Answer is: alpha decay.
Nuclear reaction: ²³⁸U → ²³⁴Th + α (alpha particle).
Alpha particle is nucleus
of a helium-4 atom, which is
made of two protons and
two neutrons.
Alpha decay is radioactive decay in which
an atomic nucleus emits
an alpha particle (helium
nucleus) and transforms into an atom with an atomic
number that is reduced by two and mass number that is reduced by four.
