The Forbush decrease after the GLE on 13 December 2006 detected by the muon telescope at BEO – Moussala

The Basic Environmental Observatory – Moussala is located at peak Moussala, 2925 m a.s.l. (22 deg 35′ E, 42 deg 11′ N), Rila Mountain, Bulgaria. A muon telescope with effective area 1 square meter, using 8 water cherenkov detectors, is in operation at the observatory since August 2006. A Forbush decrease with amplitude approximately 4% in the intensity of the muon component of cosmic rays was detected after the GLE on 13 December 2006. Brief description of the instrument and the experimental results are presented.