RE: Gamma Spec

["Bradshaw, Keith" <Keith.Bradshaw@nnc.co.uk>]

> -----Original Message-----
> From:	Joseph Archer [SMTP:captainnuke@earthlink.net]
> Sent:	Monday, March 06, 2000 1:28 PM
> To:	Multiple recipients of list
> Subject:	Re: Gamma Spec
> 
> I think I would have to side with Dr. Zitouni on the question of adding
> spectra to obtain a lower LLD.
> 
> Assume a sample has x dpm in it. For any given count, you will see x +/-
> y standard deviations of activity. If on the first count, you got a
> count of x - 3 STDs, and then by sheer coincidence got x - 3 STDs on the
> second count, you conclusion would be wrong.
> 
	That's statistics for you.  You accept a certain proportion of wrong
conclusions when you set the confidence level.  I don't see any difficulty
*in principle* with adding spectra, as long as the delay between
acquisitions is short compared to the radionuclide of interest half-life,
and the backgrounds and energy calibrations are stable.  The scenario you
mention could just as well happen in the first and second halves of single
count period.  Every so often it WILL happen.   The lower LLD, or the least
relative uncertainty for a finite result, will be given by the longer count
period, irrespective of whether that count period was continuous or split,
all other things being equal.  I agree there may be difficulties IN PRACTICE
because of changes in background and energy calibration, as well as possible
software difficulties with decay corrections, but not in principle.

	<CUT>


	Personal thoughts only

	Regards

	keith.bradshaw@nnc.co.uk






	END


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