The Radiopharmaceuticals Division of Institute of Nuclear Engineering - DIRAD/IEN, produces ultrapure iodo-123 for medical aplications, using the CV- cyclotron accelerator. To produce this radionuclide, the CV-28 produces proton beams of 24 MeV energy and 20 µA current. This beam is driven to the target sample to be irradiated. This target cave, where a xenon-124 gas target is positioned, reaches the dose level limits set out by the Brazilian National Nuclear Energy Commission Standard 3.01 in its surrounding area. At its the access door there is a need to use a protection block formed by lead and polyethylene doped with boron, but this shield does not meet the minimum exposure limits of the standard in some specific points of the facilities. This report presents a methodology to determine the neutron flow in the area external to the block and study the use of materials for neutron shielding, allowing the reconstruction of the protection block with characteristics that make it more efficient, aiming mainly to reduce the dose rate in occupationally exposed individuals. The improvement of this shield is being developed through analysis of materials able to moderate, capture and absorb the neutrons, using the MCNPX code. The initial model was based on a 241Am-Be neutron source and a point detector was used. Tally F5 was used to estimate the flow in a point detector (neutron.cm-2) and in the input file a quantity converter was used to transform the flow values into environmental dose equivalent.
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