8 Neutrino Detectors
343
interactions since it is energetically impossible to produce μ’s or τ ’s. Oscillations
can then only be observed through the disappearance technique resulting in a
reduction and distortion of the expected ¯
ν e spectrum. Given the energy of reactor
¯
ν e ’s (a few MeV) and the value of the atmospheric m 2 , CHOOZ [19] was located
1000 m from a reactor complex in order to be near oscillation maximum. It used
a single large tank of liquid scintillator and was subjected to a cosmic muon rate
of 0.4 m −2 s −1 . One of the major backgrounds in this type of experiment is the
background generated by cosmic ray muons. The first line of defense is to place
the detector underground, at a depth of 300 m water equivalent (m.w.e) in the case
of CHOOZ. A muon traversing the detector does not, in itself, simulate a signal
event because the large amount of energy deposited can be well identified. However
neutrons produced by muons traversing dead areas of the detector or the surrounding
rock can elastically scatter on a proton, causing the proton and the subsequent
neutron capture to simulate the signature of a reactor event. This background can
be eliminated by vetoing on the passage of a nearby muon. In addition cosmic
muons can produce long lived isotopes such as 6 He and 9 Li which subsequently
can beta decay producing an electron and a neutron, thus simulating an antineutrino
event. This background cannot be eliminated by vetoing on the passage of a muon
because of the long lifetime of these decays (178 ms in the case of 9 Li) which
would introduce an inordinate dead time. It must be estimated and subtracted. Palo
Verde [20] was located at a shallower depth of 32 m.w.e. and chose to use acrylic
cells filled with liquid scintillator. This extra segmentation was needed to reduce the
larger muon induced neutron background caused by the larger cosmic muon flux of
22 m −2 s −1 at this depth. Instead of cadmium, these experiments have been using a
0.1% admixture of gadolinium with a large neutron absorption cross section leading
to an 84% capture fraction. Absorption in gadolinium leads to the emission of
gamma rays with a total energy of 8 MeV, within ∼30 μs and ∼6 cm of the positron
annihilation, thus providing a well recognizable delayed coincidence. The CHOOZ
target scintillator consisted of 50% by volume Norpar-15 [21] and IPB + hexanol
(also 50% by volume). The wave-length shifters were p-PTP and bis-MSB (1 g/l).
The gadolinium was introduced as a solution of Gd(NO 3 ) 3 in hexanol. Because
of oxygenation of the nitrate the 4 m light attenuation length in the scintillator
decreased with time at a rate of (4.2 ± 0.4) · 10 −3 per day. This required a careful
monitoring of the scintillator transparency using calibration sources. The light yield
was 5300 photons/MeV.
The observation of a modification of the expected ¯
ν e spectrum necessitates a
very precise knowledge of the antineutrino flux emitted by the reactor as well as of
the antineutrino interaction cross section. They failed to observe a disappearance of
antineutrinos and the limit set on this oscillation was governed by these sources of
systematics uncertainty. In order to overcome these limitations more recent reactor
oscillation experiments use a second identical detector located close to the reactor in
order to measure the expected interaction rate before the neutrinos have a chance to
oscillate. The detector used by one such experiment, Double Chooz [22], located at
the same location as CHOOZ but using, in addition, a near detector placed at 410 m
from the reactors, will be described as an example. The scintillator, amounting to
343
interactions since it is energetically impossible to produce μ’s or τ ’s. Oscillations
can then only be observed through the disappearance technique resulting in a
reduction and distortion of the expected ¯
ν e spectrum. Given the energy of reactor
¯
ν e ’s (a few MeV) and the value of the atmospheric m 2 , CHOOZ [19] was located
1000 m from a reactor complex in order to be near oscillation maximum. It used
a single large tank of liquid scintillator and was subjected to a cosmic muon rate
of 0.4 m −2 s −1 . One of the major backgrounds in this type of experiment is the
background generated by cosmic ray muons. The first line of defense is to place
the detector underground, at a depth of 300 m water equivalent (m.w.e) in the case
of CHOOZ. A muon traversing the detector does not, in itself, simulate a signal
event because the large amount of energy deposited can be well identified. However
neutrons produced by muons traversing dead areas of the detector or the surrounding
rock can elastically scatter on a proton, causing the proton and the subsequent
neutron capture to simulate the signature of a reactor event. This background can
be eliminated by vetoing on the passage of a nearby muon. In addition cosmic
muons can produce long lived isotopes such as 6 He and 9 Li which subsequently
can beta decay producing an electron and a neutron, thus simulating an antineutrino
event. This background cannot be eliminated by vetoing on the passage of a muon
because of the long lifetime of these decays (178 ms in the case of 9 Li) which
would introduce an inordinate dead time. It must be estimated and subtracted. Palo
Verde [20] was located at a shallower depth of 32 m.w.e. and chose to use acrylic
cells filled with liquid scintillator. This extra segmentation was needed to reduce the
larger muon induced neutron background caused by the larger cosmic muon flux of
22 m −2 s −1 at this depth. Instead of cadmium, these experiments have been using a
0.1% admixture of gadolinium with a large neutron absorption cross section leading
to an 84% capture fraction. Absorption in gadolinium leads to the emission of
gamma rays with a total energy of 8 MeV, within ∼30 μs and ∼6 cm of the positron
annihilation, thus providing a well recognizable delayed coincidence. The CHOOZ
target scintillator consisted of 50% by volume Norpar-15 [21] and IPB + hexanol
(also 50% by volume). The wave-length shifters were p-PTP and bis-MSB (1 g/l).
The gadolinium was introduced as a solution of Gd(NO 3 ) 3 in hexanol. Because
of oxygenation of the nitrate the 4 m light attenuation length in the scintillator
decreased with time at a rate of (4.2 ± 0.4) · 10 −3 per day. This required a careful
monitoring of the scintillator transparency using calibration sources. The light yield
was 5300 photons/MeV.
The observation of a modification of the expected ¯
ν e spectrum necessitates a
very precise knowledge of the antineutrino flux emitted by the reactor as well as of
the antineutrino interaction cross section. They failed to observe a disappearance of
antineutrinos and the limit set on this oscillation was governed by these sources of
systematics uncertainty. In order to overcome these limitations more recent reactor
oscillation experiments use a second identical detector located close to the reactor in
order to measure the expected interaction rate before the neutrinos have a chance to
oscillate. The detector used by one such experiment, Double Chooz [22], located at
the same location as CHOOZ but using, in addition, a near detector placed at 410 m
from the reactors, will be described as an example. The scintillator, amounting to