Rossi's group focused on the use of cloud chambers to study their properties and interactions. In 1948, with the aid of a multi-plate cloud chamber in which lead plates alternated with aluminium ones, Gregory, Rossi and Tinlot showed that the source of the electromagnetic component of cosmic ray interactions was predominantly energetic photons, rather than electrons. This result confirmed Oppenheimer's suggestion of 1947 that neutral pions are produced in interactions, along with charged ones, and that this component arises from their rapid decay into photons.