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.