Study of the effects induced by lead on the emulsion films of the OPERA experiment

The OPERA neutrino oscillationexperiment is based on the use of the Emulsion Cloud Chamber(ECC). In the OPERAECC, nuclear emulsionfilms 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 emulsionand lead. A low radioactivity lead is required in order to minimize the number of background tracks in emulsionsand to achieve the required performance in the reconstruction of neutrino events. It was observed that adding other chemical elementsto 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.