[ RadSafe ] Reporter's question about lower limits of detection

[Marvin Resnikoff]

Matt:
The attached report from New York State may be helpful to you.  As you see the concentration of 8.5 pCi/kg is below detectable limits, according to the AREVA lab.  Generally 8 pCi/kg in bone is what is seen in Lake Ontario. Sr-90 in the atmosphere has gone down by a factor of 1000 since 1957.  Among other factors, the detectable limit is a function of the counting time, mass and sensitivity of the instruments.  To measure Sr-90, one actually monitors the decay product Y-90; after chemically separating Sr, and one has to wait about two weeks for the Y-90 buildup.  Sr-89 is added to determine the yield.
At the high end, one trout caught in Buttermilk Creek near the former West Valley reprocessing plant measured 320,000 pCi/kg in bone and 5,400 pCi/kg in flesh.
Marvin

--- On Fri, 8/5/11, Wald, Matthew <mattwald at nytimes.com> wrote:

From: Wald, Matthew <mattwald at nytimes.com>
Subject: [ RadSafe ] Reporter's question about lower limits of detection
To: "'radsafe at agni.phys.iit.edu'" <radsafe at agni.phys.iit.edu>
Date: Friday, August 5, 2011, 11:30 AM

I have been a "lurker" on this list for a couple of years, and I write intermittently about nuclear power. 
Could someone who is expert on this subject please help me with a radiation measurement question?
What is the lower limit of detection for strontium-90 in
 fish ?
Background: 
The Vermont Department of Health samples fish from the Connecticut River. It recently reported finding strontium-90 in some samples, slightly above what it said was the lower limit of detection, 47 pCi/kg. See: http://healthvermont.gov/enviro/rad/yankee/tritium.aspx

A website called Vermont Digger, http://vtdigger.org/2011/08/02/vermont-yankee-4/, reported on the result.  But Vermont Digger also says that New York measured 8 pCi/kg. 

http://vtdigger.org/2011/08/04/shumlin-vermont-yankee-not-necessarily-source-of-strontium-90-in-fish/

The underlying issue is whether the strontium came from the Vermont Yankee
 reactor, in Vernon, near the Massachusetts border, which had a leak from an underground pipe, or whether it is from fallout or some other source.  But I have another question. 
What is the lower limit of detection? 8 picocuries per kg? 47? Some other number? And what determines the lower limit? 
I would appreciate any explanation. 

Thank you. 
--- Matt Wald


Matthew L. Wald
Washington Bureau
The New York Times
1627 Eye St NW, Suite 700
Washington, DC 20006
202-862-0363
cell: 202-997-5854
fax: 202-318-0057

http://www.nytimes.com/info/nuclear-energy/
twitter: mattwaldnyt

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