alpha particles monoenergetic?

By quantum mechanics, alpha particle is trapped in a potential well by the nucleus. Classically, it is forbidden to escape, but according to principles in quantum mechanics, it has a tiny (but non-zero) probability of "tunneling" through the potential barrier and appearing on the other side to escape the nucleus.

It is found that the energy of the alpha emitted by" tunneling effect" is related to the decay constant of the parent nuclide (the Geiger- Nuttall Law). Because the half-life of a particular nuclide is a constant, hence the "decay constant". This resulted in a constant energy of the alpha particles emitted by that niclide.