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In fast reactors, the inherent neutron source strength is often insufficient for monitoring the reactor start-up operation with ex-core detectors. To increase the subcritical neutron flux, an auxiliary neutron source sub-assembly (SSA) is generally used to overcome this prob-lem. In this study, the estimated neutron source strength and detector count rate of an antimony–beryllium-based SSA are obtained using the deterministic transport code DORT and Monte Carlo calculations. Because the anti-mony activation rate is a critical parameter, its sensitivity to the capture cross section and neutron flux spectrum is studied. The reaction cross section sensitivity is studied by considering data from different evaluated nuclear data files. It is observed that, because of the variation in the cross sections from different evaluated nuclear data files, the values of the saturation gamma (>1.67 MeV) activity and neutron strength predicted by ORIGEN2 lie within ± 2%. The obtained antimony activation rate and sensitivity to the neutron flux are partially validated by irradiating samples of antimony in the KAMINI reactor. The average one-group capture cross sections of bare and cadmium-covered 123Sb samples obtained by the ratio method are 4.0 and 1.78b, respectively. The results of the calculation predict-ing the activated neutron source strength as a function of operating time and sensitivity to the neutron spectrum in the irradiation region are also presented.