grays rads sievert difference between X-ray and neutron absorption Little Boy neutron flux Baratol Fat Man neutrons higher biological effect per unit of energy absorbed radiation dose reconstruction dosimetry radiation Nagasaki hypocenter Hiroshima
The reporting of doses in terms of absorbed energy in units of grays and rads – rather than the biologically significant, biologically weighted sievert in both the SMR and cognitive performance data – is typical. The reported threshold dose variance between the two cities is suggested to be a manifestation of the difference between X-ray and neutron absorption, with Little Boy emitting substantially more neutron flux, whereas the Baratol that surrounded the core of Fat Man filtered or shifted the absorbed neutron-radiation profile, so that the dose of radiation energy received in Nagasaki was mostly that from exposure to X-rays/gamma rays. Contrast this to the environment within 1500 meters of the hypocenter at Hiroshima, where the in-utero dose depended more on the absorption of neutrons which have a higher biological effect per unit of energy absorbed. From the radiation dose reconstruction work, the estimated dosimetry at Hiroshima still has the largest uncertainty as the Little Boy bomb design was never tested before deployment or afterward, therefore the estimated radiation profile absorbed by individuals at Hiroshima had required greater reliance on calculations than the Japanese soil, concrete and roof-tile measurements which began to reach accurate levels and thereby inform researchers, in the 1990s.
