Re: MDA and LLD

[avest@magnus.acs.ohio-state.edu (Albert Lee Vest)]

>Last year the NRC inpsected the Reactor's License and informed us
>that the MDA and the LLD must be recorded with error analysis.
>
>Of course, any measurement must have an error analysis associated
>with it.  However there are conflicting views on the recording of
>the MDA and LLD with errors.  One view says this is already a
>calculation of the error, the other view as well as the NCRP No. 58
>seems to say that it is both a number and an error.  So which is it ?

MDA, whatever confidence interval you decide to use, is based on three 
numbers:  Your counter's background rate, your counter's efficiency, and the 
sample count time.  Each of these three numbers has some uncertainty to it, 
which contributes to the uncertainty of the documented MDA.

>For those who calculate the MDA and/or LLD's for any sampling how do
>you record the results ?

A signed memo placed in the instrument's calibration file.  So far, I've got 
MDA's pinned down for OSU's thyroid monitor, and for several nuclide-peak 
regions of interest on our MCA (for leak-test swabs counted with NaI(Tl)).  
That liquid scintillation counter is going to be a battle, though.

>MDA as we define it is  4.65*squ(background/time).

Unaffected by efficiency?  At best this could be called MDCR 
(minimum-detectibility count rate), but not MDA.

>Thanks
>steve at UMCP

I hope it helps.
Albert Lee Vest    avest@magnus.acs.ohio-state.edu
health physicist        Office of Radiation Safety
(614)292-0122            The Ohio State University
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