Constancy of cosmic rays in time

The relative production ratio of ³⁹Ar/³⁶Cl in iron by 6.2 Bev protons has been measured to be 0.96 ± 0.05. This ratio is compared to the ³⁹Ar/³⁶Cl ratio of 0.98 ± 0.05 found in iron meteorites whose date of fall to the earth is known. It is concluded that the ratio of the average cosmic ray intensity during the last 300 years has been 1.02 ± 0.07 times the average cosmic ray intensity during the last 300,000 years. In this sense, cosmic ray intensity has been essentially unchanged.