Pontecorvo created a neutron source using radium and beryllium, as the Via Panisperna boys had, with paraffin wax as a neutron moderator, and measured the absorption of different minerals using methods developed by Fermi and Amaldi. By June 1941, he had a device that could differentiate shale, limestone and sandstone, and map the transitions between them. The technique may be considered the first practical application of the discovery of slow neutrons, and would still be in use decades later for well logging. He filed four patents relating to his instrumentation.