Hi Tim,
What you can do in this case is setup a time cutoff, so that any particle stops being transported after that set time.
In the case of an ISOTOPE source, the primary (decay product) is created with a time already set at the sampled decay time, which means the time cutoff can be used to remove those products that are coming from long-lived isotopes, thus reproducing somewhat a DCYTIMES philosophy (which as a reminder is not available for ISOTOPE sources)
In this particular case, we are lucky, as the half life of 241Am is much shorter than that of its daughter. You can then “isolate” 241Am decay products by setting a time cutoff at a few times 241Am half-life. This is done through the addition of the TIME-CUT card.
Please see TIME-CUT in the manual for the syntax, and be mindful of the fact time is expressed there in nanoseconds
Cheers
Philippe