8 Neutrino Detectors
345
threshold is less than 700 keV, well below the threshold of 1.022 MeV of the inverse
beta decay reaction.
Calibration of the detector is required to determine, in both detectors, the
efficiency for observing the inverse beta decay reaction, the energy scales for
positrons, neutrons and gamma, the timing of the photomultipliers and the light
transport properties. To this end gamma sources, neutron sources and laser light
flashers are used and deployed throughout the detector volumes in order to map out
the relevant parameters. In the target this is done with an articulated arm at the end
of which is mounted the calibrating device, the position of which is determined by
the length and azimuthal position of the arm. In the gamma catcher a guide tube into
which a source can be inserted at the end of a wire is used. The geometry of the tube
and the wire length determine the position of the source.
Whereas Double Chooz was the first to report a hint for a non-zero θ 13 , two
experiments have since produced the best measurements of this angle. They use
the same concept as Double Chooz but have used either a larger neutrino flux
(RENO [23]) or more detectors and more flux (Daya Bay [24]). RENO, in South
Korea, is exposed to the flux of 6 reactors in a row totalling 16.4 GW th . Its far
detector is 168 m underground and 1380 m from the central reactor whereas its
near detector is 46 m underground and 290 m from the reactor line. Its inner target
weighs 15.4 tons and is viewed by 340 photomultipliers. Daya Bay, in China, uses
eight identical detectors and is located near three reactor complexes Daya Bay,
Ling Ao I and II, a total of 17.4 GW th . Its far detector hall is 324 m underground,
1540 m from Ling Ao and 1910 m from Daya Bay and houses four detectors. One
near detector hall 363 m from the Daya Bay complex and another one about 500 m
from the Ling Ao complex each house 2 detectors. Each detector includes a 20 ton
neutrino target viewed by 192 8 photomultipliers. Their inner vetos are tanks of
water in which Cerenkov light is viewed by photomultipliers. The Daya Bay energy
resolution is σ E /E = 7.5%/
√
E. Using a variety of radioactive sources they are
able to determine the absolute neutrino energy scale to 1% and the relative energy
scale between detectors to <0.2%. The relative detection efficiency uncertainty
was 0.13% and was substantiated by comparing rates of detectors in the same
hall. Table 8.1 compares the systematic uncertainties achieved in Daya Bay to the
ones in the CHOOZ single detector experiment demonstrating the effectiveness
of a multiple detector and multiple location experiment. It should be noted that
all three experiments have observed a structure in the positron energy distribution
between 4 and 6 MeV when compared to Monte Carlo predictions based on the
present understanding of a reactor neutrino flux. This structure is also seen in their
near detectors (see for instance [25]) and its amplitude is proportional to the reactor
flux. It is therefore believed to be due to our lack of complete understanding of the
complex origin of a reactor neutrino flux.
KamLAND [26] is also an experiment observing reactor antineutrinos but studies
oscillations in the domain of the solar 2 , 7.5 × 10 −5 eV 2 , and is therefore
situated at an average distance of about 180 km from 53 Japanese power reactors
to compensate for the much smaller 2 . The same reaction and technique as
described above are used. However to observe enough events at this distance
345
threshold is less than 700 keV, well below the threshold of 1.022 MeV of the inverse
beta decay reaction.
Calibration of the detector is required to determine, in both detectors, the
efficiency for observing the inverse beta decay reaction, the energy scales for
positrons, neutrons and gamma, the timing of the photomultipliers and the light
transport properties. To this end gamma sources, neutron sources and laser light
flashers are used and deployed throughout the detector volumes in order to map out
the relevant parameters. In the target this is done with an articulated arm at the end
of which is mounted the calibrating device, the position of which is determined by
the length and azimuthal position of the arm. In the gamma catcher a guide tube into
which a source can be inserted at the end of a wire is used. The geometry of the tube
and the wire length determine the position of the source.
Whereas Double Chooz was the first to report a hint for a non-zero θ 13 , two
experiments have since produced the best measurements of this angle. They use
the same concept as Double Chooz but have used either a larger neutrino flux
(RENO [23]) or more detectors and more flux (Daya Bay [24]). RENO, in South
Korea, is exposed to the flux of 6 reactors in a row totalling 16.4 GW th . Its far
detector is 168 m underground and 1380 m from the central reactor whereas its
near detector is 46 m underground and 290 m from the reactor line. Its inner target
weighs 15.4 tons and is viewed by 340 photomultipliers. Daya Bay, in China, uses
eight identical detectors and is located near three reactor complexes Daya Bay,
Ling Ao I and II, a total of 17.4 GW th . Its far detector hall is 324 m underground,
1540 m from Ling Ao and 1910 m from Daya Bay and houses four detectors. One
near detector hall 363 m from the Daya Bay complex and another one about 500 m
from the Ling Ao complex each house 2 detectors. Each detector includes a 20 ton
neutrino target viewed by 192 8 photomultipliers. Their inner vetos are tanks of
water in which Cerenkov light is viewed by photomultipliers. The Daya Bay energy
resolution is σ E /E = 7.5%/
√
E. Using a variety of radioactive sources they are
able to determine the absolute neutrino energy scale to 1% and the relative energy
scale between detectors to <0.2%. The relative detection efficiency uncertainty
was 0.13% and was substantiated by comparing rates of detectors in the same
hall. Table 8.1 compares the systematic uncertainties achieved in Daya Bay to the
ones in the CHOOZ single detector experiment demonstrating the effectiveness
of a multiple detector and multiple location experiment. It should be noted that
all three experiments have observed a structure in the positron energy distribution
between 4 and 6 MeV when compared to Monte Carlo predictions based on the
present understanding of a reactor neutrino flux. This structure is also seen in their
near detectors (see for instance [25]) and its amplitude is proportional to the reactor
flux. It is therefore believed to be due to our lack of complete understanding of the
complex origin of a reactor neutrino flux.
KamLAND [26] is also an experiment observing reactor antineutrinos but studies
oscillations in the domain of the solar 2 , 7.5 × 10 −5 eV 2 , and is therefore
situated at an average distance of about 180 km from 53 Japanese power reactors
to compensate for the much smaller 2 . The same reaction and technique as
described above are used. However to observe enough events at this distance