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RE: A Question for Power Reactor Types and others with portal monitors.
I assume that you meant that the Mo-99 would have an
activity of 357 nCi after one week. Again, 0.15
uCi/mCi(Tc-99m) X 30 mCi(Tc-99m) = 4.5 uCi With a half
life of 2.75 days, the activity of Mo-99 would be 0.77
uCi. Of course, you are also in transient equilibrium
with Tc-99m. The combined activity would be 0.77
uCi-Mo-99 + (0.89)(0.770 uCi)-Tc-99m = 1.46 uCi.
However, manufactures are reporting Mo-99
breakthrough of only 0.03 uCi Mo-99/1 mCi Tc-99m, so
the values may be only 1/5 as great
http://www.nuclearonline.org/PI/Nycomed%20Mo%2099-Tc%2099m%20Genera.pdf
Most of the experience I have had in medical HP work
has never involved studing activities in the nCi
level. The exception is for contamination smears. I
guess that is why people in the medical fields do not
appreciate the problems with detection of
radionuclides, either at nuclear power plants or with
homeland insecutiry operations.
--- "Ayers, Rex G" <Rexg.Ayers@TycoHealthcare.com>
wrote:
> You always have a small amount of Mo-99 present. The
> capability to measure
> it with 4-5 orders of magnitude more Tc-99m is
> nearly impossible. At the
> limit 0.15 uCi/mCi, and we are never there, you
> would still have 357 uCi of
> Mo-99 after one week. Many portals, such as the NNC
> Gamma 60, can be tweaked
> to have a sensitivity as low as 20 nCi (mid-line
> with no self-shielding),
> for a radionuclide with energies in a range of
> Cs-137 where Mo-99 resides.
> This causes a Tc-99m test, depending upon compound,
> to cause portal alarms
> for several weeks.
>
> The big issue is always Tl-201 as the Tl-202 stays
> around for several months
> with a 12.2 day half-life. The Tl-204 is negligible
> due to the small number
> of atoms produced and the much longer half-life. A
> person with a heart study
> using Tl-201 can set off the portal for 4-5 weeks.
> During manufacture, the
> biggest issue is the dose rate from Tl-200.
>
> Rex Ayers, CHP
> Tyco Healthcare/Mallinckrodt
>
> -----Original Message-----
> From: Stabin, Michael
> [mailto:michael.g.stabin@vanderbilt.edu]
> Sent: Thursday, April 08, 2004 1:36 PM
> To: Sewell, Linda; Peter.Vernig@MED.VA.GOV;
> Radsafe@list.vanderbilt.edu
> Subject: RE: A Question for Power Reactor Types and
> others with portal
> monitors.
>
>
> >We have found that Tc-99 alarms our portals for 1-3
> weeks. I think it
> really depends on how much Mo-99 there is as a
> contaminant.
>
> I agree with John J - first, I know you meant
> Tc-99m, but mostly I am
> surprised that any place would have enough Moly
> breakthrough to be so
> noticeable. This is supposed to be a major QA point.
> You are right about
> the 10 half-life rule, that just gets you to
> (1/2^10), not zero, and if
> you are set to a sensitive level, this may still be
> measureable, but
> Tc-99m has a 6 hour half-life, a typical
> administration is perhaps 500
> MBq (depends on the compound), so you are down to
> 1-2 Bq in a week. So I
> am also skeptical that Tc-99m would alarm for more
> than a week, even
> with a sensitive detector, and we are back to the
> Moly hypothesis.
>
> Mike
>
>
> Michael G. Stabin, PhD, CHP
> Assistant Professor of Radiology and Radiological
> Sciences
> Department of Radiology and Radiological Sciences
> Vanderbilt University
> 1161 21st Avenue South
> Nashville, TN 37232-2675
> Phone (615) 343-0068
> Fax (615) 322-3764
> Pager (615) 835-5153
> e-mail michael.g.stabin@vanderbilt.edu
> internet www.doseinfo-radar.com
>
>
>
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