9 Nuclear Emulsions
397
reducing the analysis time of the emulsions by orders of magnitude as compared to
the early times.
The CHORUS detector [75] is a good example of a large hybrid experimental
setup combining a nuclear emulsion target with various electronic detectors. The
detector was designed to search for ν μ ↔ ν τ oscillations in the CERN WANF
neutrino beam with high sensitivity. At that time, a relatively massive ν τ was a
preferred candidate to explain the Dark Matter of the Universe. Since charmed
particles and the τ lepton have similar lifetimes, the detector was also well suited
for the observation of the production and decay of charmed particles.
Also in CHORUS nuclear emulsions acted both as neutrino target and as a high
space-resolution detector, allowing three-dimensional reconstruction of short-lived
particles. The emulsion target had an unprecedented large mass of 770 kg and was
segmented into four stacks, each consisting of eight modules, each in turn composed
of 36 plates with a size of 36×72 cm 2 . Each plate had a 90 μm plastic support coated
on both sides with a 350 μm emulsion layer [76]. Each stack was followed by a set
of scintillating fibre tracker planes. Three Changeable Sheets with a 90 μm emulsion
layer on both sides of a 800 μm thick plastic base were used as interface between
the fibre trackers and the bulk emulsion. The accuracy of the fibre tracker prediction
was about 150 μm in position and 2 mrad in the track angle. The electronic detectors
downstream of the emulsion target and the associated trackers included a hadron
spectrometer measuring the bending of charged particles in an air-core magnet, a
calorimeter where the energy and direction of showers were measured and a muon
spectrometer.
CHORUS represents a milestone in the history of nuclear emulsions for the
size of the target and of the CS, which implied very labor-intensive procedures for
emulsion gel production, pouring on the plastic bases, and development conducted
in the CERN emulsion laboratory [46], as well as for the first massive use of
automated scanning microscopes running in the Japanese and European laboratories
of the Collaboration [75].
The operation of the experiment consisted of several steps. It is worth noting
that the large-size emulsion target was replaced only once during the entire duration
of the experiment, while the CSs were periodically exchanged with new detectors,
therefore integrating tracks for a relatively short period. The best time resolution
was obviously provided by the electronic detectors. With the CS scanning, the
association between electronic detectors and emulsions took place, and tracks with
position and angle compatible with that of the electronic trackers’ predictions
were searched for in the interface emulsions. If found, these tracks were further
extrapolated into the bulk emulsion, with a much better resolution, up to the
track stopping point, with a procedure called scan-back, consisting in connecting
emulsion layers progressively more upstream. After that, a “volume scan” (net-scan)
around the presumed vertex was accomplished and repeated for all stopping tracks
until the neutrino interaction vertex was found.
In the search for charmed particle decays, a dedicated topological selection was
applied to the collected net-scan data. The analysis procedure was complemented
by the visual inspection of the selected event candidates, aimed at checking both
397
reducing the analysis time of the emulsions by orders of magnitude as compared to
the early times.
The CHORUS detector [75] is a good example of a large hybrid experimental
setup combining a nuclear emulsion target with various electronic detectors. The
detector was designed to search for ν μ ↔ ν τ oscillations in the CERN WANF
neutrino beam with high sensitivity. At that time, a relatively massive ν τ was a
preferred candidate to explain the Dark Matter of the Universe. Since charmed
particles and the τ lepton have similar lifetimes, the detector was also well suited
for the observation of the production and decay of charmed particles.
Also in CHORUS nuclear emulsions acted both as neutrino target and as a high
space-resolution detector, allowing three-dimensional reconstruction of short-lived
particles. The emulsion target had an unprecedented large mass of 770 kg and was
segmented into four stacks, each consisting of eight modules, each in turn composed
of 36 plates with a size of 36×72 cm 2 . Each plate had a 90 μm plastic support coated
on both sides with a 350 μm emulsion layer [76]. Each stack was followed by a set
of scintillating fibre tracker planes. Three Changeable Sheets with a 90 μm emulsion
layer on both sides of a 800 μm thick plastic base were used as interface between
the fibre trackers and the bulk emulsion. The accuracy of the fibre tracker prediction
was about 150 μm in position and 2 mrad in the track angle. The electronic detectors
downstream of the emulsion target and the associated trackers included a hadron
spectrometer measuring the bending of charged particles in an air-core magnet, a
calorimeter where the energy and direction of showers were measured and a muon
spectrometer.
CHORUS represents a milestone in the history of nuclear emulsions for the
size of the target and of the CS, which implied very labor-intensive procedures for
emulsion gel production, pouring on the plastic bases, and development conducted
in the CERN emulsion laboratory [46], as well as for the first massive use of
automated scanning microscopes running in the Japanese and European laboratories
of the Collaboration [75].
The operation of the experiment consisted of several steps. It is worth noting
that the large-size emulsion target was replaced only once during the entire duration
of the experiment, while the CSs were periodically exchanged with new detectors,
therefore integrating tracks for a relatively short period. The best time resolution
was obviously provided by the electronic detectors. With the CS scanning, the
association between electronic detectors and emulsions took place, and tracks with
position and angle compatible with that of the electronic trackers’ predictions
were searched for in the interface emulsions. If found, these tracks were further
extrapolated into the bulk emulsion, with a much better resolution, up to the
track stopping point, with a procedure called scan-back, consisting in connecting
emulsion layers progressively more upstream. After that, a “volume scan” (net-scan)
around the presumed vertex was accomplished and repeated for all stopping tracks
until the neutrino interaction vertex was found.
In the search for charmed particle decays, a dedicated topological selection was
applied to the collected net-scan data. The analysis procedure was complemented
by the visual inspection of the selected event candidates, aimed at checking both