Dear experts
I used a histogram sampling using source_newgen.f but when I am scoring the fluence spectrum using USRBDX and normalizing it for MeV and area, I am not getting the fluence (Y-axis) as that of input one.
Please help me to find the reason for the discrepancy, I think I am missing some concepts here.

FLUKA doesnâ€™t take into account the absolute intensity of your source histogram/spectrum. The integral value of the histogram is automatically normalized to 1.0. You will need to multiply the results with the original intensity as it must be done with a simple BEAM source.

In your case, if I integrate your histogram, I get 0.000467 electrons per decay. After applying this normalization factor to the FLUKA result, I get back the input spectrum correctly.