/run/verbose 2
/GB03/verbose 2 

/gun/particle neutron

# 10 layers, splitting with factor 2 is fine:
/GB03/numberOfLayers 10
/GB03/biasing/setSplittingFactor 2
/GB03/biasing/setApplyProbability 1.0
/run/beamOn 10

# 20 layers, splitting with factor 2 is too high
# to alleviate the "over-splitting", apply 
# splitting in only 50% of the cases:
/GB03/numberOfLayers 20
/GB03/biasing/setApplyProbability 0.5
/run/beamOn 10