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RE: MDA vs. Critical level for DOE Free Release surveys
I think this summarizes the issue very well. The survey methodology must be
based on MDA. DL is of interest in identifying when contamination is
present but doesn't confirm that it is not there; therefore, the survey
design must require that the MDA is less than the specified limit. The DL
is of interest as it may identify when contamination is present and further
evaluation may be warranted.
tom
-----Original Message-----
From: Richard Orthen [mailto:rorthen@earthsciences.net]
Sent: Wednesday, October 29, 2003 8:14 AM
To: Goff, Tom
Subject: RE: MDA vs. Critical level for DOE Free Release surveys
Absolutely it is the core of the issue as well as much confusion in the
health physics community! And unless you cherish audit findings or load
rejects by your waste broker, you must have your design MDA below the
performance criteria so that the right tail of the MDA distribution is not
significantly beyond your performance criteria.
Here's how: Within the constraints of your specific application (i.e., the
realistic background countrate, observation times, and tolerable error rates
for the measurement), the first decision is to determine if the net result
represents something other than background. This tool is the DL.
Accordingly, your survey protocol can then be applied with known confidence
to identify occurrences where the net countrate is different from
background, prompting investigation against the performance criteria. On an
a priori basis, optimize the parameters in the standard MDC equation to
yield a design result suitable for your performance criteria. Typically,
the MDC level will be no less than twice the DL, which in turn will be a
small fraction of the performance criteria (unless you are dealing with
certain low yield/energy alpha/beta emitters driving your survey protocol
off the charts due to diminishing "K" factors). Through further parameter
optimization, you can gain margin (in terms of reduced count times and the
accompanying reduced survey effort) below your performance criteria. After
all is said and done, you will have established a design MDC distribution
for activity whose left tail is unlikely not to "set off the alarm" (the
alarm state being "net activity detected"") and whose right tail is below
the performance criteria (or at least just marginally above it).
Hope this helps.
Rick Orthen
-----Original Message-----
From: Goff, Tom [mailto:Tom.Goff@wipp.ws]
Sent: Tuesday, October 28, 2003 3:54 PM
To: 'Richard Orthen'; Goff, Tom; ''RadSafe Send Message'
Subject: RE: MDA vs. Critical level for DOE Free Release surveys
It strikes me that this is the core of the issue. We need to define a
survey methodology where we have a 95% confidence of detecting contamination
at the stated value.
If we have a survey methodology that gives us a MDA at the limit, then we
have a 95% chance of detecting the contamination.
If we have a survey methodology that gives us a DL at the limit, then we can
say it is above the limit if it is greater than the DL but we cannot say
with an acceptable degree of confidence that it is below the limit.
Therefore, it seems that we should set a survey protocol, instrument
efficiency, background, survey time to give us an MDA below the limit.
Thoughts????
-----Original Message-----
From: Richard Orthen [mailto:rorthen@earthsciences.net]
Sent: Tuesday, October 28, 2003 1:22 PM
To: Goff, Tom; ''RadSafe Send Message'
Subject: RE: MDA vs. Critical level for DOE Free Release surveys
Tom,
You're on the right course. In a nutshell, never compare a sample result to
the MDA; always compare to the decision (critical level, DL or CL). This is
the mantra of MARLAP and good health physics practice. As Dan Strom put it:
"The MDA is really the 'if-it's-in-the-sample-you're-likely-to-detect-it"
level, while the DL is the
'if-you-got-a-result-above-this-it's-probably-real' level." For the rest of
the story, see http://www.pnl.gov/bayesian/Strom/Stat-CEL-DJStrom.pdf
Cheers.
Rick Orthen
Sr. Project Manager
Earth Sciences Consultants, Inc.
Export, PA
rorthen@earthsciences.net
"The best way out is always through."
-Robert Frost
-----Original Message-----
From: owner-radsafe@list.vanderbilt.edu
[mailto:owner-radsafe@list.vanderbilt.edu] On Behalf Of Goff, Tom
Sent: Tuesday, October 28, 2003 2:27 PM
To: ''RadSafe Send Message'
Subject: MDA vs. Critical level for DOE Free Release surveys
> Title:
>
> We are discussing using a critical level for free release surveys.
> Please provide answers to the following:
>
> Are you using critical value or MDA for free release surveys of
> materials from your site?
>
> Does anyone have a reference/regulatory standard on whether to use
> MDA or critical value for free release surveys?
>
> Given that an MDA results in a 95% confidence in detecting
> contamination at the MDA value, what is the corresponding confidence if
> using the critical level?
>
> I would welcome any other comments or thoughts on this.
>
> Tom Goff
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