High frequency rays of cosmic origin

It was as early as 1903 that the British physicists, McLennan and Burton(1) and Rutherford and Cooke(2) noticed that the rate of leakage of an electric charge from an electroscope within an air-tight metal chamber could be reduced as much as 30% by enclosing the chamber within a completely encircling metal shield or box with walls several centimeters thick. This meant that the loss of charge of the enclosed electroscope was not due to imperfectly insulating supports, but must rather be due to some highly penetrating rays, like the gamma rays of radium, which could pass through metal walls as much as a centimeter thick and ionize the gas inside.