RE: MicroShield Exposure Estimates

[Falo Gerald A <Gerald.Falo@APG.AMEDD.ARMY.MIL>]

Dr. Lowe,

Off the top of my head, I can think of 2 possible causes of the
discrepancies. With respect to the point source calculation, you do not
mention if your MS run included build up which is not explicitly included in
the gamma constant calculation.  Furthermore, and I am not very sure about
this point, I think that the gamma constants do not include contributions
from x-rays.  According to the revised edition of the Radiological Health
Handbook (1992) only gamma rays with energies greater than 0.01 MeV are
considered in the gamma constant.

You don't mention your source for the Ra-226 in soil numbers, but the values
in FGR 12 are calculated using Monte Carlo and transport methods and
estimate the effective dose equivalent to a person - not the external
exposure rate.  For a surface source, the numerical value for the exposure
rate is about 45% greater than the calculated effective dose equivalent
rate; that is, 1.45 R corresponds to about 1 rem (EDE). In fact, the actual
difference is likely even greater than that.

I suspect that the difference on the order of 10 to 20% that you see is not
unusual.

I hope this helps and that I have not misled you, but in the words of me
thesis advisor, "I reserve the right to be wrong."

Take care,

Jerry

Jerry Falo, Ph.D., CHP
HMJ Professional Associate
USACHPPM
Medical Health Physics Program
gerald.falo@apg.amedd.army.mil


-----Original Message-----

<snip>

We often do calculations involving NORM radionuclides and have found that
MS appears to give  exposure estimates that are larger (by 20% or more)
than one would obtain from other references e.g.. for Ra-226 in soil.  In
fact,  the MS estimate for a point Ra-226 source (and decay products) is
about 10% larger than the 0.825 R/h per Ci at 1 m given in the
Radiological Health Handbook.


Is this the "latest" value for a point Ra-226 source?  Has anybody else
found similar discrepancies with MS?



Leo M. Lowe, Ph.D.

Senior Health and Environmental Physicist

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