RE: Uranium and Thorium counting

[Simon "M." Jerome <smj1@npl.co.uk>]

Good morning everyone.

The quantification of Thorium-232 by gamma counting of the daughters is 
only possible if you know what the equilibrium state of the decay chain 
is. Monitoring Ac-228 will tell you how much Ac-228 and hence Ra-228 is 
present, as the Ac-228 takes about 2.5 days to arrive at radioactive 
equilibrium with the Ra-228 parent. The longer half life (~5.8 years) 
of Ra-228 means that it takes ~50-60 years to arrive at equilibrium 
with the Th-232 parent. In addition, outgassing of Radon-220 may 
deplete the samples in Po-216, Pb-212, Bi-212, Po-212 and Tl-208. If 
this is so, you may well under estimate the activity of Th-228 and it's 
daughters. It is possible to construct a graph to derive Th-232 content 
from Ac-228 and Tl-208 ratios, but this does not necessarily give the 
'right' result.

Some years ago, we ran an intercomparison exercise on the measurement 
of Th-232 and it's daughters in aqueous solution. Two sets of samples 
were supplied; those where the complete Th-232 decay chain was known to 
be in equilibrium and those where the decay chain was not in 
equilibrium. Good results were obtained for the equlibrium solution 
with all techniques - gamma spectometry, alpha spectrometry and neutron 
activation. For the non-equilibrium sample the only accuarate 
determination was the one obtained by neutron activation. All the 
others were way off.

But, in the end, it depends what you are tring to achieve. If you want 
to merely say present/not present, your stated approach will probably 
be OK. If you are trying to accurately quantify the levels of Th-232 
and it's daughters it will probably not be OK.

I was also interested to see that you use Eu-152 as a calibration 
source. Is this for energy and efficiency calibration? Eu-152 will give 
a very good energy calibration, but a very poor efficiency calibration, 
due to the gamma ray cascade summing.

If you want any further information, please contact me at my email 
address.

Best wishes,
Simon Jerome

National Physical Laboratory
Teddington, UK

Telephone:	+44 (0)181 943 6204
Facsimile:	+44 (0)181 943 7004
E-mail:		simon.jerome@npl.co.uk
Internet:		http://www.npl.co.uk/

>>
>>I am looking for information and references on using Gamma Spec to do 
soil
>>sample analysis for Thorium and Uranium (and daughters).  
Specifically, I am
>>trying to quantify the Thorium by counting Ac-228 and Tl-208 
(multiplied by
>>2.8), and the Uranium by counting Th-234 and Pa-234m.  I am using a 
Canberra
>>Inspector MCA with a HPGe detector, and Genie PC software, the system 
was
>>calibrated with a Eu-152 standard.  We are having trouble getting the 
proper
>>peak ID and quantifing the amounts.  Any tips would be appreciated as 
I am
>>obviously not an expert in this area.  You may respond to me at
>>RMS4941@AOL.com.
>>
>>Thanks in advance
>>
>>Raeann Shane
>>Radiological Engineer
>>
>>
>
>
>>End of message<<