[ RadSafe ] Reporter's question about lower limits of detection
Glenn R. Marshall
GRMarshall at philotechnics.com
Fri Aug 5 14:00:10 CDT 2011
Also, is the fish sample a piece of whole fish, just tissue, or just bone? That would make a big difference not only in the detection limit but also in the measured concentration--which is really what we're interested in most of the time. And attempting to calculate dose based on the Sr-90 concentration in the bones would not be very meaningful, either.
Unfortunately, the uncertainties mentioned thus far make this a complex answer--far too complex to write into a news article that the average person (myself included) is likely to stay awake through.
Suffice to say that, if the lab has expertise in the field, a good QA/QC program, and uses sound procedures, then their results are what they are. Detection limit will vary from lab to lab, from instrument to instrument, and from sample to sample. But when sample results are a few multiples of the detection limits, we tend to see pretty good agreement about the true value.
From: radsafe-bounces at health.phys.iit.edu [mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of stewart farber
Sent: Friday, August 05, 2011 1:58 PM
To: 'The International Radiation Protection (Health Physics) Mailing List'
Subject: Re: [ RadSafe ] Reporter's question about lower limits of detection
I have to respect your modesty. I have been reading articles you have frequently written for the NYTimes about nuclear related issues for many decades, so I am pleased you ask a question of some technical people about a radiation related issue. However, I must warn you, you may be sorry you asked about "Lower Limits of Detection" in radioactivity measurements. It's complicated unfortunately, and I often say to friends "Don't get me started"
There is NO specific lower limit of detection for Sr-90 in fish or any other media [or for any isotope in any media]. Wait, this gets even more complicated.
The lower limit of detection [or many other ways of expressing an analytical result] is based on many variables including the specifics of the analytical technique any given lab employs to prepare a fish sample [or any other
media: soil, vegetation, grass, etc.] for counting, AND what one is willing to accept in quoting the result as to what is decided in advance is the probability of a "false positive" and "false negative".
The sample size from which the Sr-90 is concentrated and prepared prior to "counting" can vary greatly. Has the lab extracted the Sr-90 from one gram of the sample media or a kg? This changes final sensitivity by factor of
After chemical processing to concentrate the Sr into a sample for counting, numerous variables go into calculating how much Sr-90 [and Sr-89 is present and radioactive Yttrium-90 -don't ask why yttrium is there] and other beta emitters are present on the counting planchette ready to be placed under some specific detector. So ultimately a lab is able to measure decay events ie. counts from all the beta emitters on the planchette and mathematically derive how much Sr-90 is present. But many different types of detectors exist, with any given counting system having an inherent ability to actually "see" a decay event. the actual detection sensitivity will vary.
SO "Lower limit of detection [LLD]" as only one of several ways of calculating and reporting an analytical result which is based on each of the following factors [and more]:
- Sample size from which an isotope is extracted
- How a sample is prepared for counting [how much Sr is lost in concentrating a given sample for beta counting and what contaminants are not removed chemically in preparing a sample]
- The efficiency of the specific detector which is being used to count the specific isotope involved, --How long a sample once prepped and put into a detector is counted, and
- The statistical "confidence" one is willing to specify that, based on all the variables [and uncertainties of each step of the process] that go into an actual measurement, some isotope is really in the sample.
When all is said and done, the reported lower limit of detection for Sr-90 in a very good analytical measurement can range down to about 1 pCi/kg or much less.
During the days of open air testing of nuclear weapons by the US and Soviets [thru 1963] and after 1968 when most of this nuclear bomb released radioactivity had left the stratosphere and come to deposit on the earth's surface, Sr-90 and every other fallout radionuclide was much, MUCH higher than levels one would measure today in almost all sample media. Levels of
Sr-90 in surface water, and air, and fish and other biota were much higher than today. People [and the media] who have panicked recently from trivial levels [in terms of radiation dose and risk] of measurable contamination in the US from Fukushima, overlook the fact that during the days of open air nuclear testing levels of radioactivity were hundreds to thousands of times higher in some cases. The amazing thing about measuring radiation is you can measure a few atoms of some isotope decaying with accuracy and precision, vs. the best chemical contaminant analytical technique requiring countess "billions and billions" of atoms as Sagan so enjoyed saying.
Specific to Vermont as but one example of how fallout radioactivity [of which Sr-90 and Cs-137 are but 2 major isotopes of nuclear fission] in a media can vary tremendously in a single state like Vermont, consider this.
Back in 1991, I sampled domestic, home wood ash from burning biomass [mature hardwoods] collected very near VT Yankee [VY] and far from VY [my vacation home in VT]. These wood ash samples were analyzed by ultra-sensitive gamma spectroscopy which could measure about 10 to 20 pCi/kg of Cs-137, depending on counting time in ash, or milk, etc. The sample of woodash from my wood stove in Warren, VT, over 100 miles from VY, was approximately 15,000 pCi/kg of ash +/- a small analytical uncertainty for the technique employed. The sample of woodash from burning mature hardwoods collected near VT Yankee was
1,500 pCi/kg of ash, or one-tenth --10% of the Cs-137 measured in wood ash from 100 miles away.
Did the operation of VY clean the trees nearby of Cs-137? OF COURSE NOT.
This simple set of measurements only show how variable a fallout isotope [from a nuclear bomb or a nuclear plant] can be in a given sample. The difference between the Cs-137 contamination in wood ash near and far [over
100 miles away] from VT Yankee appears to be due to differences in soil chemistry between the two locations which led to 10 times more fallout
Cs-137 radioactivity being taken up [and retained over many decades] by trees near my home in Warren, VT than were retained by trees near the nuclear plant in southern Vermont for what was essentially EQUAL fallout deposition of Cs-137 to soil from past nuclear bomb tests. That is, if you measured Cs-137 in Northern Vermont, or most other locations around New England where I got much higher Cs-137 in woodash measurement than near VY, the soil concentration of Cs-137 would be approximately the same.
Plus keep in mind that any given lab's reporting of analytical radioactivity measurements vary in analytical ability and reliability. Labs have vastly different analytical sensitivity, accuracy, and precision. Accuracy is how close a measurement is to a "known" value of an isotope which may have been added to a sample for testing. Precision is how close a series of measurements are to each other, even though a lab can have good Precision but lousy Accuracy. They may be making a very erroneous determination of an isotope [poor accuracy], but doing it in a consistent [high precision] manner. They might also make a reasonably accurate average analysis in analyzing the same sample many times, but the results can have a lot of scatter [good Accuracy in but lousy Precision].
I had the direct experience of dealing with the State of Vermont in an analogous situation to what may be a current "red herring" flap, many years ago regarding an [ultimately totally false] claim that had been made by some VT politicians and regulators that elevated airborne levels of I-131, a nuclear related isotope, had been detected in measurements the State of VT had made near Vermont Yankee . All kinds of extreme assertions that the good people of VT were at serious thyroid cancer risk from exposure to I-131 from VY's operations, and other cancers from other releases. When someone wants maximum media exposure trot out the big C -whether based on any accurate information or not. This incident I refer to goes back many years to when I was responsible for analyzing and reporting the data from the environmental radiation monitoring programs at Vermont Yankee, Maine Yankee, and Yankee Rowe nuclear power plants, and assessing power plant impacts.
I met with the VT laboratory personnel about their measurements and the State's claims of elevated airborne I-131. I was able to review their data and analytical technique. It turned out that the State of VT's reported results for claims of elevated I-131 in air near the VY nuclear plant were based on serious errors in their analytical technique for I-131. They had measured decay from a natural isotope that was present in their lab, and a
high and variable background counting rate, and reporting the counts they
measured that went into their calculation of I-131, as being from I-131 and not Radon-222 daughters and other gamma isotopes contributing to their high lab background radiation exposure rate. The lab was located in an area of high and varying background, and the variable background count rates were not properly accounted for in the initial calculations and reports of airborne I-131.
Interesting, despite false measurements having been made and false inflammatory claims made by VT politicians and government officials, the State of VT did not correct their prior erroneous claims.
Matt, please realize that I am not associated with the nuclear power industry in any manner since 1993 and have no relationship with any nuclear power interests today. These remarks are made as an interested member of the public with technical expertise in radiation detection in the area about which you inquired.
Good luck in trying to keep your reporting accurate & thanks for inquiring.
Feel free to call if you wish to discuss any aspect of your reporting of environmental radioactivity issues or public health aspects of nuclear energy.
Stewart Farber, MS Public Health
Farber Medical Solutions, LLC
Bridgeport, CT 06606
email: SAFarber at optonline.net
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 11:31 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:
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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