Recent multiplicity distributions in EMMA experiment.

M. Slupecki (1), L. Bezrukov (2), T. Enqvist (1), H. Fynbo (3), L. Inzhechik (4), J. Joutsenvaara (1), T. Kalliokoski (5), P. Kuusiniemi (1), K. Loo (5), B. Lubsandorzhiev (2), V. Petkov (2), J. Sarkamo (1), W. Trzaska (5), A. Virkajarvi (1)

(1) Oulu Southern Instutute and Department of Physics, University of Oulu, Finland
(2) Institute of Nuclear Research, Russian Academy of Sciences, Moscow, Russia
(3) Department of Physics and Astronomy, University of Aarhus, Denmark
(4) Moscow Institute of Physics and Technology, Moscow, Russia
(5) Department of Physics, University of Jyvaskyla, Finland

EMMA, an experiment designed to determine the chemical composition of cosmic rays around the knee region, is located 75 meters underground (210 m.w.e.) in the Pyhasalmi mine in Finland. It is still in the construction phase: out of planned 11 detector stations already 9 has been built and 5 has been equipped with detectors and collecting data.

According to CORSIKA simulations the lateral distribution of high energy muons (>50 GeV) in the air shower carries information about the energy and mass of the primary cosmic ray particle. The underground location of EMMA provides appropriate energy cutoff. In order to extract energy and mass of the primary particle from muons' lateral distribution EMMA needs to determine the shower arrival direction (3 tracking detector stations) and shower core position (+ 8 sampling stations) separately for each event. 

The bulk of array's detector active area consists of drift chambers (270 m^2) characterised by good spatial resolution (1 cm^2). In the central part there are 24 m^2 of scintillators (pixels: 12x12 cm^2), which provide information about muon multiplicity at the shower core, when drift chambers saturate. Additional 170 m^2 of Limited Streamer Tubes will be added as soon as they are integrated into the data acquisition system. They will improve the discrimination between muons and electrons in the sampling stations and shower core position reconstruction accuracy.