IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1P2_2D-O27

Room B1

Date/Time: July 21 18:30-18:45

Muon radiography by nuclear emulsions: data acquisition and processing
Cristiano Bozza1 , Lucia Consiglio2 , Nicola D’Ambrosio3 , Giovanni De Lellis4 , Chiara De Sio5 , Natalia Di
Marco3 , Umut Kose6 , Eduardo Medinaceli7 , Seigo Miyamoto8 , Ryuichi Nishiyama8 , Fabio Pupilli3 ,Simona Maria Stellacci1 , Chiara Sirignano7 , Hiroyuki Tanaka8 , Valeri Tyukov2
1

University of Salerno and INFN, Italy, 2 INFN Napoli, Italy, 3 INFN / LNGS, Italy, 4 University of Napoliand INFN, Italy, 5 University of Salerno, Italy, 6 INFN Padova, Italy, 7 University of Padova and INFN, Italy,
8
University of Tokyo, Japan
E-mail: kryss@sa.infn.it

Nuclear emulsions used as tracking devices as have unique features that make them suitable for muon radiographyof volcanic edifices. They are cheap and have excellent spatial and angular resolution, respectively about 0.3
micrometers and 1 mrad. They have no dead time, but on the other hand they have no time resolution. Suchspecific behaviours demand the use of specific techniques.
Once a nuclear emulsion-based detector is removed from its exposure site and the films are developed, data
readout can start. Modern automatic microscopes acquire images stored in the emulsion thickness and convert
them to sets of tracks of charged particles. Even very thin layers, of the order of 50 micron, can provide 3D
information about the local flux of muons and/or other particles present in the showers initiated by primary cosmic
rays. In the case of exposures that take several months and detector surfaces of the order of a few m2, data readout
time is only a fraction of the data collection time.
The performances of the data readout system play a prominent role. Fast emulsion scanning speed means ability
to acquire data on large areas; high statistics implies low statistical fluctuations of data, and hence good sensitivity
to the variations of the integrated flux density. Track recognition efficiency is also crucial: since emulsion films areexposed in stacks to reduce the amount of fake tracks due to ambient radioactivity and not related to cosmic rays,
the ability to reconstruct penetrating tracks depends on a power of the tracking efficiency. The basic architecture
and working principle of the European Scanning System (ESS) are described. Such system is currently used bymuon radiography research groups in Europe and Japan. The evolution of the system to scale up its speed anddata quality are also presented.
The size of data sets from a typical exposure can be 1010 singlet tracks. Detectors for muon radiography are builtof several small stacks of emulsion films (doublets, triplets, quadruplets or multiplets with or without absorbers
to stop soft components). Single layer tracks are normally correlated in the films of the same stack to reconstructpenetrating tracks. This operation dramatically increases the signal-to-noise ratio to values that range from 100:1 to
10000:1. The resulting data sets of penetrating tracks provide detailed angular spectra of integrated rock thickness.
Given the amount of data, specific algorithms have been developed and optimized.
Such processed data even allow a posteriori fine-tuning of the position, direction and inclination of the individual
sub-stacks that build the detector, with a precision of a few degrees. Finally, selection of the quality of tracks allows
filtering different components of the flux of charged particles that cross the detector, from electromagnetic showers
to muons. The method is shown with examples from data analysis activities on exposures to Unzen and Stromboli.

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