Alpha particles are emitted during the radioactive decay of

Uranium 238  

Uranium 238  → thorium-234 + α particle (He²⁺ helium nucleus)

Explanation:

A uranium-238 nucleus, which is termed the parent nucleus, experiences radioactive decay with the emission of an alpha particle to form thorium-234, which is known as the daughter nuclide by the emission of an alpha particle which is made up of to neutrons and two protons.

When nucleus of a radioactive atom decays by emitting an alpha particle, which is equivalent to a helium nucleus, to transform into a different atomic nucleus with a lesser mass (less by four)  and a reduced atomic number (reduced by two), it is said to have undergone an alpha decay or α-decay.