Multi-layer shielding optimization of a high activity ²⁴¹Am-Be mixed field irradiation facility

Neutron and gamma-ray shielding design for a 30Ci (1.11TBq) ²⁴¹Am-Be irradiation facility is studied using MCNP5 Monte Carlo simulation code. The study focuses on the optimization of the shielding layers of the previously planned neutron irradiation facility. The shielding design aims at reducing the effective dose rate down to 10 μSv/h for occupationally exposed workers as recommended by the International Commission on Radiological Protection (ICRP) (20 mSv per year for workers). The dose rate calculations consider not only ²⁴¹Am-Be produced neutrons but also gamma rays from the ²⁴¹Am decay, ¹²C∗ de-excitation, and neutron interaction with source and facility materials. Different materials (paraffin, borated-paraffin, beryllium, copper, tantalum, tungsten, zinc, bismuth, and lead) have been investigated as neutron and gamma shields. The choice of the shielding materials is based on their effectiveness (the total weight of the facility and the cost). Among several configurations, shielding layers consisting of 10 cm paraffin wax, 2.5 cm lead, and 31.5 cm borated-paraffin wax are found sufficient to meet the safety requirements, leading to a cubic facility of 104.2 cm overall length. Some irradiation parameters are estimated for the final design; namely neutron and gamma spectra as well as flux and absorbed dose components. In addition, the occupational exposure is assessed considering ICRP exposure conditions to calculate the effective dose rate along with the isodose distribution.