Re: Efficiency Measurement

["Scott O. Schwahn" <schwahn@CEBAF.GOV>]

At 07:07 PM 11/26/96 -0600, Mr. Sawyer wrote:
>...[snip]
>I think someone has already commented on the problem of self absorption in
>filter media with "prepared sources". If you were calibrating an instrument
that
>measured swipes, it would be appropriate to use the same media in the
>calibration. It's probably not a good idea to use a "swipe" based calibration
>when you're not measuring swipes... The other issue with this type of prepared
>source is that of depositing the activity in such a way as to maximize source
>uniformity and minimize self-absorption - but that's a whole other topic.

I think this was perhaps the most important thing said in this excellent
post by Mr. Sawyer, so I thought I'd bring it to the forefront.

>...[snip]
>but please remember that all of
>these errors add up (proper error propogation says its the square root of the
>sums of the squares of the relative errors) and it all works against you.

Sorry to correct, but please keep in mind, fellow RADSAFERS, that this is
only true when you are adding or subtracting indepdent variables (I am
ignoring covariance at the moment).  For error determination on
efficiencies, one is usually dividing count rate by activity rate,
therefore, proper propogation of error for this type of operation is (for
e=C/A): 

(sigma_e)^2  =  (sigma_C)^2  +  (sigma_A)^2
-----------     -----------     -----------
   e^2               C^2            A^2

I hope that came out all right on everybody's mail.  Again, this assumes
independence between A and C.

My best Thanksgiving wishes to all (I suppose just the Americans?) - and
please rememember to Whom proper thanks belong.

Scott O. Schwahn, CHP
Jefferson Lab, M.S. 12A1
12000 Jefferson Avenue
Newport News, VA 23606
(757)269-7551 (w)
(757)269-7363 (fax)
schwahn@jlab.org