[ RadSafe ] Reporter's question about lower limits of detection
Thompson, Dewey L
DThompson3 at ameren.com
Fri Aug 5 13:45:49 CDT 2011
I am going to attempt to reply to your request with the assumption that you don't know much of the science behind your question. I do not intend to "talk down" to you, but I am making an assumption that you want facts in a manner that you can understand them.
First. There is no single "Lower Limit of Detection" (LLD) in fish. LLD by definition is dependent upon the specifics of the counting system and sample used, and the numbers vary wildly between laboratories.
The term LLD is a statistics analysis of the probability that radioactivity is present in a given sample, "usually" quoted at "95% probability" both ways. That means that there is a 95% probability that radioactivity is in the sample if the results indicate positive, and there is a 95% probability that radioactivity is NOT in the sample if the results indicate negative. Statistics is is a rigorous mathematical discipline, however I'm sure you have heard the following axioms:
There are Lies, Damn Lies, and Statistics.
Liars Figure, and Figures Lie.
My point is that because of the complexity of the math, most people don't understand it, and as humans we tend to not trust what we don't understand. Then there are the people who think they understand, but don't, and make bad decisions or claims because they don't understand.
Now. You are asking a question of one lab says they can detect down to 8 pCi/Kg, and another lab that says their LLD is 47 pCi/kg. Both could well be correct, or both could well be silly. My next discussion is to outline the difficulty of measuring Sr-90 in a sample.
First. Sr-90 decays to Yttrium-90 with what is called "beta" decay. There is negligible gamma radiation emitted from Sr-90 or the Y-90 daughter. Sr-90 has a "half life" of 28.78 years, the Y-90 daughter has a half life of about 2.67 days. The S-90 beta has a maximum energy of .546 Mega Electron Volts (Mev), while the Y-80 beta has a maximum energy of 2.28 Mev. (Mev is a relative measure of energy, which in the end will describe the "kinetic energy" (speed) of the beta particle.
Now, why is this beta stuff important? Betas are not emitted at a fixed energy they are emitted in a spectrum with an average energy of approximately (ish) of 1/3 of the maximum energy. What is the point there? You cannot easily tell the difference between a Sr-90 beta and beta from something else. Take for example a photon emitting isotope, say Cesium 137. You can place a sample on a detector that is sensitive to photons and if it can tell you the energy of the photon, you can easily get a precise analysis of the number of .662 Mev photons that was in the sample. There will still be an LLD of the sample, again, that is driven by the specifics of the counting system, the sample size, and the background radioactivity of the system (and other variable).
Not so with Sr-90. So. How do you analyze for Sr-90? Well, first you have to chemically strip out only Strontium atoms from the sample. Then you have to allow the sample to sit for a good while taking repeated counts so that you can get an idea of the ratio of Sr-90 betas to the total number of betas released from the sample, (you have to take into account the Y-90 grow in, as well as betas that were NOT from Sr-90, but some other strontium.
My point in this? It is very difficult to analyze for Sr-90, and precise and accurate results are very easily misinterpreted. In fact, given only the data that you have stated, I would be interested to see corroboration by some side technique. For example, Cs-137 has a half life very nearly that of Sr-90 (approximately 30 years). Cesium behaves somewhat like Strontium chemically (in fact it is actually released earlier from a nuclear reactor because it has a lower boiling point). Cs-137 is easily detected due to that photon discussed above. Are these labs seeing any Cs-137? Personally, I would be suspicious of a claim that there is Sr-90 without evidence of Cs-137.
Now. That being said, I have no problem believing one lab could detect down to 8 pCi/kg. With the proper analysis technique, one can get quite a bit lower. I have seen LLDs for Sr-90 quoted in the 1-2 pCi/kg range. But it is very hard to do. And expensive.
Hope this helps.
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 10:31 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. 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.
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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