radioactivity from the PMT glass and

other external sources. Immediately outside of the topand bottom of the outer AV, two reflectors are mountedrespectively to increase the photon collection efficiency.With this design, the energy resolution is about 12% at1 MeV. In order to verify the design principles and studythe detail of the detector response, a two-zone prototypedetector with a scale of 1/3 in dimensions and some sim-plification has been constructed at the Institute of HighEnergy Physics (IHEP), Beijing. A variety of studieshave been carried out and results [2] confirmed the ex-pectations including the performance of the optical re-flectors, energy resolution and calibration, Monte Carlosimulation, and the stability of Gd-LS.

Figure 3: Schematic drawing of the Daya Bay antineutrino detectormodule.

The Daya Bay expected systematic uncertainties aresummarized in Table 2 as well as signal and backgroundrates in Table 3. With three years of data taking, the sen-sitivity of Daya Bay for sin2 2θ13 will be better than 0.01within the allow range of Δm2

31 as shown in Figure 4.

Table 2: Summary of Daya Bay uncertainties.Source UncertaintyReactor Power 0.087% (4 cores)

0.13% (6 cores)Detector (per module) 0.38% (baseline)

0.18% (goal)Signal Statistics 0.2%

3. Civil and detector construction

The Daya Bay civil construction started on 13 Oc-tober 2007. The above-ground facilities, including a

M. Wang / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 92–96 93

Table 3: Summary of Daya Bay signal and background rates for an antineutrino detector module at three experimental sites.Daya Bay Near Ling Ao Near Far Hall

Baseline (m) 363 481 from Ling Ao 1985 from Daya Bay526 from Ling Ao II 1615 from Ling Ao

Overburden (m) 98 112 350Radioactivity (Hz) <50 <50 <50Muon rate (Hz) 36 22 1.2Antineutrino Signal (events/day) 840 740 90Accidental Background/Signal (%) <0.2 <0.2 <0.1Fast neutron Background/Signal (%) 0.1 0.1 0.18He+9Li Background/Signal (%) 0.3 0.2 0.2

Figure 4: The Daya Bay sensitivity curve with three years of data.

surface assembly building and control rooms have beenconstructed and in use. As depicted in Figure 1, the totallength of the completed Daya Bay tunnel is 3.2 km. Inaddition to the three experimental halls, there are two fa-cility halls underground, one for Gd-LS production (LShall) and another for water purification and circulation(water hall). The Daya Bay near hall and the LS hallare in preparation for beneficial occupancy. The exca-vation of the Ling Ao near hall and the water hall hasbeen finished. The excavation of the far site hall is inprogress.

The production of the AD main components is alsomaking good progress, as listed below:

• 5 stainless steel vessels (SSVs) have been pro-duced and 3 others will be ready this year;

• all 8 AD support platforms are ready;

• 2 outer acrylic vessels are ready on-site and 6 oth-ers in production;

• 4 inner acrylic vessels are produced and 4 others inproduction;

• all PMTs have arrived and passed check.

Using reflectors in AD is an innovation in the detectordesign for having advantages of increasing the photoncollection and improving the uniformity of energy re-sponse in vertical. It is made of high reflectivity filmsandwiched between two 10 mm thick acrylic panels of4.2 m diameter. Great effort has been devoted to studythe performance with the prototype detector and simula-tion as well as the procedure of construction. Figure 5,for instance, shows the comparison of the AD energyresolution with and without reflectors via Monte Carlosimulation. All 8 pairs of reflectors have been producedand passed quality control.

Figure 5: Comparison of the AD energy resolution with and withoutreflectors via Monte Carlo simulation.

There are a great number of interfaces, o-rings andgaskets for each AD module. Hence its assembly iscomplicated and needs great care. In addition, leak-check is critical because different liquid will be filledin three zones. By the time this paper is written, two

M. Wang / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 92–9694

(a) (b)

Figure 6: An AD in assembly: (a) inserting a reflector to bottom; (b)lifting a PMT ladder for installation.

ADs have been assembled and passed the leak-checkand tested with dry run (to be discussed later). Figure 6shows an AD in the procedure of assembly.

Figure 7: The absorption of Gd-LS at 430 nm as a function of time for900 days.

Daya Bay will use 185 tons 0.1% Gd-doped LS withthe formula of

Gd-TMHA+LAB+PPO (3 g/L)+bis-MSB (15 mg/L),

where TMHA is trimenthylhexanoic acid, LAB is linearalkyl benzene, PPO is diphenyloxazole and bis-MSB isbis-methylstyryl benzene. The stability of the Gd-LShas been tested with the prototype detector since Jan-uary 2007 and no degradation is found, as shown inFigure 7. The device and procedure for massive pro-duction has been designed, made and tested. A batch of4 ton Gd-LS was produced successfully in March 2009.All Gd-LS will be produced in multiple 4-ton batchesand mixed in a reservoir on-site to ensure functionallyidentical ADs. The Gd-LS and undoped LS will be pro-duced and stored in a underground hall. 400 ton LABare now stored in two reservoirs; 1.4 ton PPO purifica-tion has been finished; other materials are also ready.

The water Cerenkov detector will make use of wa-terproof 8-inch PMTs. The potting and production hasbeen finished. Pressure test under water is in progress.

Figure 8: A schematic view of the water pool with ADs in far hall. Thewater pool is divided into two zones via Unistrut frames and reflectiveTyvek panels (unseen on the picture).

As shown in Figure 8, a water pool is divided into theinner and outer regions with Unistrut frames and reflec-tive Tyvek panels. After initial purification, pure waterwill take continuous circulation with speed of each poolper 10 days. The purification and circulation system hasfinished design and contract and is ready for installation.A water-pool prototype is also built in order to verifythe design principle and test material compatibility. Thestudy confirms the design and material selection [3].

To cover the near-/far-hall water pools with respectiveareas of 18 × 12 m2 and 18 × 18 m2, 189 RPC modules(about 2 × 2 m2) are needed. Each module consists of 4layers of two chambers (2.1 × 1.1 m2 and 2.1 × 1.0 m2

respectively). In total 1600 RPC bare chambers havebeen produced and most of modules have been assem-bled. Efficiencies of all tested modules meet require-ment (> 95%), as shown in Figure 9. The RPC supportstructures for Daya Bay near hall have been producedand tested as well.

Figure 9: Efficiency as a function of high voltage for three RPC mod-ules.

The PMT front-end electronics, trigger electronics,data acquisition system (DAQ) and detector control sys-

M. Wang / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 92–96 95

tem (DCS) for one AD has been prepared and tested.RPC electronics for one near hall are also producedand are being tested. The computer system for DayaBay near hall data taking has been deployed on-site.The Daya Bay computing model requires real-time datatransfer from Daya Bay to IHEP in Beijing and then toLBNL in the US and a dedicated networking system, asshown in Figure 10, for the purpose has been set up andin use.

Figure 10: The Daya Bay computing model.

Daya Bay dry run is a full chain test consisting of anassembled AD, automatic calibration units (ACUs) de-ployed with LEDs, PMT front-end electronics and trig-ger electronics, DAQ, DCS, off-line system on-site andoff-site including automated data transfer and physicsquality monitor as well as data analysis. By mid-October this year, a few dry runs have been performedwith the first and second assembled ADs. Many testsand data analyses were carried out, such as the compar-ison of the Monte Carlo simulation with data, calibra-tion of PMT parameters, validation of the identicalnessof ADs and uniformity of their energy response, and soon. Figure 11 shows some results as examples.

(a) (b)

Figure 11: Examples of data analysis for dry run: (a) the comparisonof cosmic muon event rates as the function of trigger thresholds be-tween data and simulation; (b) charge distributions of LED events andcosmic muon events.

4. Summary

The goal of the Daya Bay experiment is to determinethe value of sin2 2θ13 with a sensitivity better than 0.01

at the 90% confidence level. The collaboration consistsof about 240 collaborators from 38 institutions in sixcountries and regions. With such international effort,we are making good progress in both civil constructionand detector construction. A few full chain tests of ADswithout liquid filled have been performed successfully.Daya Bay is now preparing for commissioning. TheDaya Bay near hall physics ready is expected in Spring2011 and all near and far halls physics ready in Autumn2012.

Acknowledgments

The Daya Bay experiment is supported in part bythe Ministry of Science and Technology of China(contract no. 2006CB808100), the Chinese Academyof Sciences, the National Natural Science Foundationof China (Project number 10890090), the Guangdongprovincial government, the Shenzhen Municipal gov-ernment, the China Guangdong Nuclear Power Group,the Research Grants Council of the Hong Kong Spe-cial Administrative Region of China (Project numbers400805, 703307, 704007 and 2300017), the focusedinvestment scheme of CUHK and University Devel-opment Fund of The University of Hong Kong, theMOE program for Research of Excellence at NationalTaiwan University and NSC fund support, the UnitedStates Department of Energy (DE-AC02-98CH10886,DE-AS02-98CH1-886, DE-FG02-92ER40709, DE-FG02-07ER41518, DE-FG02-91ER40671, DE-FG02-08ER41575, DE-FG02-88ER40397 and DE-FG02-95ER40896), the U.S. National Science Foundation(Grants PHY-0653013, PHY-0650979, PHY-0555674and NSF03-54951), the Alfred P. Sloan Foundation,the Ministry of Education, Youth and Sports of theCzech Republic (Project numbers MSM0021620859and ME08076), the Czech Science Foundation (Projectnumber GACR202/08/0760), and the Joint Institute ofNuclear Research in Dubna, Russia. The speaker ac-knowledges the financial support of Taishan Scholarshipby the Shandong provincial government.

References

[1] X.-H. Guo, et al., Daya Bay proposal – A precision measurementof the neutrino mixing angle θ13 using reactor antineutrinos atDaya Bay, 2006. hep-ex/0701029.

[2] Z.-M. Wang, et al., Study of a prototype detector for the Daya Bayneutrino experiment, Nucl. Instrum. Meth. A602 (2009) 489–493.

[3] H.-Q. Lu, et al., Circulation model for water circulation and pu-rification in a water Cerenkov detector, Chinese Physics C33(2009) 567–571.

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