8 Neutrino Detectors
349
steel sphere on which are mounted 2200 photomultipliers. The sphere is filled with
a pseudocumene solvent with a quencher acting as a shield for radioactivity coming
mainly from the tubes and is itself immersed in a water Cerenkov tank viewed by
an additional 200 photomultipliers to identify cosmic ray muons. The light yield
of the detector is 500 photoelectrons/MeV actually recorded. Timing information
from the photomultipliers allow the spatial reconstruction of the event and hence the
determination that it occurred within the fiducial volume. The α and β + components
of natural radioactivity can be reduced by pulse shape discrimination whereas
the β − and γ components are indistinguishable from the signal. A reduction and
thorough understanding of the background has allowed them to observe solar
neutrinos with an energy as low as 150 keV and, hence, make the first direct
observation of pp fusion solar neutrinos as well as measure the solar beryllium line
and geoneutrinos [35]. After a year during which the background has been reduced
through six cycles of water extraction the radiopurity levels are now 2.7 × 10 −18 for
14 C/ 12 C and, at 95% CL, <9.7 × 10 −19 g · g −1 for uranium and <1.2 × 10 −18 g · g −1
for thorium. This will allow improved measurements of solar and geoneutrinos as
well as a new very short baseline neutrino oscillation project, SOX [36].
SOX is intended to search for oscillations of decay ν e or ¯
ν e from a radioactive
source into sterile neutrinos at the level of m 2 of 1 eV 2 . The sources being
considered are 51 Cr and 144 Ce, with the latter already approved. The 144 Ce would
be placed in a pit under the Borexino detector. The small size of the 3.7–5.0 PBq
source (about 1 L) coupled with the large 7 m size of the detector and its good spatial
resolution of 12 cm and energy resolution of 3.5% would allow the observation of
oscillation waves as a function of L/E within the detector as well as an overall
measurement of ¯
ν e disappearance. Given an existing detector, the most taxing task
is the source. It would be produced in a Russian laboratory from the reprocessing
of nuclear fuel and must then be extensively shielded and transported to the Gran
Sasso by a circuitous route for safety reasons.
Totally active liquid scintillator detectors have also been used in accelerator
experiments producing higher energy neutrinos. MiniBooNE [37], looking for
ν μ → ν e oscillations in the Fermilab Booster neutrino beam in order to investigate
the LSND signal [38], is exposed to neutrinos of about 1 GeV. The detector consists
of 800 tons of mineral oil (CH 2 ) held in a spherical tank. The density of the oil
is 0.86 g · cm −3 and has an index of refraction of 1.47. The light attenuation in this
medium varies from a few cm at 280 nm to 20 m at 400 nm. The inner region (575 cm
radius) is viewed by 1280 8-inch photomultipliers held on an optical barrier that
separates it from a 35 cm thick outer region. This outer region, itself viewed by 240
tubes is used to veto events caused by charged particles entering the detector and to
tag events that include particles exitting the detector in order to identify contained
events. Cosmic ray events are greatly reduced by restricting the triggers to those
occurring within a 19.2 μs window starting 4.4 μs before the 1.6 μs long beam spill.
The energy of an event is related to the total amount of light observed. ν μ and ν e
events are identified by the flavour (muon or electron) of the lepton in the final state
CC interaction. Muons are distinguished from electrons using the light pattern of
their Cerenkov rings as shown in Figs. 8.7 and 8.8. Muons give a sharp ring filled on
349
steel sphere on which are mounted 2200 photomultipliers. The sphere is filled with
a pseudocumene solvent with a quencher acting as a shield for radioactivity coming
mainly from the tubes and is itself immersed in a water Cerenkov tank viewed by
an additional 200 photomultipliers to identify cosmic ray muons. The light yield
of the detector is 500 photoelectrons/MeV actually recorded. Timing information
from the photomultipliers allow the spatial reconstruction of the event and hence the
determination that it occurred within the fiducial volume. The α and β + components
of natural radioactivity can be reduced by pulse shape discrimination whereas
the β − and γ components are indistinguishable from the signal. A reduction and
thorough understanding of the background has allowed them to observe solar
neutrinos with an energy as low as 150 keV and, hence, make the first direct
observation of pp fusion solar neutrinos as well as measure the solar beryllium line
and geoneutrinos [35]. After a year during which the background has been reduced
through six cycles of water extraction the radiopurity levels are now 2.7 × 10 −18 for
14 C/ 12 C and, at 95% CL, <9.7 × 10 −19 g · g −1 for uranium and <1.2 × 10 −18 g · g −1
for thorium. This will allow improved measurements of solar and geoneutrinos as
well as a new very short baseline neutrino oscillation project, SOX [36].
SOX is intended to search for oscillations of decay ν e or ¯
ν e from a radioactive
source into sterile neutrinos at the level of m 2 of 1 eV 2 . The sources being
considered are 51 Cr and 144 Ce, with the latter already approved. The 144 Ce would
be placed in a pit under the Borexino detector. The small size of the 3.7–5.0 PBq
source (about 1 L) coupled with the large 7 m size of the detector and its good spatial
resolution of 12 cm and energy resolution of 3.5% would allow the observation of
oscillation waves as a function of L/E within the detector as well as an overall
measurement of ¯
ν e disappearance. Given an existing detector, the most taxing task
is the source. It would be produced in a Russian laboratory from the reprocessing
of nuclear fuel and must then be extensively shielded and transported to the Gran
Sasso by a circuitous route for safety reasons.
Totally active liquid scintillator detectors have also been used in accelerator
experiments producing higher energy neutrinos. MiniBooNE [37], looking for
ν μ → ν e oscillations in the Fermilab Booster neutrino beam in order to investigate
the LSND signal [38], is exposed to neutrinos of about 1 GeV. The detector consists
of 800 tons of mineral oil (CH 2 ) held in a spherical tank. The density of the oil
is 0.86 g · cm −3 and has an index of refraction of 1.47. The light attenuation in this
medium varies from a few cm at 280 nm to 20 m at 400 nm. The inner region (575 cm
radius) is viewed by 1280 8-inch photomultipliers held on an optical barrier that
separates it from a 35 cm thick outer region. This outer region, itself viewed by 240
tubes is used to veto events caused by charged particles entering the detector and to
tag events that include particles exitting the detector in order to identify contained
events. Cosmic ray events are greatly reduced by restricting the triggers to those
occurring within a 19.2 μs window starting 4.4 μs before the 1.6 μs long beam spill.
The energy of an event is related to the total amount of light observed. ν μ and ν e
events are identified by the flavour (muon or electron) of the lepton in the final state
CC interaction. Muons are distinguished from electrons using the light pattern of
their Cerenkov rings as shown in Figs. 8.7 and 8.8. Muons give a sharp ring filled on