S-Space College of Medicine/School of Medicine (의과대학/대학원) Dept. of Radiation Applied Life Science (대학원 협동과정 방사선응용생명과학전공) Theses (Ph.D. / Sc.D._협동과정 방사선응용생명과학전공)
Radioactivity Determination of Sealed Beta-Emitting Sources by Surface Dose Measurements and Monte Carlo Simulations
베타 동위원소 선원의 표면흡수방사선량 측정과 몬테카를로 전산모사를 통한 방사능 교정 시스템의 개발에 관한 연구
- 의과대학 협동과정 방사선응용생명과학전공
- Issue Date
- 서울대학교 대학원
- 학위논문 (박사)-- 서울대학교 대학원 : 협동과정 방사선응용생명과학전공, 2015. 2. 예성준.
- Purpose: This study aims to determine an unknown radioactivity of a sealed pure-beta source using a conversion factor that can be derived from Monte Carlo (MC) simulations of the source. The conversion factor is defined as the ratio of surface dose rate to radioactivity (cGy/s•Bq). Thus, in order to determine an unknown radioactivity the measured surface dose rate of the source is divided by the calculated conversion factor.
Method: To validate this proposed method we started with the standard pure-beta source of a known radioactivity and compared this value with the radioactivity determined by this method. The surface dose rates of sealed standard sources of Sr/Y-90 and Si/P-32 were measured by an extrapolation chamber (EC) and also calculated by a Monte Carlo (MC) code. The radioactivities of the two standard sources were traceable to the primary calibration laboratory. A measured conversion factor was derived from the measured surface dose rate at a known radioactivity. A calculated conversion factor was derived from the surface dose per history. The detector efficiency, stopping power ratio and some other correction factors were also calculated by the MC simulations. The uniformity of radioactivity distribution on the surface of the source was measured. Finally, a sealed test source of Sr/Y-90 was manufactured by the HANARO reactor group of KAERI (Korea Atomic Energy Research Institute). The developed method was applied to determine the radioactivity of the source.
Results: The calculated value of stopping power ratio was approximately 1.112 for the Sr/Y-90 standard source and 1.115 for the Si/P-32 source. The detector efficiencies for four different surface-to-detector distances (SDDs) were calculated to correct the measured surface dose rates. The measured surface dose rates were 4.65 × 10-05 cGy/s and 2.25 × 10-05 cGy/s for the Sr/Y-90 and Si/P-32 standard sources, respectively. The calculated conversion factors were 1.21 × 10-8 cGy/s•Bq and 1.07 × 10-8 cGy/s•Bq for the Sr/Y-90 and Si/P-32 standard sources, respectively. The radioactivity of the standard Sr/Y-90 source determined by this method was 3.995 kBq, which was 2.0% less than the value certified by the primary calibration laboratory (4.077 kBq). For Si/P-32 the determined radioactivity was 2.102 kBq, which was 6.6% larger that the certificated radioactivity (1.971 kBq). The overall uncertainty involved in this method was determined to be 7.4%. Considering the uncertainty associated with our measurements and simulations, the determined radioactivities were in good agreement with the certified values. This indicates that this method based on our hypothesis be validated. The radioactivity of the Sr/Y-90 test source determined by this method was 4.166 kBq, while the apparent radioactivity reported by KAERI was 5.803 kBq. This difference might result from the lack of accuracy in determining the radioactivity of the original liquid and from the uncertainty in the amount of weighed aliquot. In addition, a possible 3D distribution of radioactivity in the absorbent disk of the source, which was approximated by a 2D distribution in the MC simulations, might contribute to this difference.
Conclusions: The developed method can be conveniently used to determine an unknown radioactivity of a sealed pure-beta source without limitations on its physical size and strength of radioactivity.