These rays are emitted during the decay of certain versions of uranium, iodine, cesium and other elements.

Now when you look at this you see these people handling the uranium pellets. Now uranium 235 in this form is very low in activity because uranium235 has a very long half-life.

Most famously, uranium-238 decays into lead-206 with a half life of 4.5 billion years, though radioactive rubidium and samarium are also useful for dating things billions of years old.