If you are only interested in the time between the produced secondary and its detection then @maire1 ‘s solution is easiest. This will still give an “offset” if the seconday is from a daughter, granddaughter, etc. You could write or modify RE01’s UserTrackingAction example to grab all secondaries after checking the particle ID. That would allow you to kill all neutrons based on the parent or reset the global timer of the particle when Pu240 (and only Pu240) decays/fissions/etc.
What specifically are you interested in with Pu240? Is the Pu240 fission interesting because it can seed neutrons for Pu239 induced fission (ala LANL’s BeRP ball) or just in the spontaneous fission of Pu240 itself? If it is the latter then you can modify what I suggest in this thread to “force” spontaneous fission for every single event (Cf252 has a SF BR of 3%). That will increase your statistics for S.F events by 1E4%