Hi Ding,
a few questions for clarification:
- the input as you sent it cannot be run, there is a DCYSCORE card is trying to modify a detector that does not exis (w_CL74). Make sure this is solved on your end.
- you are looking at a quite rare event, the activity of Eu152 from a concrete that only contains a tiny fraction of europium (~1e-6). That is not an issue of course, but does mean that longer run times and biasing techniques are inevitable
- it looks like your proton beam is not on axis with the copper target, which is resulting in only a small amount of neutrons (approximately 0.017 neutrons per primary protons, as you can see in the run summary of the .out file)
Please check the above points and let me know if it is all intentional. I tried with 1e7 primary particles, 20 spawns, and 2 cycles, and I have achieved satisfactory statistics. Of course you will need to decide according to your requirements and resources. Please let me know which of the scoring card you were evaluating when you encountered low statistics after the long run, as well as any cuts or projections you applied.
Regarding, the LAM-BIAS card, low-energy neutrons (En < 20 MeV) use a tabulated approach for their cross section and the biasing card does not apply to them, unfortunately. See this post and its reply.
Thank you
Stefano