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Helium is found in deposits of uranium and thorium because their isotopes - Uranium-238 and Thorium-232 - are atoms with large nuclei. To achieve a more stable state, these atoms undergo radioactive decay. This decay produces alpha particles (helium nuclei), beta particles (electrons), and gamma rays. Helium atoms are formed when alpha particles combine with electrons and are thus found in deposits.

Helium is thought to be the most abundant element in the universe, as it is produced by the fusion reaction that occurs in the cores of stars. On Earth, however, helium is extremely rare, accounting for only 5.2 ppm of the atmosphere. The majority of the helium on Earth is derived from radiological sources. Deep underground, as elements such as uranium and thorium decay into thorium and radium, respectively, $\mathrm{Î\pm }$-particles consisting of two protons and two neutrons are released and trapped. As these -particles absorb electrons from their surroundings, they form stable helium atoms.

$\text{U238}→\text{Th234 +}\mathrm{Î\pm }\text{; U235}→\text{Th231 +}\mathrm{Î\pm }\text{; Th232}→\text{Ra228 +}\mathrm{Î\pm }$

Hence, Alpha particles combine to form helium atoms. To achieve stability, uranium and thorium emit these particles and the Particles of Alpha Gamma Ray are obtained.