The fission of ${}^{235}\mathrm{U}$ can be triggered by the absorption of a slow neutrons by a nucleus. Similarly, a slow proton cannot also be used. This statement is 

Because the neutron has no electric charge, it experiences no electric repulsion from a ${\mathrm{U}}^{235}$ nucleus. Hence a slow-moving neutron can approach and enter a ${\mathrm{U}}^{235}$ nucleus, thereby providing the excitation needed to trigger fission. By contrast a slow-moving proton feels a strong repulsion from a ${\mathrm{U}}^{235}$ nucleus. It never gets close to the nucleus, so it cannot trigger fission