Measurement of electron antineutrino oscillation based
on 1230 days of operation of the Daya Bay experiment
F. P. An,
1
A. B. Balantekin,
2
H. R. Band,
3
M. Bishai,
4
S. Blyth,
5,6
D. Cao,
7
G. F. Cao,
8
J. Cao,
8
W. R. Cen,
8
Y. L. Chan,
9
J. F. Chang,
8
L. C. Chang,
10
Y. Chang,
6
H. S. Chen,
8
Q. Y. Chen,
11
S. M. Chen,
12
Y. X. Chen,
13
Y. Chen,
14
J.-H. Cheng,
10
J. Cheng,
11
Y. P. Cheng,
8
Z. K. Cheng,
15
J. J. Cherwinka,
2
M. C. Chu,
9
A. Chukanov,
16
J. P. Cummings,
17
J. de Arcos,
18
Z. Y. Deng,
8
X. F. Ding,
8
Y. Y. Ding,
8
M. V. Diwan,
4
M. Dolgareva,
16
J. Dove,
19
D. A. Dwyer,
20
W. R. Edwards,
20
R. Gill,
4
M. Gonchar,
16
G. H. Gong,
12
H. Gong,
12
M. Grassi,
8
W. Q. Gu,
21
M. Y. Guan,
8
L. Guo,
12
X. H. Guo,
22
Y. H. Guo,
23
Z. Guo,
12
R. W. Hackenburg,
4
R. Han,
13
S. Hans,
4
,*
M. He,
8
K. M. Heeger,
3
Y. K. Heng,
8
A. Higuera,
24
Y. K. Hor,
25
Y. B. Hsiung,
5
B. Z. Hu,
5
T. Hu,
8
W. Hu,
8
E. C. Huang,
19
H. X. Huang,
26
X. T. Huang,
11
P. Huber,
25
W. Huo,
27
G. Hussain,
12
D. E. Jaffe,
4
P. Jaffke,
25
K. L. Jen,
10
S. Jetter,
8
X. P. Ji,
28,12
X. L. Ji,
8
J. B. Jiao,
11
R. A. Johnson,
29
D. Jones,
30
J. Joshi,
4
L. Kang,
31
S. H. Kettell,
4
S. Kohn,
32
M. Kramer,
20,32
K. K. Kwan,
9
M. W. Kwok,
9
T. Kwok,
33
T. J. Langford,
3
K. Lau,
24
L. Lebanowski,
12
J. Lee,
20
J. H. C. Lee,
33
R. T. Lei,
31
R. Leitner,
34
J. K. C. Leung,
33
C. Li,
11
D. J. Li,
27
F. Li,
8
G. S. Li,
21
Q. J. Li,
8
S. Li,
31
S. C. Li,
33,25
W. D. Li,
8
X. N. Li,
8
Y. F. Li,
8
Z. B. Li,
15
H. Liang,
27
C. J. Lin,
20
G. L. Lin,
10
S. Lin,
31
S. K. Lin,
24
Y.-C. Lin,
5
J. J. Ling,
15
J. M. Link,
25
L. Littenberg,
4
B. R. Littlejohn,
18
D. W. Liu,
24
J. L. Liu,
21
J. C. Liu,
8
C. W. Loh,
7
C. Lu,
35
H. Q. Lu,
8
J. S. Lu,
8
K. B. Luk,
32,20
Z. Lv,
23
Q. M. Ma,
8
X. Y. Ma,
8
X. B. Ma,
13
Y. Q. Ma,
8
Y. Malyshkin,
36
D. A. Martinez Caicedo,
18
K. T. McDonald,
35
R. D. McKeown,
37,38
I. Mitchell,
24
M. Mooney,
4
Y. Nakajima,
20
J. Napolitano,
30
D. Naumov,
16
E. Naumova,
16
H. Y. Ngai,
33
Z. Ning,
8
J. P. Ochoa-Ricoux,
36
A. Olshevskiy,
16
H.-R. Pan,
5
J. Park,
25
S. Patton,
20
V. Pec,
34
J. C. Peng,
19
L. Pinsky,
24
C. S. J. Pun,
33
F. Z. Qi,
8
M. Qi,
7
X. Qian,
4
N. Raper,
39
J. Ren,
26
R. Rosero,
4
B. Roskovec,
34
X. C. Ruan,
26
H. Steiner,
32,20
G. X. Sun,
8
J. L. Sun,
40
W. Tang,
4
D. Taychenachev,
16
K. Treskov,
16
K. V. Tsang,
20
C. E. Tull,
20
N. Viaux,
36
B. Viren,
4
V. Vorobel,
34
C. H. Wang,
6
M. Wang,
11
N. Y. Wang,
22
R. G. Wang,
8
W. Wang,
38,15
X. Wang,
41
Y. F. Wang,
8
Z. Wang,
12
Z. Wang,
8
Z. M. Wang,
8
H. Y. Wei,
12
L. J. Wen,
8
K. Whisnant,
42
C. G. White,
18
L. Whitehead,
24
T. Wise,
2
H. L. H. Wong,
32,20
S. C. F. Wong,
15
E. Worcester,
4
C.-H. Wu,
10
Q. Wu,
11
W. J. Wu,
8
D. M. Xia,
43
J. K. Xia,
8
Z. Z. Xing,
8
J. Y. Xu,
9
J. L. Xu,
8
Y. X u,
15
T. Xue,
12
C. G. Yang,
8
H. Yang,
7
L. Yang,
31
M. S. Yang,
8
M. T. Yang,
11
M. Ye,
8
Z. Ye,
24
M. Yeh,
4
B. L. Young,
42
Z. Y. Yu,
8
S. Zeng,
8
L. Zhan,
8
C. Zhang,
4
H. H. Zhang,
15
J. W. Zhang,
8
Q. M. Zhang,
23
X. T. Zhang,
8
Y. M. Zhang,
12
Y. X. Zhang,
40
Y. M. Zhang,
15
Z. J. Zhang,
31
Z. Y. Zhang,
8
Z. P. Zhang,
27
J. Zhao,
8
Q. W. Zhao,
8
Y. B. Zhao,
8
W. L. Zhong,
8
L. Zhou,
8
N. Zhou,
27
H. L. Zhuang,
8
and J. H. Zou
8
(Daya Bay Collaboration)
1
Institute of Modern Physics, East China University of Science and Technology, Shanghai
2
University of Wisconsin, Madison, Wisconsin 53706
3
Department of Physics, Yale University, New Haven, Connecticut 06520
4
Brookhaven National Laboratory, Upton, New York 11973
5
Department of Physics, National Taiwan University, Taipei
6
National United University, Miao-Li
7
Nanjing University, Nanjing
8
Institute of High Energy Physics, Beijing
9
Chinese University of Hong Kong, Hong Kong
10
Institute of Physics, National Chiao-Tu ng University, Hsinchu
11
Shandong University, Jinan
12
Department of Engineering Physics, Tsinghua University, Beijing
13
North China Electric Power University, Beijing
14
Shenzhen University, Shenzhen
15
Sun Yat-Sen (Zhongshan) University, Guangzhou
16
Joint Institute for Nuclear Research, Dubna, Moscow Region
17
Siena College, Loudonville, New York 12211
18
Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616
19
Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
20
Lawrence Berkeley National Laboratory, Berkeley, California 94720
21
Department of Physics and Astronomy, Shanghai Jiao Tong University,
Shanghai Laboratory for Particle Physics and Cosmology, Shanghai
22
Beijing Normal University, Beijing
23
Xi’an Jiaotong University, Xi’an
24
Department of Physics, University of Houston, Houston, Texas 77204
PHYSICAL REVIEW D 95, 072006 (2017)
2470-0010=2017=95(7)=072006(46) 072006-1 © 2017 American Physical Society
25
Center for Neutrino Physics, Virginia Tech, Blacksburg, Virginia 24061
26
China Institute of Atomic Energy, Beijing
27
University of Science and Technology of China, Hefei
28
School of Physics, Nankai University, Tianjin
29
Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221
30
Department of Physics, College of Science and Technology, Temple University,
Philadelphia, Pennsylvania 19122
31
Dongguan University of Technology, Dongguan
32
Department of Physics, University of California, Berkeley, California 94720
33
Department of Physics, The University of Hong Kong, Pokfulam, Hong Kong
34
Faculty of Mathematics and Physics, Charles University, Prague
35
Joseph Henry Laboratories, Princeton University, Princeton, New Jersey 08544
36
Instituto de Física, Pontificia Universidad Católica de Chile, Santiago
37
California Institute of Technology, Pasadena, California 91125
38
College of William and Mary, Williamsburg, Virginia 23187
39
Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute,
Troy, New York 12180
40
China General Nuclear Power Group, Shenzhen
41
College of Electronic Science and Engineering, National University of Defense Technology, Changsha
42
Iowa State University, Ames, Iowa 50011
43
Chongqing University, Chongqing
(Received 31 October 2016; publis hed 6 April 2017)
A measurement of electr on antineutrino oscillation by the Daya Bay Reactor Neutrino Experiment
is described in detail. Six 2.9-GWth nuclear power reactors of the Daya Bay and Ling Ao nuclear
power facilities served as intense sources of
ν
e
’s. Comparison of the
¯
ν
e
rate and energy spectrum
measured by antineutrino detectors far from the nuclear reactors (∼1500–1950 m) relative to detectors
near the reactors (∼350–600 m) allowed a precise measurement of
¯
ν
e
disappearance. More than
2.5 million
¯
ν
e
inverse beta-decay interactions were observed, based on the combination of 217 days of
operation of six antineutrino detectors (December, 2011–July, 2012) with a subsequent 1013 days using
the complete configuration of eight detectors (October, 2012–July, 2015). The
¯
ν
e
rate observed at the far
detectors relative to the near detectors showed a significant deficit, R ¼ 0.949 0.002ðstatÞ
0.002ðsystÞ. The energy dependence of
¯
ν
e
disappearance showed the distinct variation predicted by
neutrino oscillation. Analysis using an approximation for the three-flavor oscillation probability yielded
the flavor-mixing angle sin
2
2θ
13
¼ 0. 0841 0.0027ðstatÞ0.0019ðsystÞ and the effective neutrino
mass-squared difference of jΔm
2
ee
j¼ð2.50 0.06ðstatÞ0.06ðsystÞÞ × 10
−3
eV
2
. Analysis using
the exact three-flavor probability found Δm
2
32
¼ð2.45 0.06ðstatÞ0.06ðsystÞÞ × 10
−3
eV
2
assuming
the normal neutrino mass hierarchy and Δm
2
32
¼ð−2.56 0.06ðstatÞ0.06ðsystÞÞ × 10
−3
eV
2
for the
inverted hierarchy.
DOI:
10.1103/PhysRevD.95.072006
I. INTRODUCTION
Recent experiments have resulted in significant advances
in our understanding of neutrinos. Although neutrinos were
considered massless within the standard model, abundant
evidence of lepton flavor violation by neutrinos strongly
implies small but nonzero masses. A long-standing disparity
between measurement and models of the solar ν
e
flux was
corroborated by successive radiochemical
[1–3] and water-
Cherenkov [4,5] experiments. Variation of the ratio of
atmospheric ν
μ
to ν
e
provided initial evidence for dis-
tance-dependent neutrino disappearance
[6]. Subsequent
observation of the disappearance of ν
μ
produced in particle
accelerators confirmed atmospheric ν measurements
[7].
A comparison of the solar ν
e
to the total solar ν flux showed
that the apparent disappearance was a consequence of the
conversion of ν
e
’s to other neutrino flavors
[8,9].
Disappearance of
¯
ν
e
’s emitted by nuclear reactors demon-
strated a distinct dependence on the ratio of propagation
distance to antineutrino energy, L=E
ν
, cementing neutrino
flavor oscillation as the explanation for the observed flavor
violation
[10].
The rich phenomena of neutrino flavor oscillation arise
from two remarkable characteristics of neutrinos: small
differences between the masses of the three neutrino states,
m
1
≠ m
2
≠ m
3
, and an inequivalence between neutrino
flavor and mass eigenstates. Produced in a flavor eigenstate
*
Department of Chemistry and Chemical Technology, Bronx
Community College, Bronx, New York 10453, USA.
F. P. AN et al. PHYSICAL REVIEW D 95, 072006 (2017)
072006-2
by the weak interaction, a neutrino state evolves as a
coherent superposition of mass eigenstates. Interference
between the phases of each mass component results in the
oscillation of the neutrino flavor. The flavor oscillates with
phases given by Δm
2
ji
L=4E
ν
, where L is the propagation
distance, E
ν
is the neutrino energy, and Δm
2
ji
¼ m
2
j
− m
2
i
is
the difference of the squared masses. The amplitude of
flavor oscillation is determined by the amount of mixing
between the flavor and mass eigenstates. Using the unitary
Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix,
U
PMNS
, a neutrino with flavor α can be expressed as a
combination of mass states, jν
α
i¼
P
i
U
αi
jν
i
i. In the three-
flavor model, U
PMNS
is commonly parametrized using three
mixing angles, θ
12
, θ
23
, θ
13
, and an off-diagonal CP-
violating phase δ
CP
. With sensitivity to the small neutrino
mass separations, oscillation experiments have provided
strong evidence for three distinct neutrino mass states ν
i
with masses m
1
, m
2
, and m
3
. Evidence for matter-enhanced
resonant flavor conversion of solar neutrinos has shown
that Δm
2
21
cosð2θ
21
Þ > 0. Whether m
3
is much lighter or
heavier than m
1
and m
2
, also referred to as the neutrino
mass hierarchy, is currently unknown and is the focus of a
broad experimental program
[11]. Direct measurements of
decay kinematics and indirect cosmological observations
are currently consistent with massless neutrinos, implying
that the absolute masses are less than ∼1 eV. Neutrino
mass qualitatively alters the standard model, for example
by inhibiting renormalization or by requiring a new global
symmetry [12,13].
The Daya Bay Reactor Neutrino Experiment set out to
answer the question: Does the ν
3
mass eigenstate mix with
the electron neutrino state ν
e
? This is equivalent to asking
whether the parameter θ
13
is nonzero. Solar and reactor
experiments have established significant mixing between
the ν
e
and ν
1;2
states, given by sin
2
ð2θ
12
Þ¼0.846 0.021
[14]. Atmospheric and accelerator experiments yielded
nearly maximal mixing of the ν
μ
and ν
2;3
states, with
sin
2
ð2θ
23
Þ¼0.999
þ0.001
−0.018
[14]. Previous searches found no
evidence of
¯
ν
e
disappearance at ∼1 km from reactors,
limiting sin
2
2θ
13
≤ 0.17 at the 90% C.L.
[15,16].
Measurement of θ
13
provides a key parameter of a new
standard model which incorporates massive neutrinos, and
may allow a deeper insight into the flavor and mass structure
of nature. A nonzero value for θ
13
also makes it possible for
future experiments to determine the neutrino mass hierarchy
and to search for neutrino CP violation
[11].
Nuclear reactors produce an intense and pure flux of
¯
ν
e
’s, which is useful for experimental searches for θ
13
.
Approximately 2 × 10
20
¯
ν
e
’s per second are emitted per
gigawatt of thermal power, with a steeply falling energy
spectrum showing minuscule flux above 10 MeV.
Section
V gives further details of
¯
ν
e
emission by nuclear
reactors. Reactor
¯
ν
e
are most commonly detected via
inverse beta decay (IBD),
¯
ν
e
þ p → e
þ
þ n: ð1Þ
Convolving the energy spectrum with the IBD cross section
[17] results in an expected spectrum which rises with
neutrino energy from the 1.8 MeV interaction threshold,
peaks at ∼4 MeV, and falls to a very low rate above 8 MeV.
Charge-current interactions of
¯
ν
μ
or
¯
ν
τ
at these energies are
forbidden by energy conservation; hence oscillation is
observed as a reduction, or disappearance, of the expected
¯
ν
e
signal. In the three-flavor model of neutrino oscillation,
the survival probability of detecting an
¯
ν
e
of energy E
ν
at a
distance L from the production source can be expressed as
P
sur
¼ 1 − cos
4
θ
13
sin
2
2θ
12
sin
2
Δ
21
− sin
2
2θ
13
ðcos
2
θ
12
sin
2
Δ
31
þ sin
2
θ
12
sin
2
Δ
32
Þ; ð2Þ
where Δ
ji
≃ 1.267Δm
2
ji
ðeV
2
ÞLðmÞ=E
ν
ðMeVÞ. The
KamLAND experiment measured the first term, demon-
strating large-amplitude disappearance of reactor
¯
ν
e
with an
oscillation length of ∼60 km. Atmospheric and accelerator
ν measurements of jΔm
2
32
j predict an oscillation length of
∼1.6 km for the latter terms. At this distance, the two
oscillation phases Δ
31
and Δ
32
are indistinguishable.
Therefore, the expression can be approximated using a
single effective
¯
ν
e
disappearance phase Δ
ee
,
P
sur
≃ 1 − cos
4
θ
13
sin
2
2θ
12
sin
2
Δ
21
− sin
2
2θ
13
sin
2
Δ
ee
; ð3Þ
which is independent of the neutrino mass hierarchy. Here
the definition of Δm
2
ee
is empirical; it is the mass-squared
difference obtained by modeling the observed reactor
¯
ν
e
disappearance using Eq.
(3). The mass-squared differences
obtained by modeling an observation using either Eq.
(2)
or Eq. (3) are expected to follow the relation Δm
2
ee
≃
cos
2
θ
12
jΔm
2
31
jþsin
2
θ
12
jΔm
2
32
j
[18]. Based on current
measurements, jΔm
2
31
j ≃ jΔm
2
ee
j2.3 × 10
−5
eV
2
and
jΔm
2
32
j ≃ jΔm
2
ee
j ∓ 5.2 × 10
−5
eV
2
, assuming the normal
(upper sign) or inverted (lower sign) mass hierarchy.
Previous searches for oscillation due to θ
13
were limited
by uncertainty in the
¯
ν
e
flux emitted by reactors
[15,16].
A differential comparison with an additional detector
located near the reactor was proposed to overcome this
uncertainty
[19]. With a far-versus-near detector arrange-
ment, sensitivity to neutrino oscillation depends on relative
uncertainties between detectors in the number of target
protons N
p
,
¯
ν
e
detection efficiency ϵ, and distances from the
reactor L. If these relative uncertainties are well controlled,
small differences in the oscillation survival probability P
sur
become detectable in the ratio of the number of
¯
ν
e
interactions in the far relative to near detector,
MEASUREMENT OF ELECTRON ANTINEUTRINO … PHYSICAL REVIEW D 95, 072006 (2017)
072006-3
N
f
N
n
¼
N
p;f
N
p;n
L
n
L
f
2
ϵ
f
ϵ
n
P
sur
ðE
ν
;L
f
Þ
P
sur
ðE
ν
;L
n
Þ
: ð4Þ
Three experiments were constructed based on this
technique: the Daya Bay
[20], RENO [21], and Double
Chooz experiments
[22]. In March, 2012, the Daya Bay
experiment reported the discovery of
¯
ν
e
disappearance due
to a nonzero value of θ
13
[23]. Oscillation due to θ
13
has
since been confirmed by the other experiments [24,25],as
well as by other techniques
[26,27]. The relatively large θ
13
mixing has also allowed measurement of jΔm
2
ee
j from the
variation of the disappearance probability versus
¯
ν
e
energy
[28]. Compatibility of the mass-squared difference with
that obtained from the disappearance of accelerator and
atmospheric ν
μ
’s with GeV-energies firmly establishes the
three-flavor model of neutrino mass and mixing.
This paper provides a detailed review of the Daya Bay
measurement of neutrino oscillation. Section
II gives an
overview of the experiment. The calibration and charac-
terization of the experiment are presented in Sec. III.
Identification of reactor
¯
ν
e
interactions, signal efficiencies,
and assessment of backgrounds are discussed in Sec.
IV.
Section V presents an analysis of neutrino oscillation using
the measured
¯
ν
e
rate and spectra, while Sec.
VI contains
concluding remarks.
II. EXPERIMENT DESCRIPTION
The relative measurement of oscillation, as summarized
in Eq.
(4), motivated much of the design of the Daya Bay
experiment. The disappearance signal is most pronounced at
the first oscillation minimum of P
sur
. Based on existing
accelerator and atmospheric ν
μ
measurements of Δm
2
32
, this
corresponded to a distance L
f
≈ 1.6 km for reactor
¯
ν
e
with a
mean energy of ∼4 MeV. Significant
¯
ν
e
disappearance in
the near detectors would have reduced the overall sensitivity
of the far-to-near comparison, so L
n
was kept to ∼500 mor
less. The use of identically designed modular detectors
limited variationsinrelative number of target protonsN
p
and
efficiency ϵ between detectors. Situating detectors at a
sufficient depth underground reduced muon-induced neu-
trons and short-lived isotopes, the most prominent back-
grounds for reactor
¯
ν
e
detection. Statistical sensitivity
increases with
¯
ν
e
flux, target size, and detector efficiency,
arguing for the use of intense reactors and large detectors.
The campus of the Daya Bay nuclear power plant near
Shenzhen, China was well suited for this purpose. At the
time of this measurement the facility consisted of six
2.9 GWth pressurized water reactors and produced roughly
3.5 × 10
21
¯
ν
e
=s, making it one of the most intense
¯
ν
e
sources on Earth. Steep mountains adjacent to the reactors
provided ample shielding from muons produced by cosmic
ray showers. Underground experimental halls were exca-
vated to accommodate 160 tons of fiducial target mass for
¯
ν
e
interactions, equally divided between locations near and
far from the reactors. With this arrangement, a total of
∼2000
¯
ν
e
interactions per day were detected near to, and
∼250 far from, the reactors, with muon-induced back-
grounds contributing less than 0.5%. The target mass was
divided between eight identically designed modular anti-
neutrino detectors (ADs). Installing at least two ADs in
each experimental hall allowed side-by-side demonstration
of <0.2% variation in
¯
ν
e
detection efficiency between
detectors. A confirmation of the side-by-side performance
of the first two ADs was given in
[29]. These basic
characteristics have yielded measurements of sin
2
2θ
13
with ∼4% precision and jΔm
2
ee
j with ∼3% precision, as
discussed in this paper. This section provides an abbre-
viated description of the Daya Bay experiment, while a
more detailed description is given in
[30].
The reactors at Daya Bay were arranged in two clusters:
the Daya Bay cluster hosted two reactors (D1 and D2),
while the Ling Ao cluster hosted four (L1, L2, L3 and L4).
Correspondingly, four near detectors were divided between
two near experimental halls (EH1 and EH2) near the two
clusters. The remaining four detectors were installed in a
single far hall (EH3). The locations of the experimental
halls were determined to optimize sensitivity to θ
13
,
considering reactor locations and mountain topography.
While uncertainties in reactor flux were not completely
canceled as would happen for the case of a single reactor,
this arrangement of detectors reduced the far-to-near flux
ratio uncertainty to ≤0.1% (see Sec.
V). The layout of the
six reactors and three experimental halls is shown in Fig.
1.
FIG. 1. Layout of the Daya Bay experiment. The Daya Bay and
Ling Ao nuclear power plant (NPP) reactors (red circles) were
situated on a narrow coastal shelf between the Daya Bay coastline
and inland mountains. Two antineutrino detectors installed in
each underground experimental hall near to the reactors (EH1 and
EH2) measured the
¯
ν
e
flux emitted by the reactors, while four
detectors in the far experimental hall (EH3) measured a deficit in
the
¯
ν
e
flux due to oscillation. The detectors were built and
initially tested in a surface assembly building (SAB), transpor ted
to a liquid scintillator hall (LS Hall) for filling, and then installed
in an experimental hall.
F. P. AN et al. PHYSICAL REVIEW D 95, 072006 (2017)
072006-4
When comparing the measurements between near and
far detectors, the largest relative correction was from the
baselines of the detectors, as seen in Eq. (4). Accurate
surveys of the experiment site allowed precise correction
for this effect. Surveys consisted of total station electronic
theodolite measurements combined with supplemental
global positioning system (GPS) measurements. Lacking
GPS reception underground, surveys of the experimental
halls and access tunnels relied on redundant total station
measurements. Table
I provides the surveyed reactor and
detector coordinates, where X is due north and Z is vertical
at the survey origin. Uncertainties in the survey results were
18 mm in each coordinate, dominated by the precision of
the GPS measurements and the tension between GPS and
total station survey results.
¯
ν
e
emission was distributed
throughout the fuel elements of each reactor core, spanning
a region 3.7 m in height and 3 m in diameter. Reactor
models established the horizontal centroid of
¯
ν
e
emission to
within 2 cm of the geometric center of each core. With the
centroid determined, the spatial variation of the distribution
of
¯
ν
e
emission within the core had negligible impact to the
oscillation measurement.
The combination of organic liquid scintillator with
photomultiplier tubes (PMTs) results in a powerful tech-
nique for reactor
¯
ν
e
detection. Scintillator contains protons
(as
1
H) which serve as targets for
¯
ν
e
inverse beta-decay
interactions [see Eq.
(1)]. Scintillators simultaneously
function as a sensitive medium, emitting photons in
response to ionization by the products of IBD interactions.
Detection of the photons using PMTs allows a calorimetric
measurement of the prompt positron energy deposition.
This energy is the sum of the IBD positron kinetic plus
annihilation energy, E
prompt
¼ T
e
þ
þ 2m
e
, where m
e
is the
mass of the electron. The initial
¯
ν
e
energy can be accurately
estimated using E
ν
≃ E
prompt
þ 0.8 MeV, based on the
kinematics of inverse beta decay. The IBD neutron gen-
erally carries only a small fraction of the initial kinetic
energy, Oð10 keVÞ. The neutron thermalizes and is then
captured on a nucleus within the scintillator in a time of
Oð100 μsÞ. The resulting nucleus rapidly deexcites by
emitting one or more characteristic γ rays. Detection of
this subsequent pulse of scintillation light from the delayed
neutron capture γ rays efficiently discriminates
¯
ν
e
inter-
actions from background.
The eight antineutrino detectors of the Daya Bay experi-
ment relied on this technique, and were designed to
specifically limit potential variations in response and
efficiency between detectors. Each detector consisted of
a nested three-zone structure, as shown in Fig.
2. The
central
¯
ν
e
target was 20 tons of linear-alkyl-benzene-based
liquid scintillator, loaded with 0.1% of
nat
Gd by mass
(GdLS). Details of the production and composition of the
scintillator are discussed in
[31]. Gadolinium (Gd) effi-
ciently captures thermalized neutrons, emitting a few γ rays
with a total energy of ∼8 MeV per capture. The relatively
high capture energy enhanced discrimination of the signal
from backgrounds produced by natural radioactivity, pri-
marily at energies ≲5 MeV. Gd loading also provided a
physical method to fiducialize the detector, allowing
efficient rejection of
¯
ν
e
interactions which occurred outside
TABLE I. The surveyed coordinates of the geometric centers of the nuclear reactor cores and antineutrino detectors of the Daya Bay
experiment. The detectors are labeled as AD1 through AD8, according to their order of as sembly and installation. The X coordinate is
due north, whil e the Z coordinate is vertical at the survey origin. Coordinates were determined from a combination of total station
electronic theodolite and GPS measurements, with a precision of 18 mm. The corresponding neutrino oscillation baselines for each
reactor-detector pair are provided. The approximate rock overburden of each experimental hall and the mass of the GdLS antineutrino
target in each detector are also given in both meters and meters-water equivalent. The average thermal power of each reactor core, in
gigawatts, is given separately for the six detector and eight detector periods.
Reactor
D1 D2 L1 L2 L3 L4
¯
W
6AD
th 2.082 2.874 2.516 2.554 2.825 1.976
¯
W
8AD
th 2.514 2.447 2.566 2.519 2.519 2.550
X [m] 359.20 448.00 −319.67 −267.06 −543.28 −490.69
Y [m] 411.49 411.00 −540.75 −469.21 −954.70 −883.15
Z [m] −40.23 −40.24 −39.73 −39.72 −39.80 −39.79
Hall Depth [m(mwe)] Detector Target [kg] X [m] Y [m] Z [m] Baseline [m]
EH1 93 AD1 19941 3 362.83 50.42 −70.82 362.38 371.76 903.47 817.16 1353.62 1265.32
(250) AD2 19967 3 358.80 54.86 −70.81 357.94 368.41 903.35 816.90 1354.23 1265.89
EH2 100 AD3 19891 4 7.65 −873.49 −67.52 1332.48 1358.15 467.57 489.58 557.58 499.21
(265) AD8 19944 5 9.60 −
879.15 −67.52 1337.43 1362.88 472.97 495.35 558.71 501.07
EH3 AD4 19917 4 936.75 −1419.01 −66.49 1919.63 1894.34 1533.18 1533.63 1551.38 1524.94
324 AD5 19989 3 941.45 −1415.31 −66.50 1917.52 1891.98 1534.92 1535.03 1554.77 1528.05
(860) AD6 19892 3 940.46 −1423.74 −66.50 1925.26 1899.86 1538.93 1539.47 1556.34 1530.08
AD7 19931 3 945.17 −1420.03 −66.49 1923.15 1897.51 1540.67 1540.87 1559.72 1533.18
MEASUREMENT OF ELECTRON ANTINEUTRINO … PHYSICAL REVIEW D 95, 072006 (2017)
072006-5