8 Neutrino Detectors
351
oil with 5% pseudocumene. Since one of its physics goals is ν e appearance it must
be fine-grained enough to identify electrons and distinguish them from the showers
produced from the decay photons of π 0 mesons. To this end, each plane corresponds
to a sampling frequency of only 17% of a radiation length. The WLS fibres are in
the shape of a loop and are read at the end opposite the loop. Avalanche photodiodes
with a quantum efficicency of about 80% are used and detect 40 photoelectrons for
a minimum ionizing particle crossing a tube at the far end. They are produced in
arrays of 16 diodes each with a cross-section of 1.8 × 1.0 mm 2 and must be run at
a temperature of −15 ◦ C. Each diode observes both ends of a fibre. The detector
has an overall mass of 14 kilotons, consisting 70% of scintillator and the remainder
of PVC. Its overall length is 67 m, with a cross section of 15.7 × 15.7 m 2 . The
detector is located on the surface but the impact of cosmic rays is mitigated by the
short beam spill of 10 μs and the speed of the photodiodes. Nonetheless, to reduce
the electromagnetic component of cosmic rays, the detector is covered by a 3 m
overburden of concrete and barite.
Totally active tracking detectors can also be made of extruded solid scintillator
bars usually read with WLS fibres embedded in a hole or a groove made in
the scintillator. An example of such a detector is SciBar [40], a 15 ton detector
consisting of 14,336 strips each of dimensions 1.5 × 2.5 × 300 cm 3 and using 64pixel multianode photomultipliers. It was first used in Japan on the KEK neutrino
beam line and then moved to the NuMI beam line at Fermilab in the US.
Coherent Elastic Neutrino-Nucleus Scattering, CEνNS , is a process in which the
neutrino interacts with the whole nucleus rather than with individual nucleons [41],
leaving the nucleus whole and carrying very little energy since the momentum
transfer must be small. The recoiling nucleus subsequently produces secondary
recoils and scintillation light. The CEνNS cross section is several orders of
magnitude larger than, for instance, the IBD cross section as it depends on the square
of the number of neutrons in the nucleus. However, the smallness of the energy
release made the process impossible to measure until recently. The COHERENT
experiment [42] overcame this difficulty by using a 14.6 kg sodium-doped CsI
crystal 34 cm long. The heavy cesium and iodine nuclei, provide the large cross
sections and the large scintillation light yield necessary to detect low energy recoil
nuclei down to a few keV. The crystal was read by a super bialkali low background
Hamamatsu R877-100. The source of neutrinos was the decay of pions and muons
produced by the Spallation Neutron Source at Oak Ridge National Laboratory.
Protons on target(POTs) were delivered in 1 μs long spills at a rate of 60 Hz resulting
in 4 × 10 18 isotropically emitted neutrinos per day. The detector was placed in a
basement corridor at a location, Fig. 8.9, that provided 12 m of neutron-moderating
concrete and gravel in the direct line of sight to the SNS target, thus reducing
neutron-induced recoil nuclei background (NIN) to an acceptable level. Cosmic
rays were also reduced with an 8 m water overburden. The detector was enclosed
in high-density polyethylene, to reduce NIN, as well as in both low activity and
in standard lead. Muon vetos and water tanks containing a neutron moderator
completed the shielding of the detector. The photomutiplier signals were amplified
and digitized at 500 MSamples/s over 70 μs intervals starting 55 μs before the POT
351
oil with 5% pseudocumene. Since one of its physics goals is ν e appearance it must
be fine-grained enough to identify electrons and distinguish them from the showers
produced from the decay photons of π 0 mesons. To this end, each plane corresponds
to a sampling frequency of only 17% of a radiation length. The WLS fibres are in
the shape of a loop and are read at the end opposite the loop. Avalanche photodiodes
with a quantum efficicency of about 80% are used and detect 40 photoelectrons for
a minimum ionizing particle crossing a tube at the far end. They are produced in
arrays of 16 diodes each with a cross-section of 1.8 × 1.0 mm 2 and must be run at
a temperature of −15 ◦ C. Each diode observes both ends of a fibre. The detector
has an overall mass of 14 kilotons, consisting 70% of scintillator and the remainder
of PVC. Its overall length is 67 m, with a cross section of 15.7 × 15.7 m 2 . The
detector is located on the surface but the impact of cosmic rays is mitigated by the
short beam spill of 10 μs and the speed of the photodiodes. Nonetheless, to reduce
the electromagnetic component of cosmic rays, the detector is covered by a 3 m
overburden of concrete and barite.
Totally active tracking detectors can also be made of extruded solid scintillator
bars usually read with WLS fibres embedded in a hole or a groove made in
the scintillator. An example of such a detector is SciBar [40], a 15 ton detector
consisting of 14,336 strips each of dimensions 1.5 × 2.5 × 300 cm 3 and using 64pixel multianode photomultipliers. It was first used in Japan on the KEK neutrino
beam line and then moved to the NuMI beam line at Fermilab in the US.
Coherent Elastic Neutrino-Nucleus Scattering, CEνNS , is a process in which the
neutrino interacts with the whole nucleus rather than with individual nucleons [41],
leaving the nucleus whole and carrying very little energy since the momentum
transfer must be small. The recoiling nucleus subsequently produces secondary
recoils and scintillation light. The CEνNS cross section is several orders of
magnitude larger than, for instance, the IBD cross section as it depends on the square
of the number of neutrons in the nucleus. However, the smallness of the energy
release made the process impossible to measure until recently. The COHERENT
experiment [42] overcame this difficulty by using a 14.6 kg sodium-doped CsI
crystal 34 cm long. The heavy cesium and iodine nuclei, provide the large cross
sections and the large scintillation light yield necessary to detect low energy recoil
nuclei down to a few keV. The crystal was read by a super bialkali low background
Hamamatsu R877-100. The source of neutrinos was the decay of pions and muons
produced by the Spallation Neutron Source at Oak Ridge National Laboratory.
Protons on target(POTs) were delivered in 1 μs long spills at a rate of 60 Hz resulting
in 4 × 10 18 isotropically emitted neutrinos per day. The detector was placed in a
basement corridor at a location, Fig. 8.9, that provided 12 m of neutron-moderating
concrete and gravel in the direct line of sight to the SNS target, thus reducing
neutron-induced recoil nuclei background (NIN) to an acceptable level. Cosmic
rays were also reduced with an 8 m water overburden. The detector was enclosed
in high-density polyethylene, to reduce NIN, as well as in both low activity and
in standard lead. Muon vetos and water tanks containing a neutron moderator
completed the shielding of the detector. The photomutiplier signals were amplified
and digitized at 500 MSamples/s over 70 μs intervals starting 55 μs before the POT