Here's why:

* Alpha decay is a type of radioactive decay where an atomic nucleus emits an alpha particle, which is essentially a helium nucleus consisting of two protons and two neutrons.

* Pu-240 has 94 protons and 146 neutrons.

* U-236 has 92 protons and 144 neutrons.

Therefore, the difference in mass and atomic number between Pu-240 and U-236 corresponds exactly to the composition of an alpha particle (2 protons and 2 neutrons).