Abstract : The 252Cf radioisotope and 241Am-Be are intense neutron emitters that are readily encapsulated in compact, portable, sealed sources. They have wide range of applications in laboratories, industries and medicines, because of their high flux and reliable neutron spectrum. They are cost-effective neutron sources for Prompt Gamma Neutron Activation Analysis (PGNAA). PGNAA method can be used in medicine for neutron radiography and body chemical composition analysis. 252Cf and 241Am-Be sources generate not only neutrons, but also emit high-energy that are useless when using PGNAA method. Also the sample in medical treatments is a human body that is forced to be under the harmful bombardments of undesirable gammarays. In addition accumulations of these high-rate gamma-rays in the detector volume eventuate simultaneous pulses that can be piled up and distort spectra in the region of interest (ROI). All of these restrictions forced us to attenuate these gamma-rays in a practical way without being concerned about losing the neutron flux or significant undesirable alteration in the neutron spectrum. Gamma-rays contribute 68% of the total activity from a 252Cf fission neutron source and 37.5% from a 241Am-Be. To reduce these gamma-ray components, spherical shields of variable radius, made of Pb, Bi and Hg separately, were considered that enclosing neutron source. Gamma-ray shielding effects and optimum radius of spherical shield and the proper material have been investigated and compared with the unfiltered case, bare source, by Monte Carlo simulation, using MCNP 4C code.

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