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From what I read the radiation levels in Nagasaki today are equivalent to the natural background radiation levels in the surrounding areas. On the other hand, the radiation levels at the Trinity test site are x10 the natural background level.

I understand that the device used in Trinity was the same as the one dropped on Nagasaki. So why the huge difference? Is it all attributed to the different detonation altitudes? Or are there other factors?

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    $\begingroup$ I think it's all due to the altitude. 30 m for the Trinity vs 500 m for the Fat Man. Much more dirt would be irradiated for the former. $\endgroup$
    – OON
    Jun 16, 2019 at 12:26
  • $\begingroup$ I visited the Trinity site as a child in 1963. The crater had been buried with a layer of dirt. But I still found a small fragment of glass melted from the sand when the bomb exploded. I took it home and kept it. I lost it many years ago. $\endgroup$
    – mmesser314
    Jun 16, 2019 at 15:22

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The Nagasaki blast altitude was set as a compromise between minimizing the generation of radioactive fallout (setting the detonation altitude high) versus the need to maximize blast damage (setting it close to the ground). This could be done because the bomb was dropped from an airplane at high altitude, allowing its burst altitude to be chosen.

The Trinity test was performed on a shot tower which necessarily put it close to the ground, where the neutron flux then induced much more radioactivity in the soil.

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