Re: MicroShield Exposure Estimates

[SENES Consultants Limited <senes@senes.on.ca>]

Thanks for the informative responses to my MicroShield (MS) questions.  I have a few comments elicited by the responses.  It should be noted that I was referring only to exposure rates (R/h), not dose or effective dose equivalent rates.   

Considering first the point source of Ra-226, the MS exposure rate estimate is reduced by only about 2% when a 0.5 mm Pt filter is included (spec. grav = 24) i.e. from about 0.91 R/h per Ci at 1 m to 0.89 R/h.  The MS estimate includes build-up in air and excludes photons below 0.015 MeV.  If build-up in the Pt filter is assumed, the exposure rate is reduced to 0.84 R/h.  Which is correct?  (P.S.  Does radon escape the "point" source?)

As pointed out in responses to my first message, these estimates are not much different that the Handbook value of 0.825 R/h.  (Is this a theoretical, calculated value or a measured value?)   It is important however to know the "correct" value since Ra-266 sources are often used as calibration sources for survey instruments.

The MS estimates diverge further from values given in other references when considering Ra-226 in soil.  For example, NCRP No. 94 (1987) gives a value equivalent to 1.8 uR/h per pCi/g in soil (based on an old Beck et al. paper) and ICRU Report 53 (1994) ("Gamma -Ray Spectrometry in the Environment) gives  a  calculated value of about 1.9 R/h.  However, MS gives 2.4 uR/h (build-up in the source included), or about 33% larger than the NCRP value (while using the same soil composition).  The MS estimate is 2.7 uR/h when build-up in air is assumed.  Which is correct?  Both estimates are higher than the other references.  All these estimates assume complete radioactive equilibrium i.e. no effect of radon escape.  (Note that the exposure rate is independent of density for the same pCi/g concentration in thick sources.)

Again, these differences may appear to be relatively small but can be practically significant , for example when developing soil clean-up or remediation criteria.

Relative to effective dose estimates where irradiation geometry, energy and body size are all important, UNSCEAR (1993) references calculations that show the effective dose from exposure to U-238 series radionuclides in the ground (i.e. Ra-226) is about 0.6 rem per R for adults and 0.8 rem per R for infants.

Anyone who is interested in these issues may respond directly to me if they feel the detailed discussions are not of  interest to the general Radsafe community.



Leo M. Lowe, Ph.D.
Senior Health and Environmental Physicist

SENES Consultants Limited
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