The source page

Specifying the radiation source consists of defining the actual alpha emitters, as well as defining how the emitters are deposited. As an example, radioactive particles in an air filter consist of particles (of non-active material) that encapsulate the radioactive nuclides. These particles in turn are embedded into a matrix of yet another material - the filter itself. This scheme of defining the radiation source allows for various different type of simulations, from radiochemically prepared samples (no particles, no source matrix) to rapidly collected samples that exhibit severe self-absorption effects.

For specifying the alpha emitting radionuclides to be used in simulation
see Radionuclides in the source

For specifying the deposition of the emitters
see Source properties