Edited to clear up my typos.
I think you need a light element like Be. What it does is capture alpha particles and then emit neutrons. Be is good as it has a low neutron capture cross section.
Americium would make a good alpha source, but polonium 210 is even better. Very low gamma activity, but an amazingly potent alpha source, due to it's half life.
The reaction between Be and an alpha particle is: