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View Diary: Radioactivity, part 3: rads, rems, etc. (110 comments)

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  •  400 mSv / hr = 40 rem / hr (0+ / 0-)

    I believe the diary may be attempting to quote this figure, i.e. 400 mSv / hr, a measurement from near the Unit 3 reactor building that was widely reported yesterday.  For example, by the IAEA at 15 March 2011, 11:25 UTC:

    http://www.iaea.org/...

    This is 40 rem / hour, or 40,000 mrem / hr, not 400 mrem / hr.  Now the levels have reportedly fallen significantly.

    As an aside, I'll point out that most instruments do not actually measure effective dose equivalent units (rem or Sievert), but measure charge released in some material, e.g. air, and the readings can be subject to interpretive errors. An ion chamber survey instrument that measures roentgens (or coulombs of charge per kg dry air) can be used to estimate whole-body gamma dose equivalent (in rem) by multiplying by a conversion coefficient that, for many common energies of gamma rays, and in the popular understanding, is approximately 1 +/- 20%.  BUT, if you take that ion chamber outside and expose it to beta-emitting fallout, the beta particles will contribute to the measured ionization in a way that is not proportional to their biological effect (which is mostly on the shallow tissues).  It's easy to imagine the measurement of 40 rem / hr being not strictly accurate, or at least being subject to high uncertainty.  However, that's a huge number relative to 400 mrem / hr and was bad news when it came out.  

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