Issue with Multi-Group Fixed-Source Penetration in OpenSn: Flux Only in Source Region, No Penetration Observed

[Binji-Wang]

I am currently using OpenSn to simulate a multi-group fixed-source penetration problem. In my model, a volumetric source is added only to the leftmost layer (block_ids = [6]), while the right side consists of uniform material. I hope to observe the phenomenon of neutrons penetrating through multiple material layers from the source region, with flux gradually decreasing along the way. However, in practice, no matter how much I reduce SIGMA_T or increase the transfer parameters, the flux remains concentrated only in the source region, and the flux in the following material regions is almost zero, with no obvious penetration as expected. I have carefully checked the xs files, block_ids, and the correspondence between material region numbers, and I am sure the source is added to the leftmost layer. The material parameters are also set to extremely low absorption and high scattering (transfer). Is this normal numerical behavior for OpenSn in fixed-source problems, or am I missing something in my setup or parameter choices? Are there any example cases or experiences of similar fixed-source penetration problems that could be shared? Thank you very much!