When muons are used to scan matter

The flux of cosmic muons at ground level is relatively modest: about 150 muons/m²/s but its energy spectrum spreads over 1 TeV which explains that they can be used to scan items with very various sizes from a few cm to several hundreds meters as the higher their energy, the deeper into the matter the muons go. There are mainly 3 techniques used in muon scanning: nuclear emulsions, plastic scintillators and gaseous detectors, combined with 2 ways of scanning: transmission or deflection scanning. Muons have recently be used to scan huge structures like Egyptian pyramids. The low intensity of the muon flux implies very long exposure time up to 3 months for a pyramid. In the international collaboration 'ScanPyramids', the 3 techniques have been used simultaneously to scan the Kheops pyramid and have agreed on the presence of a void above the Great Gallery. (A.C.)