radioactivity
Main sources of internal heat are Magmatism and Radioactivity.
radioactivity
Some is fossil heat, some from radioactivity.
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Gamma
The Type Of Radiation That It Produce Is The Gamma Ray!
It is called radioactivity.
The main hazard is its radioactivity. It is so radioactive it will vaporize itself because of the tremendous heat made by its radioactivity, but you don't have to worry about it since you can't even collect a nanogram of it.
The primal heat from the deep interior of the Earth is mainly due to radioactive decay, with a small contribution due to gravity compression.
Gamma radioactivity would be the most dangerous because, unlike alpha particles that can be blocked by paper or skin, and beta particles which can be shielded by foil, gamma radioactivity would need a lead shield. Plus gamma radioactivity effects organic material.
nuclear plants because the fission reaction releases lots of radioactive particles causing radioactivity..