8 Neutrino Detectors
365
8.3.3.2 Fine-Grained
CHARM II [94] was a detector designed to measure sin
2 θ W in ν μ − e scattering.
The scattered electron is produced very forward unlike background events arising
from ν-nucleon events. A cut of Eθ 2 ≤ 1 MeV and θ < 10 mrad, where E and θ are
the scattered electron energy and production angle to the beam direction was used to
reject this background, necessitating a very good angular resolution. Hence, glass,
a low Z material to minimize multiple scattering, was selected as target material.
Each of the 420 48 mm thick glass plates was followed by a plane of 352 plastic
streamer tubes with a 1 cm pitch. The wires were readout in digital mode and 18 mm
wide cathode strips glued to the outside of the tubes in a direction orthogonal to
the wires were readout in analog mode to provide a measure of the energy and
centroid of the electron showers. Consecutive modules had their strip and wire
orientations rotated by 90 ◦ and consecutive modules with the same orientation had
their wire spacing shifted by half the wire pitch. A scintillator plane was inserted
in the detector after every 5 glass plates. The total mass of the calorimeter was 692
tons covering a volume of 3.7 × 3.7 × 15.4 m 3 . An electron angular resolution,
σ θ /θ , varying between 15–20 (mrad)/
√
E(GeV ) over the 2–24 GeV energy range
of the experiment was achieved[95] as well as a vertex resolution of about 22 mm.
The ability to discriminate the electrons from ν μ − e scattering from background
is demonstrated in Fig. 8.14. A muon spectrometer consisting of six of the CDHS
modules followed the glass target and provided a momentum resolution of 14% at
20 GeV/c and an angular resolution at the vertex of 18 mrad/E(GeV).
8.3.4 Emulsions
Detectors based on the photographic emulsion technique have a sub-micron spatial
resolution and are therefore the detector of choice when searching for secondary
vertices related to charmed particles or τ leptons. Until recently, this technique
was limited because of the difficulty in scanning the emulsion. However recent
developments in fast microscopes have revived it. Although ν μ disappearance in
atmospheric neutrinos was widely believed to be due to ν μ → ν τ interactions,
it needed to be demonstrated through ν τ appearance in a ν μ beam. This was
undertaken, using emulsions, by E531 [96] at Fermilab and by CHORUS [97] at
CERN. At the neutrino energy of these experiments the τ travels only about 1–
2 mm. CHORUS, the more sensitive of the two experiments used a 770 kg emulsion
target built out of plates consisting of a 90 μm plastic base holding 350 μm thick
emulsion layers on either side. The target was divided into four stacks each one
followed by three interface emulsion sheets and a scintillating fibre tracker. Other
sheets of emulsions were interleaved between the stacks and were changed several
times throughout the data-taking in order to be exposed to fewer tracks and therefore
ease the track reconstruction in the bulk emulsion. The fibres (more than 1 million)
were read out by 58 optoelectronic readout chains each consisting of four image
365
8.3.3.2 Fine-Grained
CHARM II [94] was a detector designed to measure sin
2 θ W in ν μ − e scattering.
The scattered electron is produced very forward unlike background events arising
from ν-nucleon events. A cut of Eθ 2 ≤ 1 MeV and θ < 10 mrad, where E and θ are
the scattered electron energy and production angle to the beam direction was used to
reject this background, necessitating a very good angular resolution. Hence, glass,
a low Z material to minimize multiple scattering, was selected as target material.
Each of the 420 48 mm thick glass plates was followed by a plane of 352 plastic
streamer tubes with a 1 cm pitch. The wires were readout in digital mode and 18 mm
wide cathode strips glued to the outside of the tubes in a direction orthogonal to
the wires were readout in analog mode to provide a measure of the energy and
centroid of the electron showers. Consecutive modules had their strip and wire
orientations rotated by 90 ◦ and consecutive modules with the same orientation had
their wire spacing shifted by half the wire pitch. A scintillator plane was inserted
in the detector after every 5 glass plates. The total mass of the calorimeter was 692
tons covering a volume of 3.7 × 3.7 × 15.4 m 3 . An electron angular resolution,
σ θ /θ , varying between 15–20 (mrad)/
√
E(GeV ) over the 2–24 GeV energy range
of the experiment was achieved[95] as well as a vertex resolution of about 22 mm.
The ability to discriminate the electrons from ν μ − e scattering from background
is demonstrated in Fig. 8.14. A muon spectrometer consisting of six of the CDHS
modules followed the glass target and provided a momentum resolution of 14% at
20 GeV/c and an angular resolution at the vertex of 18 mrad/E(GeV).
8.3.4 Emulsions
Detectors based on the photographic emulsion technique have a sub-micron spatial
resolution and are therefore the detector of choice when searching for secondary
vertices related to charmed particles or τ leptons. Until recently, this technique
was limited because of the difficulty in scanning the emulsion. However recent
developments in fast microscopes have revived it. Although ν μ disappearance in
atmospheric neutrinos was widely believed to be due to ν μ → ν τ interactions,
it needed to be demonstrated through ν τ appearance in a ν μ beam. This was
undertaken, using emulsions, by E531 [96] at Fermilab and by CHORUS [97] at
CERN. At the neutrino energy of these experiments the τ travels only about 1–
2 mm. CHORUS, the more sensitive of the two experiments used a 770 kg emulsion
target built out of plates consisting of a 90 μm plastic base holding 350 μm thick
emulsion layers on either side. The target was divided into four stacks each one
followed by three interface emulsion sheets and a scintillating fibre tracker. Other
sheets of emulsions were interleaved between the stacks and were changed several
times throughout the data-taking in order to be exposed to fewer tracks and therefore
ease the track reconstruction in the bulk emulsion. The fibres (more than 1 million)
were read out by 58 optoelectronic readout chains each consisting of four image