Experimental study of muon bundles observed in the Fréjus detector.

The present study is based on a sample of 407 775 single muons and 12 559 muon bundles with zenith angles smaller than 60{degree}, observed in the 12.3 m{times}6 m{times}6 m Frejus proton-decay detector, at a depth of 4850 hg/cm{sup 2}. The variation of the vertical muon intensity with depth is given. Using a maximum-likelihood method, the muon lateral distribution of underground bundles is investigated as a function of muon multiplicity and zenith angle. The same method yields the true multiplicity distribution at the site depth, corrected for all detection effects.