How to use class G4BOptrForceCollision as a variance reducing technique?

I do a simulation using neutron to bomb a thin target of boron-10 with thickness 2e-6 m, the energy of charged ion Li-7 and He-4 off the target are record, when 1e8 events has finished, the spectrum is below:

it seems the statistics is poor, so i use class G4BOptrForceCollision as a variance reducing technique,
the key codes in main() are:
G4GenericBiasingPhysics* biasingPhysics = new G4GenericBiasingPhysics();

the key codes in Detector class are:
G4BOptrForceCollision* fcl=new G4BOptrForceCollision(“neutron”,“ForceCollision”);

the result of simulation is shown below:

it seems the biasing algorithm is not work well.
Maybe i use the class G4BOptrForceCollision incorrectly, how to use the class G4BOptrForceCollision correctly?