Background: Activity-size distribution of fallout particles plays an important role in fallout prediction project. It also suggests how radioactive nuclides attached by carriers such as soil particles or debris and helps us know why fractionation happens. It should be determined by sampling

measurement and analysis

but not all this work well done in each ground explosion. Purpose: Feasibility of work to calculate the lower limit of particle size with special cumulate activity fraction by off-site contours of residual gamma radiation and dynamic model that can trace fallout particles was examined. Methods: Off-site contours of residual gamma radiation from Small Boy test were digitized

and the cumulative activity fraction in each area was estimated. Based on the spatial distribution of particles of different sizes deposited on the ground surface

which is calculated with a dynamic fallout prediction code

the lower limit of particle size with special cumulate activity fraction was discovered. Results: The results show that cumulative activity contribution from particles with diameter of a few hundred micrometer was 14% less than obtained with Bridgeman Equation. Conclusion: It’s feasible to calculate cumulative activity-size distribution of fallout particles from off-site contours of residual gamma radiation

and this work can help us to know activity-size distribution in different ground nuclear explosions.Cited