Anomalous behavior of neutron refraction index in a perfect crystal near Bragg resonance was studied. This phenomenon is connected with the resonance behavior of potential of neutron interaction with crystal near the Bragg reflex. The amplitude of this resonance is equal to magnitude of g-harmonic of neutron interaction potential Vg and width is about the Bragg width of reflex. Recently, it was shown that for the case of noncentrosymmetric crystal this effect result in a large electric field acting on a neutron (value of the field can reach about 10⁸V/cm) . This effect is planed using to search for the electric dipole moment of a neutron. If the degree of crystal imperfect is less than the Bragg reflection width (case of perfect crystal) the width of the reflex is determined by the own width of crystal reflex that is about 10^-5 of the neutron energy. The value of g-harmonics of interaction of neutron with crystal Vg and optical potential of the interaction of neutron with crystal V0 are usually about the same. Therefore the variation of neutron energy on a 10^-5 of its value will change significantly a potential of neutron interaction with crystal.