A Theoretical and Experimental Dose Rate Study at a Multipurpose Gamma Irradiation Facility in Ghana

Abstract

Radiation dose rate monitoring out at the Radiation Technology Centre (RTC) of the Ghana Atomic Energy Commission (GAEC) to establish the safety or otherwise of staff at the occupied areas is presented. The facility operates a rectangular source of Co-60 gamma with an having activity of 27.4kCi as at March 2015 and has 14 workers. The aim of the research was determine by means of practical and theoretical evaluations shielding effectiveness of the irradiation chamber. This was to ensure that occupationally exposed workers are not over exposed or their exposures do not exceed the regulatory limits of 7.5μSv/h or 50mSv per annum. The study included dose rate measurements at controlled areas, evaluation of personnel dose history, comparison of experimental and theoretical values and determination of whether the shielding can support a. 18.5PBq (500kCi) Co-60 source. Practical dose rate measurements when the source was in the irradiation position was carried out using a Thermo Scientific Rad-Eye Gamma Survey Meter in the controlled areas of the facility which included the control room, electric room, deionizer room, on top of the roof of irradiation chamber (specifically above the roof plugs) and the two entrances to the irradiation chamber; the personnel door and the goods door. Background reading was found to be 0.08±0.01μSv/h whilst the average dose rates at the two entrances to the irradiation chamber (ie.,- the personnel door and the goods door) were measured to be 0.090μSv/h and 0.109μSv/h respectively. Practical measurements at the roof plugs produced average values of 0.135μSv/h. A particular point on the roof marked as plug-3 produced a relatively higher dose rate of 8.151μSv/h due probably to leakage along the cable to the drive motor. Measurements in the control room, electrical room and deionizer room had average readings of 0.116μSv/h, 0.089μSv/h and 0.614μSv/h respectively. All these average values were below the regulatory limits of 7.5μSv/h and 50mSv/yr. The practical dose rate measurements were consistent with previous measurements. Theoretical calculations of dose rates in the irradiation chamber were computed using the F-line software provided by the Hungarian suppliers of the facility. It computes dose rates based on the dimensions and parameters of the source and uses the line source approximation method. Respective values obtained for the personnel and goods door, rooftop, deionizer room and outside the chamber were 0.082μSv/h, 0.076μSv/h, 0.080μSv/h, 0.193μSv/h and 0.07 μSv/h which indicates that the theoretical estimations of dose rates were generally lower than the measured values. Personnel dose (Thermolumiscent dosimeters) history for a period of 12 months (January to December 2013) was analyzed to estimate yearly doses received by radiation workers of the facility. Collective doses of Hp(10); 2.22mSv and Hp(0.07); 2.29mSv were obtained from the analysis. They were well below limits approved by the regulatory authority.