dose rate from plane source

[Michael McNaughton <mcnaught@lanl.gov>]

I have not seen a rule of thumb for the dose rate for a plane source, so I
derived the following simple rule of thumb.
If E is the average gamma energy per emission in MeV,
a is the activity in Ci/m^3,
h is the half-value layer in meters,
pi = 3.14, and ln(2) = 0.693,
then the dose rate in R/h = (pi)(E)(a)(h)/ln(2)

I would appreciate any comments.

mike
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