Dear @hanhechong,
This problem has already been addressed several times on the forum. As an example, have a look at this thread and its solution Low energy neutron xsec not found for some media - #2 by horvathd
Dear @hanhechong,
This problem has already been addressed several times on the forum. As an example, have a look at this thread and its solution Low energy neutron xsec not found for some media - #2 by horvathd