Shielding studies on a total-body neutron activation facility

Background: Prompt gamma neutron activation analysis (PGNAA) is known as a non-invasive technique capable of measuring elemental concentration in voluminous samples in a short period of time. Also it is a valuable diagnostic tool for total body elemental measurements. 252Cf and 241Am-Be sources which are usually used in this method, generate not only neutrons, but also emit high-energy and unwanted gamma-rays. Because the patient must be located against the neutron source, patient dose during an analysis is an important concern when using this technique.Materials and Methods: Gamma-rays were attenuated without losing the neutron flux or significant alteration in the neutron spectrum. A relatively safe body chemical composition analyzer was designed with an optimal spherical gamma-ray shield, enclosed to the neutron source. Effects of gamma-ray shielding and optimum radius of spherical Pb shield was investigated and compared with the unfiltered bare source, using MCNP4C code. Then, the gamma ray dose equivalent per source neutron rate (user defined parameter) in the soft tissue is calculated for different radiuses of spherical Pb shield, for both neutron sources. Results: A decreasing flux of gamma-ray was observed when the radius of the spherical Pb shields increased. The value of this reduction was about 94% for 252Cf source when a lead spherical shield of radius 4 cm was used while the reduction was about 50% for 241Am-Be source with the same spherical shield. For a spherical Pb shield of radius 4 cm, reduction of the gamma dose equivalent per source neutron rate was about 8.44×10-17 Sv when the neutron source was 252Cf and about 1.24×10-16 Sv when the neutron source was 241Am-Be. Conclusion: Results show using optimum gamma-ray shield geometry can reduce the patient absorbed dose per incident neutron in a body chemical composition analyzer.