9 Nuclear Emulsions
391
chamber followed by a detector of electromagnetic showers and a muon identifier. A
veto counter upstream discriminated against interactions in the emulsion produced
by charged particles. About 250 neutrino interactions were predicted by the spark
chamber. Given its vertex position resolution, a volume of about 0.7 cm 3 was
visually scanned around the prediction for about one third of the events; 16 of them
were located and fully reconstructed in the emulsions and one of them was found
with a topology consistent with that of charm.
A search for charmed particles in neutrino interactions was carried out at CERN
in 1977 with stacks of nuclear emulsions placed in front of the entrance window
of the Big European Bubble Chamber (BEBC) [34], filled with liquid hydrogen
and placed in a magnetic field of 3.5 T. A veto-coincidence counter system was
added in front of BEBC for this purpose. The emulsion stacks were made of 3150
pellicles of ILFORD emulsion, each 600 μm thick. The quality of the emulsion as
well as the high level of muon track background precluded any systematic scanning
along the track. A “surface” scan was therefore carried out for the bulk of the
events with 200× and 300× objective lenses, over an emulsion volume centred
on the predicted vertex position of 5×31 mm 2 for 7 plates. A total of 206,000
BEBC pictures were analysed, leading to 935 neutrino interaction vertices inside
the emulsion, 523 of which identified as charged current events. After kinematical
and topological cuts, 169 charged current interactions were selected, 8 of them
being identified as neutrino-induced charmed particles. The experiment reported
the first direct observation of a charmed baryon decay [35] and of a neutral charmed
particle [36].
The E531 experiment [32] was proposed in 1978 at Fermilab to study the
properties of charmed particles and their production mechanism in neutrino interactions [37]. The neutrino beam was produced by 350 GeV protons for a first exposure
(7.2 × 10 18 protons on target) and by 400 GeV protons for the second one (6.8 ×
10 18 protons on target). The overall beam composition was 92.3% ν μ , 7.0% ¯
ν μ ,
0.5% ν e and 0.2% ¯
ν e . The active neutrino target was made of nuclear emulsions
where short-lived particles were detected with micrometer accuracy. The decay
products were then measured by means of an electronic spectrometer, thus making
E531 the first hybrid particle physics experiment.
The emulsion target consisted of 22.6 l in the first run and of 30 l in the second
one; it was made of modules composed of plates with 300 μm emulsion layers
coated on both sides of 70 μm thick polystyrene foils. Downstream of the emulsion
modules, two large lucite plates 800 μm thick, coated on both sides with 75 μm
emulsion layers, acted as interface emulsion films, so establishing the new detector
concept of the Changeable Sheets (CS). Tracks reconstructed by electronic detectors
were first searched for in these interface films and then followed back in the bulk
target up to the neutrino interaction vertex. The CS were replaced every 2 or 3 days
of data taking in order to limit the number of accumulated background tracks that
would have affected the efficiency of finding the interaction vertex in the target.
Downstream of the target, a magnet equipped with high-resolution drift chambers
provided the track prediction in the CS with an accuracy of about 150 μm and
1 mrad. A time-of-flight detector made of two scintillator planes located 2.7 m
391
chamber followed by a detector of electromagnetic showers and a muon identifier. A
veto counter upstream discriminated against interactions in the emulsion produced
by charged particles. About 250 neutrino interactions were predicted by the spark
chamber. Given its vertex position resolution, a volume of about 0.7 cm 3 was
visually scanned around the prediction for about one third of the events; 16 of them
were located and fully reconstructed in the emulsions and one of them was found
with a topology consistent with that of charm.
A search for charmed particles in neutrino interactions was carried out at CERN
in 1977 with stacks of nuclear emulsions placed in front of the entrance window
of the Big European Bubble Chamber (BEBC) [34], filled with liquid hydrogen
and placed in a magnetic field of 3.5 T. A veto-coincidence counter system was
added in front of BEBC for this purpose. The emulsion stacks were made of 3150
pellicles of ILFORD emulsion, each 600 μm thick. The quality of the emulsion as
well as the high level of muon track background precluded any systematic scanning
along the track. A “surface” scan was therefore carried out for the bulk of the
events with 200× and 300× objective lenses, over an emulsion volume centred
on the predicted vertex position of 5×31 mm 2 for 7 plates. A total of 206,000
BEBC pictures were analysed, leading to 935 neutrino interaction vertices inside
the emulsion, 523 of which identified as charged current events. After kinematical
and topological cuts, 169 charged current interactions were selected, 8 of them
being identified as neutrino-induced charmed particles. The experiment reported
the first direct observation of a charmed baryon decay [35] and of a neutral charmed
particle [36].
The E531 experiment [32] was proposed in 1978 at Fermilab to study the
properties of charmed particles and their production mechanism in neutrino interactions [37]. The neutrino beam was produced by 350 GeV protons for a first exposure
(7.2 × 10 18 protons on target) and by 400 GeV protons for the second one (6.8 ×
10 18 protons on target). The overall beam composition was 92.3% ν μ , 7.0% ¯
ν μ ,
0.5% ν e and 0.2% ¯
ν e . The active neutrino target was made of nuclear emulsions
where short-lived particles were detected with micrometer accuracy. The decay
products were then measured by means of an electronic spectrometer, thus making
E531 the first hybrid particle physics experiment.
The emulsion target consisted of 22.6 l in the first run and of 30 l in the second
one; it was made of modules composed of plates with 300 μm emulsion layers
coated on both sides of 70 μm thick polystyrene foils. Downstream of the emulsion
modules, two large lucite plates 800 μm thick, coated on both sides with 75 μm
emulsion layers, acted as interface emulsion films, so establishing the new detector
concept of the Changeable Sheets (CS). Tracks reconstructed by electronic detectors
were first searched for in these interface films and then followed back in the bulk
target up to the neutrino interaction vertex. The CS were replaced every 2 or 3 days
of data taking in order to limit the number of accumulated background tracks that
would have affected the efficiency of finding the interaction vertex in the target.
Downstream of the target, a magnet equipped with high-resolution drift chambers
provided the track prediction in the CS with an accuracy of about 150 μm and
1 mrad. A time-of-flight detector made of two scintillator planes located 2.7 m