I found no DOSE-EQ outside room

Dear FLUKA users, I met a problem when using FLUKA. I am simulating a room with a shielding thickness of 120cm hematite concrete. The radioactive source is Co-60, and the activity is 3.7e15 Bq. When outdoor DOSE-EQ is below 2.5uSv/h, I can ensure that this shielding thickness is safe for workers. I used region importance biasing (BIASING) when simulating, but I found no DOSE-EQ outside the room. Actually, I also used thinner bins and increased the number of particles to have a better result. But they did not work. Could you please give me some suggestions if possible? Thanks so much:)
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I have a few remarks concerning your question and your input. First of all I suggest to revise your importance biasing map, and organizing it such that the importance factor stays constant for the given source-to-region distance, eg use RPP bodies to differentiate between the layers, not slabs. To understand it better you can refer to the BIASING lecture: 15_Biasing_2022_ULB.pdf (cern.ch). Secondly to get good results (good statistics) for such a thick shielding (with 50% iron) you need to cumulate a decent number of primaries, knowing that you reduce DOSE-EQ by many orders of magnitude. Or once you have a correct biasing map you could also apply more aggressive biasing.
I hope this helps,

Best regards,