There are several technological applications for the transuranium elements (Z > 92). An important one is in smoke detectors, which can use the decay of a tiny amount of americium-241 to neptunium-237. What subatomic particle is emitted from that decay process? Please explain your answer.

Please help me out, I can't understand this!

the difference in the masses is four amu ... the mass of an alpha particle

you can google ... americium-241 decay ... to check