8 Neutrino Detectors
355
transformation rather than a disappearance it remained to prove that the overall
flux of neutrinos, including all three flavours, was as predicted by the standard
solar model. This was achieved by SNO [51] by measuring neutral current reactions
which can occur for all three flavours since they do not have the energy constraint
of charged currents, namely the mass of the appropriate produced charged lepton.
It’s heavy water (D 2 O) target made it sensitive to three neutrino reactions, including
neutral current reactions:
• Elastic scattering on electrons: ν e,μ,τ + e − → ν e,μ,τ + e −
• Charged current absorption of neutrinos by deuterons: ν e + d → e − + p + p
• Neutral current disintegration of the deuteron with a threshold of 2.2 MeV:
ν e,μ,τ + d → ν e,μ,τ + n + p. This reaction could not be observed in light
water because of the binding energy of oxygen being larger than the solar
neutrino energies.The neutron was detected by absorption on deuterium or on
35 Cl in added MgCl 2 . At a later stage of the experiment an array of 3 He filled
proportional tubes was added providing a highly efficient neutron detection
through the reaction 3 He + n → p + 3 H + 764 keV.
Because of the overall neutron production of only a few tens per day, care had
to be given to select radiopure materials. The detector is located in a mine at
a depth of 6000 m.w.e, thus reducing the cosmic ray background to 70/day. It
consists of an acrylic sphere containing 1000 tons of heavy water viewed by 9438
photomultipliers. It is immersed in a structure containing light water for shielding
and support. The proportional counters were placed in the heavy water in a lattice
with 1 m spacing. The counters were 5.08 cm in diameter and filled with 85% 3 He
and 15% CF 4 at a pressure of 2.5 atm. Electrons were detected by the Cerenkov light
they emitted. These included those produced in the primary interaction as well as
those produced through Compton scattering on electrons of photons emitted through
neutron absorption.
Cerenkov detectors are also the technique of choice for cosmological neutrinos.
The scarcity of these very high energy neutrinos requires the use of large naturally
occurring target and detection media such as a lake [53] or sea water [54–57] or
Antarctic ice [58, 59] which can be instrumented with photomultipliers at the scale
of 1 km 3 . The photomultipliers are connected into vertical strings and lowered in the
water or, in the case of ice, into holes melted using hot water. The strings have to
be located at great depths to shield the detector from downgoing cosmic muons.
This necessitates the inclusion of the photomultipliers in pressure vessels. They
must also be in regions of high light transmission in order to maximize the spacing
of photomultipliers and reduce the cost. The most advanced of these detectors is
ICECUBE [59] in the Antarctic. It consists of 86 strings positioned in a 125 m
hexagonal grid at a depth between 1450 and 2450 m. Each string includes 60 digital
optical modules (DOM). Each DOM is a 35 cm pressure vessel containing a 25 cm
diameter pmt, a wave form digitizer, a fast ADC and electronics self-triggering at
the level of 1/4 of a photoelectron. Digital information is sent to the surface. It
is complemented by a 1 km 2 surface array consisting of 160 ice-filled tanks. The
average absorption and scattering lengths of the ice at the detector depth are 110 m
355
transformation rather than a disappearance it remained to prove that the overall
flux of neutrinos, including all three flavours, was as predicted by the standard
solar model. This was achieved by SNO [51] by measuring neutral current reactions
which can occur for all three flavours since they do not have the energy constraint
of charged currents, namely the mass of the appropriate produced charged lepton.
It’s heavy water (D 2 O) target made it sensitive to three neutrino reactions, including
neutral current reactions:
• Elastic scattering on electrons: ν e,μ,τ + e − → ν e,μ,τ + e −
• Charged current absorption of neutrinos by deuterons: ν e + d → e − + p + p
• Neutral current disintegration of the deuteron with a threshold of 2.2 MeV:
ν e,μ,τ + d → ν e,μ,τ + n + p. This reaction could not be observed in light
water because of the binding energy of oxygen being larger than the solar
neutrino energies.The neutron was detected by absorption on deuterium or on
35 Cl in added MgCl 2 . At a later stage of the experiment an array of 3 He filled
proportional tubes was added providing a highly efficient neutron detection
through the reaction 3 He + n → p + 3 H + 764 keV.
Because of the overall neutron production of only a few tens per day, care had
to be given to select radiopure materials. The detector is located in a mine at
a depth of 6000 m.w.e, thus reducing the cosmic ray background to 70/day. It
consists of an acrylic sphere containing 1000 tons of heavy water viewed by 9438
photomultipliers. It is immersed in a structure containing light water for shielding
and support. The proportional counters were placed in the heavy water in a lattice
with 1 m spacing. The counters were 5.08 cm in diameter and filled with 85% 3 He
and 15% CF 4 at a pressure of 2.5 atm. Electrons were detected by the Cerenkov light
they emitted. These included those produced in the primary interaction as well as
those produced through Compton scattering on electrons of photons emitted through
neutron absorption.
Cerenkov detectors are also the technique of choice for cosmological neutrinos.
The scarcity of these very high energy neutrinos requires the use of large naturally
occurring target and detection media such as a lake [53] or sea water [54–57] or
Antarctic ice [58, 59] which can be instrumented with photomultipliers at the scale
of 1 km 3 . The photomultipliers are connected into vertical strings and lowered in the
water or, in the case of ice, into holes melted using hot water. The strings have to
be located at great depths to shield the detector from downgoing cosmic muons.
This necessitates the inclusion of the photomultipliers in pressure vessels. They
must also be in regions of high light transmission in order to maximize the spacing
of photomultipliers and reduce the cost. The most advanced of these detectors is
ICECUBE [59] in the Antarctic. It consists of 86 strings positioned in a 125 m
hexagonal grid at a depth between 1450 and 2450 m. Each string includes 60 digital
optical modules (DOM). Each DOM is a 35 cm pressure vessel containing a 25 cm
diameter pmt, a wave form digitizer, a fast ADC and electronics self-triggering at
the level of 1/4 of a photoelectron. Digital information is sent to the surface. It
is complemented by a 1 km 2 surface array consisting of 160 ice-filled tanks. The
average absorption and scattering lengths of the ice at the detector depth are 110 m