RE: Thorium Mantles as Check Sources

["Heinmiller, Bruce" <heinmillerb@aecl.ca>]

This may be of only academic interest, but I assume that thorium for use in
mantles is chemically separated from all the non-thorium daughter nuclides.
Assuming equilibrium in the earth before separation, that would result,
after separation, in equal activities (but no longer in equilibrium) of
Th-232 and 1.9-year Th-228.  The Th-228 daughters would build up to
equilibrium in a couple weeks, so you don't see the activity increasing by
the time you buy a mantle.  What you should see if you look over a period of
even months (and if it's relatively freshly separated thorium) is decreasing
activity as the Th-228 decays much faster than it's replaced from Th-232
decay, as one of the intermediaries is 5.7-year Ra-228.  And then after a
few years (I haven't run the equations) I would expect the activity to
increase again as the Ra-228 builds up.  If you've got a 20-year-old mantle,
you should have the whole chain pretty much back in equilibrium.  Also, you
should be able to date a mantle (or at least the final separation before its
manufacture, and if the separation's specific to thorium) by gamma
spectroscopy, as Ac-228 has nice gamma rays for comparison with those of the
Th-228 daughters.

I once had someone from the U.S. tell me at an intercomparison in France
that his lab had imposed onerous packaging requirements on calibration
sources that he'd wanted to take with him, and he'd decided to substitute
with a thorium lantern mantle.  However, while there were no packaging
requirements for a mantle, there were onerous packaging requirements for
part of a mantle shaped and sealed into the appropriate geometry for
counting, because he'd thereby have created "a source".

Bruce Heinmiller CHP
heinmillerb@aecl.ca

> ----------
> From: 	Tad Blanchard[SMTP:Tad.M.Blanchard.1@gsfc.nasa.gov]
> Reply To: 	radsafe@romulus.ehs.uiuc.edu
> Sent: 	Monday, September 21, 1998 1:33 PM
> To: 	Multiple recipients of list
> Subject: 	Thorium Mantles as Check Sources
> 
> If you have a calibrated instrument and attach a lantern mantle to it and
> immediately log the contact reading (or at a measured distance), why
> wouldn't this be a good "operational" check source.  
> 
> The Thorium won't lose much activity over time (14,100,000,000 half life).
> 
> It seems this would be a relatively inexpensive check source that would
> not
> require regulatory agency involvement, routine inventories, leak tests,
> etc.
> 
> 
> 
> 
> 
> ************************** /^\   /^\ ***********************************
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