Re[3]: Th-232 air sample analysis -Reply

[Rodney Bauman <rodney_bauman@wssrap-host.wssrap.com>]

Mike,  The answer to all your questions is basically "Yes." 

The primary purpose of the decay period is to rid the sample of the large
quantity of thoron daughters which occupy the sampled air space due to previous
thoron migration (originating from Th-232 that was never collected on the air
sample).  If you do not allow for such a decay period, the short-lived thoron
daughter products collected on the filter will be disproportionately greater
than what would be expected due to equilibrium only.

A 7-day waiting period also provides sufficient time for all short-lived thoron
daughters to come into equilibrium with the thoron.  However, I don't think that
this is required due to an undetermined amount of thoron that would escape from
the filter.  I do not know what fraction is expected to escape, however, for
counting purposes, it would be conservative to assume that 100% escapes.

As you pointed out, an alpha spec system can easily filter out such phenomenon
by setting an appropriate energy window.  If measurements are to be made
"real-time," then alpha spec is your only option.

Rodney Bauman, CHP, RRPT
rodney_bauman@wssrap-host.wssrap.com

____________________Reply Separator____________________
Subject:    Re[2]: Th-232 air sample analysis -Reply 
Author: <radsafe@romulus.ehs.uiuc.edu>
Date:       9/21/99 9:23 AM

Rodney,  I'm feeling a bit slow this AM (OK, *every* AM)....

Since you're dealing with Th-232, wouldn't you be continuously
feeding the thoron daughters?  That is, if you've got Th-232 on the
filter, the thoron progeny won't dissipate.  Right?  (I told you I'm
feeling slow).

What are your thoughts on this?  Do you use the 7-day decay to rule
out natural background only?  And if you see thoron progeny after
7-days, you attribute it to the Th-232 chain?

Continuous air monitoring for Th-232 is generally accomplished by
lowering the ROI window to exclude the radon/thoron tails.  Since
there is about a 2 to 4 MeV difference in alpha energies btw Th-232
and radon/thoron progeny, this can be accomplished fairly easily. 
This technique can also be readily transferred to a lab analysis.

My own personal/non-corporate thoughts,
v/r
Michael
mford@pantex.com
TX Radiation Advisory Board

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