In this paper, a procedure was proposed for calculating the neutron yield and the spectrum of the Am-Be source for different granular size of the source mixture. With the combined use of the calculation codes SOURCES-4C and MCNP5, computational models of the source differing in the structure of the mixture and the energy spectrum were presented. By calculating of neutron transport, an analysis of the appearance and loss of neutrons in the sources and in the steel shell of the source was carried out. As a result of the work, it became clear how the grain size of the source mixture affects the neutron yield
