RE: A Question for Power Reactor Types and others with portal monitors.

["Ayers, Rex G" <Rexg.Ayers@TycoHealthcare.com>]

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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