Kunow, H.; Witte, M.; Wibberenz, G.; Hempe, H.; Mueller-Mellin, R.; Green, G.; lwers, B.; Fuckner, J. Cosmic Ray Measurements on Board Helios 1 from December 1974 to September 1975: Quiet Time Spectra, Radial Gradients, and Solar Events. J Geophys 1976, 42, 615-631.