Diurnal Variations in the Intensity of Cosmic Rays Underground

The number of coincidences between two trays of Geiger-counters located at a depth of 8.46×${10}^4$ g ${\mathrm{cm}}^{-2\mathrm{}}$ underground was recorded each hour, and these data were analyzed for variation of cosmic-ray intensity with solar and sidereal time. (A total of 7.4×${10}^5$ $\mu$-mesons was observed whose average energy is estimated to be 2×${10}^{11}$ ev.) In each case the standard deviation of the observed coincidence rates from the mean rate is not larger than the standard deviation expected from a normal distribution. The first harmonic amplitudes of the solar and sidereal variations indicate that the data are equally consistent with an assumed daily sinusoidal variation in the coincidence rates of ∼0.5 percent and with the absence of such variation. The difficulties involved in deducing directional properties of the primary cosmic radiation from harmonic analysis of variations in secondary radiation underground are discussed.