Experiment Proposal - opera - Infn

After the exposure to cosmic rays, bricks are disassembled in the different components in a dark
room. Emulsions films are printed with reference marks and numbers. A printer is used with a mask
with lattice pattern and a LCD shutter for numbering. Then all emulsion films are put in special racks
for automatic processing. In order to keep constant the dimensions of the films, also this room has to be
climatised with the same conditions as the assembly laboratory.
The emulsion films are processed by an automatic machine. This is equipped with 6 tanks for presoaking,
developing, stopping, fixing, washing and alcohol bathing. By means of a crane, each rack
filled with emulsion films is soaked in each bath continuously. In order to maintain conditions stable, a
continuous replacement of the different solutions and their temperature control are necessary. Therefore,
an upper level floor mixing-room and temperature controlled storage tanks are required. Processed films
are dried in a cabin with mild air flowing. The expected number of processed films per day is about
1700. A large stock room of ∼ 100 m 2 for chemicals is needed together with at least two storage tanks
for waste solutions. One is used for the fixer, for silver recovering. The expected power consumption is
10 kW .About2 ton of water are needed.
For a long emulsion exposure, like that scheduled for OPERA, it is crucial to have the lowest possible
level of background. Background grains and tracks in emulsion could mainly arise from cosmic rays
(outside the hall) and from environmental radioactivity (mainly inside the hall, due to the long exposure
time. This background shows up as Compton or photoelectrons from low-energy gamma rays. Tracks
from particles associated to the beam, both muons and hadrons, are on the contrary too rare to constitute
a problem and are not considered any further.
Although the above mentioned sources of background cannot simulate either a triggered interaction
or a τ decay, they can contribute with wrong or false candidate tracks (real tracks and ghosts) and their
piling-up with time may increase the scanning load and reduce the visibility through the emulsions.
Ambient radioactivity in the underground hall must be kept low. This level of activity has been
measured and mapped in the past, as example by the MACRO and BOREXINO experiments, in order
to identify possible hot spots. It turned out that the main source is the concrete of the floor. As the
measurements were performed with electronic counters and as is difficult to extrapolate the results to
emulsion films, test exposures are being performed to assess the issue. A small dark-room has been built
in the Gran Sasso underground laboratory for this purpose. Tests on radioactivity are in progress, both
with counters 28 and with emulsions.
11.2 Infrastructure for the emulsion readout facilities
It is envisaged to setup two to three stations for vertex location. One can foresee a scheme in which one
or two stations are located in Europe and one in Japan. The stations have to specially organised to be
fault tolerant in order to process the selected bricks.
A schematic illustration of one of these stations as planned at the Nagoya University is shown in
Fig. 180. The laboratory has a surface of about 150 m 2 and includes several facilities in order to allow
the processing rate of more than 10 bricks/day. A stock room for the emulsion films mounted on cassettes
is required.
28 Courtesy of C. Arpesella, Gran Sasso Laboratory.
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