A Precision Measurement of θ13 with Reactor

Antineutrinos at Daya Bay, China

- A Short Overview -

The Daya Bay Collaboration

May 25, 2005

1 Motivation

Recent discoveries have shown that neutrinos have mass and change flavor. Mixing amongthe mass states has been observed in atmospheric and solar neutrino experiments such asSuper-Kamiokande and SNO as well as in experiments using man-made neutrino sourcesincluding KamLAND and K2K. In the neutrino mixing matrix, all but two parametershave been measured: the smallest mixing angle, θ13, and the value of the CP violatingphase, δCP , are not yet known. To date, the most sensitive limit on θ13 is sin2 2θ13 <

0.17 for ∆m213 = 2.5 × 10−3 eV2 as measured by the CHOOZ [1] reactor anti-neutrino

disappearance experiment with a baseline of 1.05 km.Reactor anti-neutrino experiments have played a critical role in the 50-year long his-

tory of neutrinos, from the discovery of the anti-neutrino by Reines and Cowan to thefirst observation of reactor anti-neutrino disappearance at KamLAND. The importanceof reactor neutrino experiments continues to grow and the value of a measurement ofsin2 2θ13 to better than 0.01 (“a 1% measurement”) has been well documented [2]. Themagnitude of θ13 has implications for CP symmetry breaking in the lepton sector since allphysical effects of CP violation scale with sin θ13, and the discovery of non-zero θ13 willdefine the physics potential of future accelerator-based experiments. Observation of theoscillation probability of reactor anti-neutrinos over a baseline of ∼2 km allows the pre-cise determination of the θ13 mixing angle without ambiguities. The technical successesof both CHOOZ, and KamLAND provide the basis for a next-generation reactor neutrinoexperiment with significantly greater sensitivity to θ13.

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2 1 MOTIVATION

The goal of the Daya Bay [3] experiment is a measurement of sin2 2θ13 to 0.01 orbetter, an order of magnitude better sensitivity than the CHOOZ limit quoted above.The experiment measures the flux of anti-neutrinos from the Daya Bay and Ling Aoreactors via the inverse beta-decay reaction, and a deficit compared to the expected 1/L2

dependence is a signature for neutrino oscillations. Inverse beta-decay consists of thecapture of an electron anti-neutrino on a proton (hydrogen) resulting in the production ofa positron and a neutron (νe +p → n+e+). The number of inverse beta-decay reactions isdetermined by counting the coincidence of the energy deposited by the positron (1 MeV to8 MeV) followed by the energy release (∼8 MeV) from the neutron capture on gadolinium∼30 µs later. The most serious backgrounds are from cosmic rays.

The reactor complex near Daya Bay, China, about 70 km northeast of the HongKong airport, is an excellent site for the experiment, and one of only a few appropriatesites worldwide. Accomplishing the experiment requires powerful reactors in a geologicalsetting that allows for large underground neutrino detectors with significant overburden.Daya Bay is a nuclear power plant adjacent to high hills. The precision on sin2 2θ13 willbe improved over previous experiments by:

• Using a relative measurement of the neutrino interactions rates in detectors placedat appropriate distances to cancel common systematic uncertainties.

• Choosing optimal detector sites within the tunnel complex to maximize the sensi-tivity to sin2 2θ13. At Daya Bay we are planning to deploy multiple detectors at”near“, ”mid“, and ”far“ sites with distances of ∼0.5, 1, and 2 km from the reactorcores respectively.

• Using larger detectors to increase statistics.

• Locating the detectors deeper underground to suppress cosmogenic backgrounds.

• Using carefully designed shielding around the detectors that reduces backgroundsemanating from muon interactions in the surrounding rock, including an activeexternal muon tagging system around the detector

• Reducing the systematic error in the relative acceptance between the near, mid, andfar detectors by:

– Cross calibration with the same set of triggered and passive calibration sourcesand using the characteristic energy of the signal from the capture of the neutronfrom the inverse beta decay reaction.

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– Surrounding the central anti-neutrino target volume with a “gamma catcher,”improving the energy resolution uniformity and better defining the target mass.

– Using movable detectors that can be interchanged.

– Constructing the detectors at the near, mid, and far sites from multiple detectormodules of identical design.

An international collaboration between scientists from China, Russia, and the UnitedStates are conducting design and engineering studies for this project. The Chinese fundingagencies and the Daya Bay nuclear power plant operators have expressed their supportand are working with the Institute of High Energy Physics in Beijing (IHEP) to establishthe feasibility of the project. Geological exploration of the site and civil engineeringstudies for the Daya Bay project have begun. They are being conducted by the Institutefor Geology and Geophysics (IGG), Beijing. Detailed investigations of the site geology,including bore hole studies with participation of US geologists, engineers, and scientists,will be underway by June 2005.

2 Experimental Goal

The goal of the Daya Bay experiment is a measurement of sin2 2θ13 to 0.01 or better.Figure 1 shows the survival probability for electron anti-neutrinos emitted from a nuclearreactor as a function of distance from the power plant. The rapidly oscillating (red)curve is the survival probability for mono-energetic anti-neutrinos at the peak of detectedanti-neutrino energy (4 MeV), and the smoother (blue) curve accounts for the reactoranti-neutrino spectrum. The amplitude of the small oscillations corresponds to sin2 2θ13,taken from the current CHOOZ limit.

Accounting for the 1/L2 falloff of the flux, the optimal distance for the far detectoris the first maximum in the oscillation probability near 2 km. Measuring the amplitudeof the oscillation, and thereby the quantity sin2 2θ13 is the goal of the experiment. Ameasurement of sin2 2θ13 to 1% or better will require high statistical precision and goodmeasures to reduce systematic uncertainties. The stringent design criteria for the next-generation reactor experiment make it difficult to adapt previously used reactor sites tomeet the more demanding modern requirements. The presently developed experimentalplan aims at both measuring the rate deficit and exploiting the spectral distortion signalto determine sin2 2θ13.

4 2 EXPERIMENTAL GOAL

Distance (km)

Surv

ival P

robabili

ty

Daya Bay midnear far detector locations

Figure 1: Survival probability of reactor electron anti-neutrinos measured as a functionof distance from the reactor core. The rapidly oscillating (red) curve is for 4 MeV anti-neutrinos, and the smoother (blue) curve accounts for the reactor anti-neutrino energyspectrum. The amplitude of the small oscillations correspond to the CHOOZ limit onsin2 2θ13. The large oscillation amplitude corresponds to the dominant θ12 oscillationmeasured at KamLAND. The arrows refer to the locations of the detectors at the DayaBay site. The value of ∆m2

13 is taken as 2.5 × 10−3 eV2.

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3 Experimental Requirements

The need for high statistics demands powerful reactors and large detectors with targetmasses weighing tens of tons. Controlling systematic uncertainties will necessitate twoor more detector locations outfitted with detector modules of identical design to reducerelative errors in the flux measurements. Finally, the backgrounds must be understoodand reduced to the point where reliable subtraction can be accomplished. The originalCHOOZ experiment was able to measure the background directly because the experimentran nearly 3500 hrs with both the CHOOZ reactors off, compared to ∼4800 hrs [1] ofreactor-on time. It is unlikely that circumstances will allow for reactor-off running inany new experiment. Sufficient overburden is the most effective way for reducing thecosmic-ray induced background well below 1% in new experiments.

4 The Daya Bay Site

An ideal site for this experiment is near the Daya Bay–Ling Ao nuclear power plant inChina. The site consists of two operating power plants separated by about 1.1 km withtwo cores each. The Daya Bay–Ling Ao site has the following desirable attributes:

Figure 2: Photograph showing the four presently operating reactor cores at the Daya Baynuclear power plant. The two Daya Bay cores are shown in the foreground on the leftand the Ling Ao cores can be seen in the far distance on the right. Picture courtesy ofChina Guangdong Nuclear Power Group (CGNPG) [5].

• The reactor complex has a total power output of 11.6 GW (thermal), and a thirdpower plant with an additional 5.8 GW (thermal) of power is under construction

6 4 THE DAYA BAY SITE

Figure 3: Picture of the Daya Bay quarry and mountains directly next to the reactors,separated by the access road in the foreground.

and will be commissioned by 2010. With a total power of 17.4 GW (thermal), DayaBay will very likely rank as one of the top five most powerful nuclear power plants inthe world. As it operates today, it has nearly twice the power of the Diablo Canyonor Braidwood reactor complexes in the US and ∼50% more power than the CHOOZsite in France.

• The complex directly abuts a mountain range (Figure 3) that provides protectionfrom cosmic rays that are the main source of backgrounds in the experiment. Tun-nels in the mountain can provide between 100 m (mean depth of 270 meters ofwater equivalent) and 400 m (mean depth of 1200 mwe) of granite overburden toshield the detectors from cosmic rays, for distances between 250 m to 2.5 km fromthe reactor cores, respectively. A possible configuration of tunnels and detectors isshown in Figure 4.

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Figure 4: Satellite photograph of the Daya Bay–Ling Ao nuclear power plant complex inChina showing the location of the reactors, a possible tunnel layout and possible locationsfor the near and far detectors. The operating reactor cores (Daya Bay and Ling Ao) arehighlighted with solid dots, and the location of the future Ling Ao II plant is indicated withopen dots. The white dots indicate possible locations for the near, mid, and far detectors,connected by tunnels. In this configuration the total tunnel length is approximately 3 km;with other possible configurations the tunnels are between 2-3 km long.

8 5 SUMMARY

5 Summary

The Daya Bay-Ling Ao nuclear power plant in China provides a unique opportunity toconduct the next-generation reactor neutrino experiment with multiple detectors makinga precise measurement of the subdominant θ13 oscillation with reactor anti-neutrinos. TheDaya Bay reactor anti-neutrino experiment will be based on a strong partnership betweenscientists from China, Russia, and the United States. The Institute of High Energy Physics(IHEP) in Beijing is enthusiastic to host this experiment and to collaborate with colleaguesfrom the US and Russia. Home to the Chinese electron-positron collider (BPEC) andthe BES spectrometer, IHEP has the capability to construct large-scale, sophisticatedapparatus for contemporary experiments in particle physics. Physicists among the IHEPstaff are collaborators on the Palo Verde, KamLAND, MiniBooNE and Texono neutrinoexperiments.

The US groups involved in the Daya Bay project have been leaders in the successfulKamLAND, SNO, and Gallex neutrino experiments, and have many years of experiencein low-background, underground neutrino physics. Their experience in the design and op-eration of neutrino experiments will be invaluable for the success of the Daya Bay project.The US collaborators have considerable experience in the development of stable scintil-lators containing gadolinium, design and construction of calibration sources and theirdeployment apparatus, electronics, trigger systems, data acquisition and analysis, andlow-background radioactive test facilities. Most importantly, as members of KamLANDand SNO the US groups have a thorough understanding of the potential cosmogenicbackgrounds and how they can best be mitigated. The Daya Bay collaboration involvesscientists from two national laboratories, Lawrence Berkeley National Laboratory andBrookhaven National Laboratory. The support and experience of these laboratories isimportant to ensure the management and coordination of the project on the US side.

In addition, there is strong support from local universities in Hong Kong. Their prox-imity to the power plant makes them ideal hosts for the operation of the experiment andallows students, postdocs, and scientists to be integrated into the academic environmentin Hong Kong.

Scientists from Russia have joined the Daya Bay collaboration with years of experiencein scintillator development and nuclear physics experiments. This includes some of theoriginal proponents of a θ13 reactor neutrino experiment from the Kurchatov Institute inMoscow.

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6 Frequently Asked Questions

• What is the typical event rate of anti-neutrinos interactions in a reactor

θ13 experiment?

The neutrino interaction rate depends on the rate of fission reactions in the reactor(thermal power output), the composition of the reactor core, and the number ofhydrogen atoms per ton of target. A useful rule of thumb is: one event per tonper day per GW (thermal) at 1 km distance, assuming no neutrino oscillations and100% efficiency. The CHOOZ rate was about 20% lower than this value and therate in KamLAND is 10% higher, mostly because of the different hydrogen contentof the detectors.

• What does a detector in the Daya Bay experiment look like?

The experiment measures the rate and energy spectrum of electron anti-neutrinosfrom the reactors via inverse beta-decay as a function of distance from the reactor.A detector module consists of (see Figure 5):

1. A central volume of gadolinium-loaded liquid scintillator in an acrylic vessel,surrounded by:

2. A “gamma catcher” or shell of undoped (i.e. no gadolinium) liquid scintillatorto capture photons escaping from the central volume. The gamma catcher iscontained by a second acrylic vessel, which is in turn surrounded by:

3. A mineral oil buffer held in a stainless steel vessel.

4. An array of photomultiplier (PMT) tubes mounted on the outside of the stain-less steel vessel and immersed in the mineral oil to measure the energy depositedin both the central scintillator volume and the gamma catcher. The mineraloil shields the central volume and gamma catcher from radioactive decay inthe glass of the PMT’s, stainless steel vessel, and the surrounding rock.

5. A sophisticated external muon identifier with an absorber equivalent to about2 m of water to measure and reduce backgrounds induced by cosmic rays and toabsorb gamma rays (principally from uranium, thorium and potassium decays)coming out of the surrounding rock.

6. Precautions to limit the introduction of radon or other radioactive contami-nants, including special treatments to seal the detector halls from radon gas inthe surrounding rock, air handling systems to flush radon from the experimen-tal halls and clean-room environment to reduce particulate contamination.

10 6 FREQUENTLY ASKED QUESTIONS

Figure 5: Cut-away view of a detector showing the three vessels with the Gd-loadedscintillator in the central acrylic vessel, a second acrylic vessel holding unloaded scintillator(“gamma catcher”), and the outermost stainless steel vessel holding mineral oil “buffer.”

The dimensions of the detectors are constrained by the underground tunnel facil-ities. As part of the design and engineering studies, we are considering detectormodules with central target masses between 20 ton and 60 ton each. These detectormodules can be deployed in several different confifurations: A “near-far” configura-tion, where two “near” detectors effectively monitor the reactor fluxes, and a “far”detector (consisting of multiple modules identical to the near detectors with a totaltarget mass of ∼100 ton) measures the electron anti-neutrino survival probability. Asecond “mid-far” configuration is also under active study, and may achieve equiva-lent sensitivity with reduced complication. A “near-mid” configuration can be usedduring an early phase of the experiment while tunnel construction for the far site isstill being completed. We are hopeful that all configurations can economically beexploited simultaneously in a a uniquely convincing experiment at Daya Bay.

• Why gadolinium?

Adding gadolinium to the central target volume offers three advantages:

1. At a concentration of 0.1% by weight, it reduces the capture time of neutrons

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from inverse beta decay from 200 µs on hydrogen to 30 µs on Gd. Hence, theaccidental double-coincidence background rate is reduced by a factor of seven.

2. The energy release of a neutron capture on Gd is approximately 8 MeV, com-pared to 2.2 MeV for neutron capture on hydrogen. Because of this higherenergy release, that can only occur in the Gd-loaded central volume, the targetmass is better defined and the low-energy backgrounds from other sources (e.g.radioactive decay in the surrounding rock) can be excluded.

3. The capture of a neutron on hydrogen results in the production of a single 2.2MeV gamma ray, whereas the capture of a neutron on Gd results in a cascade ofseveral gamma rays, on average 3-4. The average location of these gamma raysis better localized with respect to the position of the prompt positron allowingmore efficient identification of the inverse beta-decay events. This allows fora study of the directionality of the candidate anti-neutrino events, potentiallyreducing systematic errors.

• Why horizontal access tunnels?

A reactor θ13 experiment requires the use of multiple underground neutrino detec-tors at distances of 0.2− 2 km from a power plant. The design of the experimentalfacilities depends critically on the topography of the reactor site. A variety of con-cepts involving vertical shafts or horizontal tunnels have been considered. Horizontalaccess tunnels into nearby mountains offer many advantages:

1. Horizontal tunneling is a well-established technology and used worldwide forthe construction of road tunnels. A train or road-size tunnel is sufficiently largeto provide access to the underground detector halls and to allow the movementof the neutrino detectors between the detector halls.

2. Horizontal tunnels allow the neutrino detectors to be completely assembledand tested on surface before moving them into their underground position.This minimizes construction underground and accelerates the installation andcommissioning schedule of the experiment.

3. Horizontal tunnels with roads or rails that connect the underground detectorhalls provide an easy method of interchanging the location of the detectors andmoving them to different baselines. Several underground halls in the tunnel canprovide detector locations with discrete baselines. One can imagine construc-tion of several underground halls that allow the optimization of the detectorbaseline. (We note that the oscillation length related to the θ13 transition

12 6 FREQUENTLY ASKED QUESTIONS

depends on the mass splitting ∆m213. The uncertainty in this parameter trans-

lates to a change in baseline of a few hundred meters, and requires maximumflexibility when locating the underground experimental halls.)

4. Horizontal tunnels provide underground connections between the detector hallsand allow the neutrino detectors to stay underground during the entire periodof the experiment. When a detector is moved it is not necessary to bring itto the surface, thus minimizing the risk of exposure to cosmic rays that cangenerate radioactive isotopes in the construction materials and generate long-lived backgrounds. Underground movement of the detectors also protects themfrom undesirable changes in environmental conditions, such as temperature.

• What are the most important backgrounds and how are they minimized?

Among the many backgrounds, the two most serious backgrounds are generated bycosmic rays. Shallow detectors may have unacceptable deadtime if they rely on amuon veto to remove cosmic ray events.

– Although rare, cosmogenically produced 9Li is a difficult background becausethe beta decay of the nucleus is followed by neutron emission, exactly mimickingthe inverse beta-decay signal [4]. The half-life of 9Li is 178 msec and theelectron energy from its beta decay is in the range of interest for inverse betadecay. Extrapolations from KamLAND, where the measured rate is ∼ 1.59Li decays per kton per day at 2700 mwe, indicate that most, if not all, theCHOOZ correlated background can be accounted for by 9Li. An external muonidentifier can be used to measure the rate and exclude 9Li events from the finaldata sample via their correlation in time and location with crossing muons.Because the lifetime of 9Li is 257 msec, a veto period of 1-2 seconds overthe entire detector is required or a veto on events within ∼1 meter radiusof the muon track. The latter “localization veto” may be ineffective in deepinelastic events (∼10% of the total muon triggers at KamLAND), and forthese events the entire detector volume may need to be vetoed. The best wayto minimize this background is to locate the detectors deep underground sothat the production rate of 9Li is < 1% of the signal.

– Spallation neutrons from cosmic rays in the detector or the surrounding rockcan create energy deposits in the detector that are identical to the doublecoincidence of the inverse beta-decay signal. They are reduced to less than1% of the signal rate by adequate overburden. They are further excluded byremoving candidate events that occur in conjunction with the passage of a

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cosmic ray in the detector or muon shield. The rate of events in the muonshield can be used to estimate the neutron flux from the surrounding rockentering the active detector volume, allowing the precise determination of thiscontribution to the background.

• How do movable detectors reduce systematic errors?

The measurement of θ13 relies on the determination of the anti-neutrino interactionrate in multiple detectors at different baselines. The observed signal rate will dependon the detector target mass, the distance of the detector from the nuclear reactor,the detection efficiency, and indistinguishable cosmic-ray induced backgrounds.

1. Small variations in the target mass of the detectors can lead to apparent differ-ences in the observed event rate. The CHOOZ experiment was able to measurethe hydrogen content of the target to only 0.8%. The residual uncertainty inthis measurement is due to the volatility of the organic compounds comprisingthe target mass [1]. Many strategies have been proposed to improve on thissystematic error: better chemical handling techniques, better monitoring of theflow and mass when filling the target,“simultaneous” filling of the near and fardetectors, etc. Newer experiments will use many of the techniques to controlthe chemical composition of the detector target but a residual systematic errorwill remain. Interchanging detector positions is a powerful method to calibratesignal variations due to differences in the detector target mass.

2. The mountainous terrain allows greater overburden for the far detector, cre-ating a situation where the signal-to-background ratio in the near and fardetectors is approximately equal. The larger overburden at the far detectorsite reduces the rate of cosmic-ray background, largely compensating for the1/L2 dependence of the anti-neutrino flux in the signal-to-background ratio.Interchanging detector locations will allow for measuring the same signal-to-background ratio in different detectors.

We are actively considering an experimental design that allows the movement andexchange of detectors at different baselines so that the relative neutrino detectionefficiencies of the detectors can be calibrated in the presence of the same neutrinoflux and the same backgrounds. This concept of a relative detector calibrationwill take into account detector systematics such as variations in the target mass.We do not require the detector response to be the same after moving detectors.An extensive calibration program using triggered and passive calibration sourcesas well as the neutron capture signal on Gd will allow us to (re)determine critical

14 REFERENCES

parameters of the detector response such as attenuation lengths and the absoluteenergy calibration.

Movable detectors and a comprehensive calibration program are the only guaranteethat a change in the observed event rate in detectors at different baselines is due toneutrino oscillation and not due to systematic uncertainties in the target mass, thedetermination of backgrounds, or the relative detector efficiencies.

References

[1] M. Apollonio, et.al., Euro. Phys. J. C27 (2003) 331.

[2] K. Anderson, et al.,“White Paper Report on Using Reactors to Search for a Value ofθ13,” Jan. 2004, arXiv:hep-ex/0402041.

[3] Daya Bay Collaboration, “A Precision Measurement of sin2 2θ13 with Reactor An-tineutrinos at Daya Bay, China,” May, 2005.

[4] We refer to this background as 9Li only, but based on measurements from KamLAND,it may also include a smaller, indistinguishable contribution from 8He decay with ahalf-life of 119 msec.

[5] China Guangdong Nuclear Power Group (CGNPG)http://www.cgnpc.com.cn/

[6] Project web sites:http://theta13.lbl.gov/ (US)http://bes.ihep.ac.cn/dayawane/ (China)http://theta13.phy.cuhk.edu.hk/ (Hong Kong)

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A The Daya Bay Collaboration

At present the Daya Bay Collaboration consists of some 70 scientists from China, Russia,and the United States:

X. Guo, N. Wang, R. Wang, Y. WangBeijing Normal University, Beijing 100875, China

A. Garnov, R.L. Hahn, M. YehBrookhaven National Laboratory, Upton, NY, 11973, USA

R.D. McKeown, C. Mauger, C. JillingsCalifornia Institute of Technology, Pasadena, CA 91125, USA

L. Hou, B. Xing, Z. ZhouChina Institute of Atomic Energy, Beijing 102413, China

M.C. Chu, W.K. Ngai, A. TangChinese University of Hong Kong, Hong Kong, China

J. Cao, H. Chen, C. Jiang, J. Li, Y. Lu, Y. Ma, X. Meng, Y. Wang, Z. Wang, C. Yang,J. Zhang, Z. ZhangInstitute of High Energy Physics, Beijing 100039, China

Y. Wu, J. YangInstitute of Theoretical Physics, Beijing 100080, China

B.L. Young, K. WhisnantIowa State University, Ames, Iowa 50011, USA

Yu. Gornushkin, I. Nemchenok, A. Olchevski, E. YakushevJoint Institute of Nuclear Research, Dubna, Russia

V.I. Aleshin, Yu.V. Gaponov, V.I. Kopeikin, V.P. Martemyanov, L.A.Mikaelyan, V.G.Tarasenkov,V.N.VyrodovKurchatov Institute, Moscow, Russia

B.E. Berger, P. Decowski, S.J. Freedman, B.K. Fujikawa, K.M. Heeger, L. Hsu, R.W. Kadel,Yu.G. Kolomensky, K.B. Luk

16 A THE DAYA BAY COLLABORATION

Lawrence Berkeley National Laboratory/University of California at Berkeley, Berkeley,CA 94720, USA

X. Li, Y. XuNankai University, Tianjin 300071, China

H. Niu, L. NiuShenzhen University, Shenzhen 518060, China

Q. SuTsinghua University, Beijing 100084, China

H.Z. HuangUniversity of California, Los Angeles, CA 90095, USA

K.S. Cheng, J.K.C. Leung, C.S.J. PunUniversity of Hong Kong, Hong Kong, China

K.H. LauUniversity of Houston, Houston, Texas 77204, USA

D. Howell, J.C. PengUniversity of Illinois, Urbana-Champaign, Illinois 61801, USA

X. JiUniversity of Maryland, College Park, MD 20742, USA

H. Liang, G. Jin, J. Wang, Q. Wang, X. Yu, Y. ZhouUniversity of Science and Technology of China, Hefei 230026, China

T. ZhaoUniversity of Washington, Seattle, Washington 98101, USA