| [Purpose] In nuclear event fast external dose estimation can provide dose reference for early medical treatment of radiated individuals and theoretic reference for nuclear event protocol development. In order to enhance security of emergency response personel, high dose rate areas can be identified with the estimation. This study focused on development of fast external dose estimation method, establishment of external dose database and development of fast external dose estimation software including 26 radioactive isotopes as radiation sources.[Methods] In total,26 radioactive isotopes were chosen according to IAEA reports and Chinese standards. Radiation type and energy of each isotope was defined according to isotope data manuals.MCNPX was chosen as Monte Carlo simulation workshop. According to ICRP No.103 report,15 tissues were chosen for simulation. According to ICRP No.74 report, 5 external radiation models were set up. Radiation source sprayed rays to individual at limited solid angle, so angle factorΩ/4πwas established. The air attenuated rays when they were travelling to individual, so air-attenuating factor C was established. After all these factors were considered, MCNPX models were set and simulated. Monte Carlo simulation method was developed for organ/tissue dose estimation. The whole process of development of organ/tissue dose estimation method was demonstrated.Analysis of MCNPX results led to split external dose estimation into two parts: beta-ray-caused part and gamma-ray-caused part. With isotope activity factor, radiated time factor, angle factor and air-weakening factor considered, final external dose estimation methods for beta-ray-emitting isotopes, gamma-ray-emitting isotopes and beta-gamma-mixing-ray-emitting isotopes were set. Effective dose estimation was developed based on tissue weighting factors from ICRP No.103 report. In the end, fast external dose estimation software was developed.[Results] After 1300 hours of MCNPX simulations, all results were collected as organ/tissue dose per source particle, with mnemonic Y. Among MCNPX results, when source emitted beta rays, skin doses per particle were at least 3 magnitudes greater than other results. So if the activity of source isotope is not too high, only skin dose should be considered when source emits beta rays. For beta sources, formula for calculating external organ/tissue dose was developed, while only skin dose was taken into consideration for effective dose.For gamma sources, formula for calculating external organ/tissue dose was developed, while weighting-average method from ICRP No.103 report was the method for calculating effective dose.For sources emitting both beta rays and gamma rays, to specify the ratio of beta-ray-caused dose to gamma-ray-caused dose, models emitting beta rays and models emitting gamma rays were developed and simulated. Beta rays and gamma rays were both considered in the formula for external organ/tissue dose calculation, while formula for effective dose was developed based on organ/tissue dose formula.Analysis of external dose estimation examples when 32P,201Tl or 137Cs was the source gave a conclusion that external dose caused by beta rays was too little to care about when the distance between source and individual was longer than 2 m; for the same source, external dose caused by gamma rays was mainly angle factor's puppet when the distance was no longer than 50 m; when the distance was 5 m, external dose was 1% of that when the distance was zero.[Conclusion] Based on MCNPX results, after taking into account of angle factor and air-attenuating factor, external dose estimation method for 26 radioactive isotopes was developed. Formulas for external organ/tissue dose and effective dose were developed with great compatibility for different type of source. External organ/tissue doses caused by isotope 60Co were estimated and compared with published values, proved the validity of external dose estimation method.
|
PDF Full Text Request