Experiment Proposal - opera - Infn

8 Test measurements and results
8.1 Measurements on ECC brick components
8.1.1 Emulsion films
In additions to the extensive studies of emulsion films from Fuji presented in Section 4.2.2, we also
performed test measurements with industrial emulsion films produced by the Fomos company. 17 Bricks
made with these films were exposed to muons at the CERN SPS.
The films are made up of two 55 µm emulsion layers coated on both sides of a 180 µm plastic base.
The emulsion gel has a dilution factor of three. The measured sensitivity is of 25 grains/100 µm for a
minimum ionising particle. The fog density varies from 4 to 8 grains per 1000 µm 3 , depending on the
development conditions. These features are compatible with the OPERA requirements.
The beam test data allowed us to study, in particular, edge effects. The results are shown in Fig. 148
and indicate that the track finding efficiency stays rather constant near the film edge. A drop in the
efficiency is measured for distances lower than ∼ 300 µm from the edge.
We also determined the angular resolution of the emulsion films, by measuring the angles of penetrating
muons in two emulsion films separated by 1 mm of lead. This is shown in Fig. 149.
8.1.2 Lead radioactivity measurements
With the aim of identifying suitable materials for the brick lead plates, we performed extensive measurements
of the radioactivity of commercial lead samples supplied by companies in Japan, USA and
Europe.
The α and β rays emissions were measured at the Nagoya University by using emulsion films and a
gas proportional counter with 2π geometrical acceptance and surrounded by cosmic ray veto counters.
Ameasurementoftheγ radioactivity was performed by high purity germanium detectors (HPGe)
placed in the Modane Underground Laboratory (Fréjus tunnel). 18 A detailed GEANT 3.21 simulation of
all the decay products of the 210 Pbradioactivity, namely the Pb → Bi → Pochain, was also performed.
These tools had been successfully tested in the ultra-low radioactivity domain of the double beta decay
NEMO experiment.
Recognition of α particles in emulsion films is rather straightforward, since α’s leave short blacktrajectories
in the emulsion layers. By counting these tracks one can estimate the emission level. Conversely,
the estimate of the β radioactivity level is not trivial, due to Compton electrons produced from
the natural γ ray background. Therefore, the reported emulsion measurements only refer to the α emission.
In the case of gas proportional counter, insted, α and β can be separately measured by signal pulse
height discrimination.
The results of the measurements are summarised in Table 31. The α level as measured by a proportional
counter well agrees with the measurement done with emulsion films. As the source of the emission
17 Fomos, 7 Leningradsky prospect, 125167, Moscow, Russia.
18 We thank F. Hubert and P. Hubert from the CENBG laboratory, France, and H. Ohsumi from the SAGA
University for the lead radioactivity measurements.
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