Background Radiation
Primoridal Radiation
As mentioned before, we're surrounded by natural levels of radiation. This natural level of radiation is present in all rock on Earth and is called primordial radiation.
The most important primordials consist of three very long-lived isotopes with half-lives to the order of 109 years. Yes, you read right: 109 years or 1,000,000,000 years or 1 trillion years, so they're not going anywhere anytime soon. By "they," we mean the incredible threes: Potassium, Uranium, and Thorium, 40K, 238U, and 232Th. They live in the Earth's continental upper crust with an average concentration of 
, 
, and 
, respectively.
Like all the other radiation we've studied, the three primordial buddies emit a variety of byproducts: other types of nuclei, neutrons, α-particles, β-particles, and γ-rays in a wide range of energies. Potassium-40 decays by beta radiation to the stable isotope of 40Ca, or undergoes electron capture to the stable isotope of 40Ar. However, 40K is only naturally abundant in certain rock compounds at low levels of 0.0117%.
The other two main sources of primordial radiation are 238U and 232Th, which eventually decay to the stable isotopes 206Pb and 208Pb after a series of decays. Check out these two "simplified" schemes that show the number of decays each isotope undergoes before finally having a chance to catch its breath. In terms of an experiment, all this radiation output is called "background."
Experimental physicists have to make sure their experiments aren't skewed by these natural levels of radiation by measuring the background radiation in the environment of the experiments, or by minimizing background radiation through thick radiation shields. Otherwise, the primordials could hide or even bury the signal of interest, depending on the strength of the signal.