Re: Efficiency Determination

["baumbaug@nosc.mil" <baumbaug@nosc.mil>]

        Julia,

        I was going to send this reply only to you, but perhaps this may be
of "some" interest to other "radsafers".


       My Ludlum's efficiency for the below isotopes using a 44-9 Pancake
G.M. probe are as follows:

        P-32    52%
        S-35     8%
        I-125    0.01% 
        C-14     4%


        1.      Remember 1/2 of the isotopic point sources' radiations are
going the other way.  A (non-cast in concrete) rule-of-thumb is to make your
measurement, calculate the microcurie amount (ok - Becquerels) and then
double it.  However, P-32 betas are powerful enough to bounce back from the
surface of the surface and be counted - hence better than 50% efficiency.
Actually the type of surface you are measuring against "can" make a big
difference in your results.

        2.      My efficiencies are "rounded off" .

        3.      There are so many variables in this kind of measurement - I
may get different numbers if I repeat this tomorrow.

        4.      Your efficiency with other meter manufacturers may vary. 

        5.      When making a series of standards, I "personally" have a
very difficult time aliquoting/pipetting an exact CPM amount every time -
even though I've accounted for the vials being new (they often send more
initially then give you a "start" date) or for decay.  This is because  a)
the stock vial is not completely mixed up  b) a "very-very" small amount
left in/on the pipetteman tip can make a BIG difference in your results and
c) I am not the worlds greatest "pipetter".
        
        6.      There was something else, but I think I have CRS
disease.....I can't remember...


        I hope this helps,

        
        Joel

 P.S.  The STD Disclaimer about my Bosses, the Navy, the Fed. Govt. and my
state of mind....

At 11:20 AM 11/26/96 -0600, you wrote:
>Dear Radsafers,
>
>I have recently obtained a Ludlum Model 3/44-9 and am in the process of
>performing efficiency calculations for various isotopes in use at my
>facility, namely P-32, S-35, I-125, and C-14.  In determining efficiency, I
>plan to take a measured amount of each isotope, place it on a disc
>smear, determine the count rate, and perform the necessary
>calculations.
>
>Yesterday the above method gave me a 30.6% efficiency for P-32.
>
>I would like to know if this is within the expected range for this
>instrument and if this is an efficient (so to speak) method for determining
>efficiency.
>
>I would also appreciate a list of expected efficiency ranges for the
>Model 3/44-9 for the isotopes I have listed if anyone is using similar
>instrumentation and has these numbers handy.
>
>In Anticipation, 
>Julia Sober
>Acting RSO
>Northern Illinois University
>DeKalb, IL 60115
>
>jsober@niu.edu
>Phone (815) 753-1093
>Fax (815) 753-0461
>
>
Joel T. Baumbaugh (baumbaug@nosc.mil)
Naval Research and Development (NRaD)
San Diego, CA., U.S.A.