[ RadSafe ] Reporter's question about lower limits of detection
radbloom at comcast.net
radbloom at comcast.net
Fri Aug 5 12:46:09 CDT 2011
The lower limit of detection is established for a counting (detection) system before you start counting (or detecting). It depends on the equipment, interferences (background radioactivity, electronic noise, etc.) available counting time, and more (e.g., sample manipulation). Typically, the user considers what level of radioactivity is of interest and then establishes appropriate techniques. Sample size and count time are the most typically considered parameters, but frequently sample size is standardized and count time is the variable. If there are few detectors and many samples, count time will be limited.
If an adult ate a pound of fish, the effective dose [ICRP 72 dose factors from Radiological Toolbox Version 2.0.0] would be 1.04E-4 millirem/picocurie x 1kg/2.22 pounds x the number of picocuries/kg (47 or 8 in your example): that would be 0.0022 millirem (for the 47 pCi/kg) or 0.00038 millirem (for the 8 pCi/kg), repectively. If this amount of fish were consumed once a week, the effective dose from a year of fish eating would be 52 times as large or 0.11 millirem or 0.019 millirem. The largest organ dose would be to the bone surfaces and would be just under a factor of 15 larger, or 1.7 millirem and 0.28 millirem, for the respective examples. To me, that seems like a fair amount of fish-eating in a year, but not much of a radiation dose. (For millisieverts [mSv], divide the dose numbers by 100.)
If a result is less than the lower limit of detection, this means that the actual activity concentration is less than this detection number (with typically 95- or 99-percent confidence). How much less the result is cannot be known from these counts. Data below detection limits imply the resulting calculated doses would be some amount less than the detection limits.
I hope this helps and is what you were seeking.
P.S. As always check numbers and ask questions. Transcription errors are easy to make. Thoughts and typed notes don't always match... I know someone from RadSafe will chime in if they spot an error in these quick calculations.
----- Original Message -----
From: "Matthew Wald" <mattwald at nytimes.com>
To: "radsafe at agni.phys.iit.edu" <radsafe at agni.phys.iit.edu>
Sent: Friday, August 5, 2011 11:30:41 AM
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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