404
A. Ariga et al.
The OPERA experiment has been running from 2008 to 2012 at the underground
LNGS laboratory in Italy, 730 km away from CERN where the CNGS neutrino
beam was produced. OPERA is the first very large scale emulsion experiment,
profiting from all the technological advances in the emulsion technology and in the
scanning systems described in the previous section. To give a figure, the ECC target
is made of films with a total surface of 110,000 m 2 and 105,000 m 2 lead plates.
The industrially produced, machine-coated emulsion films by FUJI provided very
uniform layer thickness and the possibility of erasing unwanted background tracks
by the refreshing technique. The scanning of the events was performed with about 40
fully automated microscopes, each of them faster by about two orders of magnitude
than those used in the CHORUS experiment [75].
The ECC target consisted of multi-layer arrays of target walls interleaved with
pairs of planes of plastic scintillator strips. A target wall (with about 10×10 m 2
cross-section) was an assembly of horizontal trays each loaded with ECC target
units called bricks. A brick consisted of 57 emulsion films interleaved with 56 lead
plates, 1 mm thick, light-tight packed. Brick dimensions were 128 × 102 × 79 mm 3
for a weight of 8.3 kg (Fig. 9.11). Interface Changeable Sheets (CS) were attached
to the downstream face of each brick. The choice of the CS geometry was such
to assemble two adjacent emulsion films as a doublet, coupled as an independent,
detachable package to the downstream face of the brick (Fig. 9.11). The use of
doublets provided the cancellation of random coincidences of tracks accumulated
during the storage and transportation and unerased by the refreshing procedure.
There were 150,000 bricks in total for a target mass of 1.25 kton. This represents
the largest ever ECC detector assembly and posed an unprecedented challenge for
the production of emulsion films and bricks, as well as for the emulsion handling,
development and analysis, i.e. scanning power. Just to give some numbers, more
than nine million emulsion films were produced and the corresponding 150,000
bricks were built by a fully robotised chain assembling films and lead plates in an
underground dark-room at LNGS. Large infrastructures were also realized at LNGS
for brick manipulation (automatic extraction from the target matrix), X-ray marking,
cosmic-ray exposure and emulsion development [100].
The principle of the experiment can be summarized as follows. At the occurrence
of a neutrino interaction, the resulting charged particle tracks are detected by the
scintillator counter planes placed behind each brick target wall, similarly to what
happens in a sampling calorimeter. The reconstruction of the “shower axis” or the
identification of a penetrating track (e.g. a muon) allows identifying the brick where
the neutrino likely interacted. At this point, the brick is extracted from the wall,
the attached CS doublet is removed and developed, while the brick, still packed, is
placed in an underground storage area waiting for the response of the CS scanning.
It is important to stress the key roles accomplished by the CS in OPERA [101]:
the first step is to confirm that the ECC brick contains the neutrino interaction;
the second step is to provide event-related tracks to be used for the ECC scanback analysis. By using Compton electrons from environmental radioactivity, the
systematic uncertainties in the relative alignment between the two CS doublet
films are reduced, thus bringing the position accuracy to the level of 1 μm [102].
A. Ariga et al.
The OPERA experiment has been running from 2008 to 2012 at the underground
LNGS laboratory in Italy, 730 km away from CERN where the CNGS neutrino
beam was produced. OPERA is the first very large scale emulsion experiment,
profiting from all the technological advances in the emulsion technology and in the
scanning systems described in the previous section. To give a figure, the ECC target
is made of films with a total surface of 110,000 m 2 and 105,000 m 2 lead plates.
The industrially produced, machine-coated emulsion films by FUJI provided very
uniform layer thickness and the possibility of erasing unwanted background tracks
by the refreshing technique. The scanning of the events was performed with about 40
fully automated microscopes, each of them faster by about two orders of magnitude
than those used in the CHORUS experiment [75].
The ECC target consisted of multi-layer arrays of target walls interleaved with
pairs of planes of plastic scintillator strips. A target wall (with about 10×10 m 2
cross-section) was an assembly of horizontal trays each loaded with ECC target
units called bricks. A brick consisted of 57 emulsion films interleaved with 56 lead
plates, 1 mm thick, light-tight packed. Brick dimensions were 128 × 102 × 79 mm 3
for a weight of 8.3 kg (Fig. 9.11). Interface Changeable Sheets (CS) were attached
to the downstream face of each brick. The choice of the CS geometry was such
to assemble two adjacent emulsion films as a doublet, coupled as an independent,
detachable package to the downstream face of the brick (Fig. 9.11). The use of
doublets provided the cancellation of random coincidences of tracks accumulated
during the storage and transportation and unerased by the refreshing procedure.
There were 150,000 bricks in total for a target mass of 1.25 kton. This represents
the largest ever ECC detector assembly and posed an unprecedented challenge for
the production of emulsion films and bricks, as well as for the emulsion handling,
development and analysis, i.e. scanning power. Just to give some numbers, more
than nine million emulsion films were produced and the corresponding 150,000
bricks were built by a fully robotised chain assembling films and lead plates in an
underground dark-room at LNGS. Large infrastructures were also realized at LNGS
for brick manipulation (automatic extraction from the target matrix), X-ray marking,
cosmic-ray exposure and emulsion development [100].
The principle of the experiment can be summarized as follows. At the occurrence
of a neutrino interaction, the resulting charged particle tracks are detected by the
scintillator counter planes placed behind each brick target wall, similarly to what
happens in a sampling calorimeter. The reconstruction of the “shower axis” or the
identification of a penetrating track (e.g. a muon) allows identifying the brick where
the neutrino likely interacted. At this point, the brick is extracted from the wall,
the attached CS doublet is removed and developed, while the brick, still packed, is
placed in an underground storage area waiting for the response of the CS scanning.
It is important to stress the key roles accomplished by the CS in OPERA [101]:
the first step is to confirm that the ECC brick contains the neutrino interaction;
the second step is to provide event-related tracks to be used for the ECC scanback analysis. By using Compton electrons from environmental radioactivity, the
systematic uncertainties in the relative alignment between the two CS doublet
films are reduced, thus bringing the position accuracy to the level of 1 μm [102].