The 252Cf radioisotope and 241Am-Be are routinely compact and portable encapsulated neutron sources that have wide range of applications in laboratories, industries and medicine 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. Unfortunately 252Cf and 241Am-Be sources generate not only neutrons but also emit high-energy and undesirable gamma-rays that are useless when using PGNAA method. Also, since the sample in medical treatments is a human body, it will be under the harmful bombardments of undesirable gamma-rays. In addition, the existence of high-rate gamma rays eventuate simultaneous pulses in the detector that can be piled up and causes a significant background and distorts 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 alteration in the neutron spectrum. In order to solve these problems, a relatively safe Body Chemical Composition Analyzer was designed that uses an optimal spherical gamma-ray shield, enclosing neutron source. Gamma-ray shielding effects and optimum radius of spherical Pb shield have been investigated and compared with the unfiltered case, bare source, using MCNP4C code. At the end the gamma ray dose equivalent per source neutron rate (user defined parameter) in the soft tissue for several radius of spherical Pb shield, for both neutron sources are calculated. Results show how using an appropriate gamma-ray filter can reduce the risk of exposure to the 252Cf and 241Am-Be neutron sources when using them in a Body Chemical Composition Analyzer.

Keywords