|Budget Amount *help
¥2,700,000 (Direct Cost : ¥2,700,000)
Fiscal Year 1994 : ¥900,000 (Direct Cost : ¥900,000)
Fiscal Year 1993 : ¥900,000 (Direct Cost : ¥900,000)
Fiscal Year 1992 : ¥900,000 (Direct Cost : ¥900,000)
The study included (1) collection A-bomb exposed rocks, roof tiles and tiles, (2) measurements of low level radioactivities induced by A-bomb neutrons, (3) benchmark test of the computer code (MCNP) by simulation experiments, (4) consideration of discrepancies between the measured data and the calculations based on DS86 and clarification of it cause.The terms from (1) to (3) were satisfactorily completed until the last year and, in this fiscal year, I considered its cause of discrepancy.
The discrepancy was firstly found by our group.The major problem was in Hiroshima A-bomb neutrons and at 1.5km DS86 calculations were only 10-30% of the measurements.The terms we checked in this study were as follows :
A.The energy spectrum of neutrons at the burst point
B.Transport calculations of the neutrons through air.
C.Benchmark test of the computer code (MCNP).
D.Measurements of air density (including humidity) by different height.
E.Verifications of activations at soil surface by simulation experiments and calculation.
F.Comparison of gamma-ray data and calculations.
As conclusions we find the problem is in the term A of Hiroshima.Namely, (a) in Nagasaki all terms (from A to F) are correct.(b) In Hiroshima terms from B to F are correct. Thus the term A is wrong.(c) The verification of the term A is impossible for us, since it include the process of the A-bomb detonation.Therefore, I performed many transport calculations changing the basic assumptions of DS86 and tried to find out the best assumption to explain the difference.As the results of the calculations I find out the assumption of DS86, that is, "before the A-bomb body was cracked, neutrons were totally emitted through thick iron material (about 20 cm thick Fe)".might be in error.I assumed 5-15% of the neutrons were emitted through a crack which divide the A-bomb body horizontally.By this assumption almost all data could be reproduced.The dose evaluation will be necessary after this.