Browsing by Author "Mahanyapane, Motlatsi Vincent"
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- ItemRadiation shielding design for sealed radioactive gamma-ray and neutron sources: measurements and modeling(Stellenbosch : Stellenbosch University, 2022-04) Mahanyapane, Motlatsi Vincent; Newman, Richard T.; Van Zyl, J. J.; Stellenbosch University. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: Adequacy of the radiation shielding implemented for sealed radioactive sources used at the University was evaluated and improved. Three different setups were investi- gated; 1. neutron activation drum, 2. radioactive-source store, and 3. gamma-ray container. Gamma-ray and neutron dose rate measurements were performed for the 5.8 GBq 241Am-Be neutron source inside the neutron activation drum in the labora- tory, as well as in and around the radioactive-source store in the Department. The radiation shielding design of the neutron activation drum and the radioactive-source store was modeled using the Monte Carlo radiation transport code FLUKA. Fur- thermore, the shielding container for gamma-ray sources was modeled and designed. Dose rates in the areas around the laboratory were at the natural background levels (0.1 μSv/h), except in the next-door laboratory close to the wall near the neutron activation drum. Dose rates were above acceptable limits inside the laboratory. The largest contribution was due to neutrons that leak out of the drum due to inade- quate shielding while gamma rays were effectively attenuated. It is recommended that at the time of neutron activation experiments, one should reduce stay times to short times when placing the neutron source inside the drum and taking it out. These times should be less than 20 minutes stay time 5 cm away from the drum side. The measured and simulated neutron dose rates were comparable, with the simulated neutron dose rates being slightly higher than the measured dose rates by a factor of 1.3 and 1.8 on the top drum surface and 5 cm horizontally away from the drum, respectively. For the radioactive-source store, measurements and simulations suggested safe ra- diation levels around the radioactive-source store with dose rates at the natural background radiation levels (0.1 μSv/h). An implication was that the next-door offices could be accessed without restrictions. Inside the radioactive-source store, however, dose rates were above acceptable levels. Therefore, it was concluded that the implemented radiation shielding was inadequate for maintaining dose rates at safe levels. The simulated gamma-ray dose rates alone were at natural background radiation levels. However, it was found from the comparison of results that gamma- ray dose rates were mainly due to the gamma-ray sources stored in the storeroom. Significantly high neutron dose rates were also resulting from the 241Am-Be neutron sources in the soil moisture gauges inside the storeroom. It was suggested that the radiation shielding for the 5.8 GBq 241Am-Be neutron source should be improved by increasing the thickness of paraffin wax around the source to 28 cm. A new gamma-ray shielding container was simulated, showing that 7 cm-thick lead attenuated gamma rays and reduced dose rates by 97 % on the container surface.