Neutron cross section above 20 MeV

I use geant4.10.5 version and I want to alpha production by incident neutron in silicon bulk.

But I run it, the alpha production rate is mismatch near 20 MeV incident neutron. (cross section drop above 20 MeV neutron)
I use FTFP_BERT_HP for Hadron inelastic physics.

I know that Neutron HP only applies below 20 MeV, but I am wondering if cross section can fit well even above 20 MeV.

It is my simulation results.

neutron energy (MeV) / alpha counts
17 / 529
19 / 517
21 / 154
23 / 174
25 / 199

The number of alpha particles is dropped above 20 MeV neutron energy …