Status of the RENO Reactor Neutrino Experiment

K.K. Joo (for the RENO Collaboration)Institute for Universe & Elementary Particles, Department of Physics, Chonnam National University, 300 Yongbong-dong, Puk-gu, Gwangju,

Korea, 500-757

Abstract

The Reactor Experiment for Neutrino Oscillation (RENO) is a reactor based neutrino oscillation experiment tomeasure the neutrino mixing angle θ13 using anti-neutrinos emitted from the Yonggwang nuclear power plant inKorea. Its thermal power output is 16.4 Gwth. The experimental setup consists of two identical 16-ton gadolinium-loaded liquid scintillator detectors. The near and far detectors are placed roughly 290 m and 1.4 km from the center ofthe reactor array, respectively. The near detector is constructed at underground of a 70 m high hill and the far detectorat underground of a 260 m high mountain. The construction of experimental halls and access tunnels for both nearand far detector sites was completed in early 2009. The experiment is planned to start data-taking from early 2011.An expected number of observed anti-neutrinos is roughly 1300 per day and 100 per day in the far and near detector,respectively. We expect that an estimated systematic uncertainty is less than 0.5%. With three years of data taken,RENO is sensitive to measure sin2(2θ13) > 0.02. Sensitivity is ten times better than the current limit obtained byChooz. In this review, current status of RENO is presented.

Keywords: RENO, Reactor neutrino, Neutrino oscillation, sin2(2θ13).

1. Introduction

In the past few years, there have been great pro-gresses in understanding the properties of neutrino. Thediscovery of neutrino oscillations is a direct indicationof physics beyond the Standard Model. It provides aunique new window to explore physics at high massscale including unification, flavor dynamics and extradimensions. Currently many activities have been per-formed to measure the parameters associated with neu-trino masses and mixings.As time passes, one flavor neutrino can be trans-

formed into another flavor neutrino by way of the wave-like property of particle. Neutrino oscillations betweenthese transformation can be expressed by three mixingangles (θ12, θ23, θ13). There are many world-wide exper-iments underway to measure the parameters associated

Email address: kkjoo@chonnam.ac.kr (K.K. Joo (for theRENO Collaboration))

with neutrino oscillations. The value of θ12 is measuredby solar neutrinos and θ23 is meausred by atmosphericneutrinos and long-baseline accelerator. So far, θ12 andθ23 are relatively well measured by several experiments[1, 2, 3]. Both angles are large, unlike mixing angles inquark sector. But the third angle, θ13 has not yet beenbeen measured precisely. It is constrained to be smallby the Chooz experiment [4]. Measurement of θ13 ispossible by using reactor neutrinos and accelerator neu-trino beams. Reactor measurement has no ambiguitiesassociated with matter effect and CP violation.We propose a new experiment on the site of Yong-

gwang nuclear power plant in Korea. RENO (ReactorExperiment for Neutrino Oscillation) is an experimentdesigned to use anti-neutrinos to measure the smallestneutrino mixing angle (θ13). The anti-neutrino fluxesfrom the nuclear reactors are measured nearby beforetheir oscillations, and measured again at a distance ofabout 1.4 km away from the reactor center. Then, a neu-

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0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved.

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doi:10.1016/j.nuclphysbps.2012.09.016

Figure 1: A schematic layout of the RENO experiment.

trino mixing parameter θ13 can be obtained by compar-ison of the two measured fluxes.

2. The RENO Experiment

The Yonggwang nuclear power plant is located in thewest coast of southern part of Korean peninsular. It isabout 400 km away from Seoul. The power plant hassix reactors. These six reactors are lined up in roughlyequal distances and span ∼ 1.3 km. The total thermaloutput is 16.4 GWth, currently the second largest in theworld.The construction of experimental halls and access

tunnels for both near and far detector was performed atthe same time and completed in early 2009. Now bothdetectors are in the middle of installation. The experi-mental setup consists of two identical detectors of 16-ton gadolinium (Gd) loaded liquid scintillator. The neardetector is placed roughly 290 m and the far detector islocated 1.4 km away from the center of the reactor ar-ray. The near detector is located underground a 70 m (∼110 mwe) high hill and the far detector is located under-ground a 260 m (∼ 450 mwe) high mountain, as shownin Fig. 1. In order to reduce the systematic uncertainty,two identical detectors are constructed at the near andfar site.

3. The RENO Detector

The RENO detector consists of four concentric cylin-drical layers from the center to the outer, these layersare the target, the γ-catcher, the buffer, and the veto asshown in Fig. 2. Dimensions of the RENO detector arelisted in Table 1. The acrylic vessel holding the targetliquid is surrounded by the γ-catcher. The target con-sists of 16 ton acrylic vessel holding the gadolinium-loaded liquid scintillator. Liquid scintillator are con-tained in the γ-catcher module of the RENO detector.Mineral oil is used in the buffer for a non-scintillatingliquid. The outermost layer is veto and is filled withpure water. Liquid scintillator consists of organic sol-vent, fluor, and wavelength shifter.

Figure 2: RENO detector. RENO detector consists of four cylindricallayers; from the center to the outermost, target, γ-catcher, buffer, andveto. Volume of neutrino target is 18.7 m3 and 16.1 mass tons.

For liquid scintillator, RENO will use linear alkylbenzene (LAB, CnH2n+1-C6H5, n = 10 ∼ 13). LAB hasa relatively good light yield, high flash point (130 oC)and large attenuation length [5]. Its density is 0.86 g/ml.LAB is obtained in Korean company from Isu chemi-cal Ltd. Furthermore, it is not harmful to health andenvironment, so that it significantly reduces the safetyconcerns. The LAB-based liquid scintillator uses lin-ear alkyl benzene as the organic solvent. PPO (2,5-diphenyloxazole) is adopted as the primary scintilla-tion solute. For secondary wavelength shifter, bis-MSB(1,4,-bis(2-methylstyryl)-benzene) is used. The amountof PPO/bis-MSB is varied and the optimal concentra-tion are determined when the solution give a maximumlight yield. 3 g/l of PPO and 30 mg/l of bis-MSB areused. A gadolinium-loaded liquid scintillator is madeby doping the liquid scintillator with 1 g/l of Gd.The target and γ-catcher vessels are made of

transparent acrylic plastic called Polymethylmethacry-late (PMMA). The molecular formula of PMMA is(C5O2H8)n. The acrylic material is Plexiglas GS233 ob-tained from the Degussa company. This acrylic is madeby a cast method.

3.1. Target

The target consists of 0.1% gadolinium-loaded liq-uid scintillator in a cylindrical acrylic container of 140cm in radius, 320 cm in height, and 25 mm in thick-ness. Total volume is 18.7 m3 and target mass is 16.1tons. The hydrogen atoms (“free proton”) in the liquidscintillator serve as the target for the inverse beta de-cay. For the detection of anti-neutrinos from reactors,we use inverse beta decay process (ν̄ + p → e+ + n).

K.K. Joo / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 97–10098

Table 1: Dimensions of the RENO detector. (*)The buffer vessel thickness is 6 mm for the top and barrel sections and 12 mm for the bottomsection.

Detector Outer Outer Thickness Material Volume MassComponent Diameter(mm) Height(mm) (mm) (m3) (tons)Target 2750 3150 – Gd-loaded LS 18.70 16.08

Target Vessel 2800 3200 25 Acrylic 0.99 1.18γ-catcher 3940 4340 570 LS 33.19 28.55

γ-catcher Vessel 4000 4400 30 Acrylic 2.38 2.83Buffer 5388 5788 694 Oil 76.46 64.22

Buffer Vessel 5400 5800 6/12* SUS 1.05 8.39Veto 8400 8890 1500 Water 354.84 354.84

The inverse beta decay process takes place on hydro-gen. The prompt positron annihilates and yields 1 ∼8 MeV of visible energy. A neutron will be capturedby hydrogen, which forms a deuterium giving off pho-tons with total energy of ∼ 2.2 MeV. Liquid scintillatorloaded with a small amount of gadolinium, which hasa very large neutron capture cross section and producesphotons with higher total energy, ∼ 8 MeV, enhancesthe delayed neutron capture signal. Neutron is captured∼ 200 μs without Gd. But with Gd, this time is ∼ 30μs. Therefore this delayed coincidence method reducesbackgrounds drastically.

Gadolinium (metal) cannot be dissolved in the or-ganic solvent. In order to dissolve the Gd into liq-uid scintillator, making Gd-compound using ligand isneeded. For the synthesis of Gd compound, neutraliza-tion chemical reactions are used as follows.

(1) RCOOH + NH3 ∗ H2O→ RCOONH4 + H2O(2) 3RCOONH4(aqueous) +GdCl3(aqueous)→

Gd(RCOO)3 + 3NH4Cl

Step (1) is neutralization reaction. 3,5,5-trimethylhexanoic acid (TMHA) is neutralized withammonium hydroxide. Second step is to mix twoaqueous solutions. When two solutions are mixed,white Gd-compound precipitated immediately. Thesereactions are very sensitive to pH. The formed solid Gdcomplexes are thoroughly rinsed with 18 MΩ ultra purewater several times and then dried. In order to studymolecular structure and chemical bonds in organiccompounds, we use Fourier Transform InfraRed (FT-IR) spectrum technique. We require four conditionsfor our final sample to make pure Gd-TMHA; noOH-group (3200 ∼ 3500 cm−1), no free acid peak (∼1700 cm−1), carboxylic peak (∼ 1420 and 1580 cm−1),and disolving capability of Gd-TMHA into LAB.

3.2. γ-catcher

The γ-catcher surrounds the target and filled with anunloaded liquid scintillator. The γ-catcher is containedin a cylindrical acrylic vessel of 200 cm in radius, 440cm in height, and 30 mm in thickness. Total voulme is33.2 m3. The purpose of γ-catcher is to detect gammarays escaping from target region and reconstruct ener-gies from positron annihilation and neutron capture sig-nal on Gd.

3.3. Buffer

In order to decrease the level of the accidental ra-dioactive backgrounds, mainly coming from the pho-tomultiplier tube (PMT) glass, outside rocks, non-scintillating buffer oil is used. The PMTs are mountedin a non-scintillating buffer. A total of 64.2 tons of min-eral oil is contained in a stainless steel vessel of 270 cmin radius, 580 cm in height, and 6 mm in thickness .Buffer plays role in keeping the signal rate below 10 Hzin the target and γ-catcher regions. A total 354 10-inchPMTs in an uniform array are mounted from the innersurface of the buffer stainless vessel. It provids 14%photo-sensitive surface coverage.

3.4. Veto

Outside the stainless steel, concrete wall is filled withpure water to operate it as a water Cerenkov detector.A 1.5 m thick water layer of 354.8 tons surrounds thewhole inner detector with a mass of about 120 tons.A total of 67 10-inch PMTs are mounted in front of aTyvek reflector on a cylindrical steel tank.

4. Sensitivity

RENO is expected to improve the sensitivity onsin2(2θ13) > 0.02 by an order magnitude relative tothe current best limit as shown in Fig. 3. This will be

K.K. Joo / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 97–100 99

Figure 3: Expected 90% CL limits with the total systematic uncer-tainty of 0.5% (red) and 1.0% (blue). The total 70% efficiency wasassumed.

achieved by reducing both statistical and systematic er-rors. The expected number of observed anti-neutrinosis roughly 1300 per day and roughly 100 per day in thenear detector and far detector, respectively. The esti-mated systematic uncertainty associated with the mea-surement is less than 0.5% and the expected statisticalerror is about 0.3%. Based on three years of data, itwill be sensitive to measure the neutrino mixing anglein the range of sin2(2θ13) > 0.02 in the 90% C.L. limit.This sensitivity is ten times better than the current limitobtained by Chooz.

5. Current Status of the RENO Experiment

Consideration of the RENO experiment was pro-posed in early 2004 based on the “White Paper for aNew Nuclear Reactor Neutrino Experiment” [6]. It wasapproved by the Ministry of Science and Technology inKorea in May 2005. Geological survey and tunnel de-sign were done in 2007. After that, the constructionof experimental halls and access tunnels for both nearand far detector sites was completed in early 2009. Inthe meanwhile, research facilities such as research con-tainer, lodging house, electricity, internet service, wa-ter supply, etc are all prepared. Detector design andspecification were carried out from 2006 ∼ 2007. From2008, detector construction has been under way. 10-inch R7081 PMT delivery from Hamamatsu was donein early 2010. PMT testing has been performed duringthe summer of 2010. Currently PMT installation is un-der way at both far and near detector at the same time.RENO will use the QBEE board (which is an

abbreviation of “QTC (charge to time converter)Based Electronics with Ethernet”) developed by Super-Kamiokande experiment [7]. The new electronics sys-tem are fast enough to record every PMT hits and areguaranteed for stable data acquisition over ten years.This is ready for taking data. Furthermore, HV system

Figure 4: RENO construction schedule. The detectors are near com-pletion, and data-taking is expected to start in early 2011.

is working for the experiment. The RENO data acquisi-tion consists of data readout using front-end electronics,event builder, software triggers, slow control monitor,event display, data logger and run control. All theseare functional. The experiment is planned to start data-taking from early 2011. The construction schedule isshown in Fig. 4.

6. Summary

The RENO experiment at Yonggwang in Korea hasbeen proposed to measure θ13. The experimental setupconsists of two 16 ton LAB-based liquid scintillator de-tectors with one at a near site, and the other at a far site.Lot of progresses in all respects have been made sincelast three years. The detector commisioning is about tostart and taking data will begin from early 2011.

Acknowledgments“This research was supported by Basic Science Re-search Program through the Nation Research Founda-tion of Korea (NRF) funded by the Ministry of Educa-tion, Science and Technology (2010-0007850).”

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