Re: skin contamination to skin dose assessment

[Bob Flood <bflood@SLAC.Stanford.EDU>]

At 10:23 AM 4/9/97 -0500, you wrote:
>with the exception of the case of a zero thickness source on bare skin
>(absorber thickness = 7 mg/cm^2), where 7 +/- 20% only bounds this curve
>up to a maximum beta energy of 750 keV or so.  For a zero-thickness source
>on bare skin, the skin dose rate per micro-curie per cm^2 is bounded
>between 8.5 and 9.2 for maximum beta energies between 750 keV and 3 MeV
>(according to Healy).
>

This is a good rule of thumb for determining skin dose once the identities
of the isotopes in the contamination are known. But that's the hard part.
Operationally, determining the isotopes in the contamination can be very
difficult, from obtaining a truly representative sample for analysis to
dealing with isotopes that cannot be detected by gamma spectrum analysis
(Sr-89, Sr/Y-90, et al.).

The detection efficiency of the standard frisker (15.5 cm2 thin window GM
tube in all its many configurations) changes with beta energy in a way that
looks like a GM plateau curve: basically, detection increases as beta
energy increases up to the point where almost every beta emitted within the
solid angle defined by the source and the detector  is detected, hence the
plateau.

The skin dose rate per uCi also exhibits this same plateau curve versus
beta energy (I have my own ideas why this happens, but I'd love to hear
some opinions). As a result, it so happens that the standard 2 inch
diameter frisker (15.5 cm2 detector surface area) is an almost energy
independent skin dose rate meter. One can simply use any matched pair of
detection efficieny and dose rate factor for a given isotope and calculate
an accurate dose rate, without knowing the actual isotopes present.

For example, let's look at a frisker survey of 10,000 cpm from
contamination on the skin. If we use (without benefit of analysis) the
detection efficiency for Co-60 (10%) and the dose factor for Co-60 (4.1
rad/h/uCi), the dose rate calculates to about 185 mrad/h (10,000 cpm =
0.045 uCi). But suppose the contamination was all really Rh-106, max beta
energy about 3.6 MeV, with a dose rate per uCi over double that of Co-60
(9.4 rad/h/uCi). Well, using the detection efficiency for a hard beta beta
like Rh-106 (25%), one gets 10,000 cpm/0.25 = 0.018 uCi, times 9,400
mrad/h/uCi, yielding 171 mrad/h. I'll accept the difference between 185 and
171 rather than try to do isotope identification any day.

If one does this calculation for a variety of isotopes from the Co-60
energy to the Rh-106 neighborhood, one gets about 175 mrad/h/10,000 frisker
cpm +/- 5%, which ain't bad for a hand held survey instrument!


Bob Flood
Stanford Linear Accelerator Center
(415) 926-3793     bflood@slac.stanford.edu
Unless otherwise noted, all opinions are mine alone.