[ RadSafe ] Reporter's question about lower limits of detection
Brennan, Mike (DOH)
Mike.Brennan at DOH.WA.GOV
Fri Aug 5 16:41:17 CDT 2011
I agree with everything everyone else has said. I would just point out
that the exuberant enthusiasm some of the answers show is because we so
seldom get asked questions, let alone the right ones. You are to be
complimented for doing so.
About the only bit that I might add is that if you were trying to
identify the source of the Sr-90 in fish, you might have to look at some
other isotopes. Probably the best way to distinguish between material
that recently came from a reactor (or bomb, but that probably would have
been noticed in other ways) is to look at the Cs-137 to Cs-134 ratio.
Cs-137 and Sr-90 have similar half lives, and are often found together
in consistent ratios in a given media within a particular area. Cs-137
and Cs-134 are produced in roughly the same amounts (on a per atom
basis) in most fission processes (though there may be ones I am not
aware of for which this is not true). Because Cs-134 has a shorter
half-life than Cs-137, you can use the ratio of the two to get a time
since the event that introduced them into the environment. Therefore,
if the ratio of Sr-90 to Cs-137 is as expected, the ratio of Cs-137 to
Cs-134 can tell you when the material entered the environment, and you
can see if that matches up with your suspected source.
<Humor>In practice it may be a little more complicated than this, but
that is mostly the rad-chemists whining.</Humor>
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Wald, Matthew
Sent: Friday, August 05, 2011 8:31 AM
To: 'radsafe at agni.phys.iit.edu'
Subject: [ RadSafe ] Reporter's question about lower limits of detection
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 ?
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.
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.
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.
--- Matt Wald
Matthew L. Wald
The New York Times
1627 Eye St NW, Suite 700
Washington, DC 20006
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