  The OPERA neutrino oscillation experiment is based on the use of the Emulsion
Cloud Chamber (ECC). In the OPERA ECC, nuclear emulsion films acting as very
high precision tracking detectors are interleaved with lead plates providing a
massive target for neutrino interactions. We report on studies related to the
effects occurring from the contact between emulsion and lead. A low
radioactivity lead is required in order to minimize the number of background
tracks in emulsions and to achieve the required performance in the
reconstruction of neutrino events. It was observed that adding other chemical
elements to the lead, in order to improve the mechanical properties, may
significantly increase the level of radioactivity on the emulsions. A detailed
study was made in order to choose a lead alloy with good mechanical properties
and an appropriate packing technique so as to have a low enough effective
radioactivity.
