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Cosmic rays are the highest energy particles found in nature. Measurements ofthe mass composition of cosmic rays between 10^{17} eV and 10^{18} eV areessential to understand whether this energy range is dominated by Galactic orextragalactic sources. It has also been proposed that the astrophysicalneutrino signal comes from accelerators capable of producing cosmic rays ofthese energies. Cosmic rays initiate cascades of secondary particles (airshowers) in the atmosphere and their masses are inferred from measurements ofthe atmospheric depth of the shower maximum, Xmax, or the composition of showerparticles reaching the ground. Current measurements suffer from either lowprecision, or a low duty cycle and a high energy threshold. Radio detection ofcosmic rays is a rapidly developing technique, suitable for determination ofXmax with a duty cycle of in principle nearly 100%. The radiation is generatedby the separation of relativistic charged particles in the geomagnetic fieldand a negative charge excess in the shower front. Here we report radiomeasurements of Xmax with a mean precision of 16 g/cm^2 between10^{17}-10^{17.5} eV. Because of the high resolution in $Xmax we can determinethe mass spectrum and find a mixed composition, containing a light massfraction of ~80%. Unless the extragalactic component becomes significantalready below 10^{17.5} eV, our measurements indicate an additional Galacticcomponent dominating at this energy range.

A large light-mass component of cosmic rays at 10^{17} - 10^{17.5} eV from radio observations