Constant SE_BSE_SPLIT_SOURCE 

pub const SE_BSE_SPLIT_SOURCE: &str = "Chen, Zou, Mao, Khan, Tokesi and Ding, Sci. Rep. 12, 18201 \
     (2022), doi:10.1038/s41598-022-20466-3 (Introduction; section Monte Carlo simulation), citing \
     Niedrig, Scanning 1, 17 (1978); E = split is counted as backscattered, as Nebula's \
     DETECTOR_GE50 flag (source/core/cpu_material_manager.h, commit a50a8e8) does";

The source of the default split, as recorded in ElectronTallyMetadata.