The design studies related to a prompt g-rays in vivo neutron activation analysis facility are described. The designs were based on two neutron sources: 252Cf and 241Am-Be, four collimator materials: polyethylene, 5% boric-acid doped polyethylene, graphite, and the heavy water, two collimator shapes: pyramidal and rectangular, and two configurations: uni lateral and bilateral. The aim of this work was improving the uniformity of the gamma production rate distribution with depth in a large biological sample. For the sample, a water phantom measuring 32 cm*16 cm*100 cm was simulated. The simulations were performed by MCNPX 2.4.0 Monte Carlo code. This study recommends the prompt g-rays IVNAA facility in the bilateral configuration with graphite as a collimator mate rial and 241Am-Be as neutron source.

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