AW: Radioisotope Efficiencies

["Franz Schoenhofer" <franz.schoenhofer@CHELLO.AT>]

Joel,



I can confirm that you wrote on Wednesday, 11 FEB 2004 at 22:33 Middle

European Wintertime, that the numbers you forwarded "are my (and my

co-worker/friend's) "best" unquenched efficiencies (w/glass vials)".



I did not contribute to this thread yet, because a comprehensive treatment

of all (or at least most) of the factors, which influence the efficiency in

LSC would require a long publication or perhaps even a PhD thesis. The term

"unquenched sample" has to be seriously questioned - when is a sample

unquenched? Then a somewhat "philosophical" remark: "Efficiency" is just a

small part of the game. Much depends what you use your LSC measurements for.

I have done for about thirty years nearly exclusively low-level and

ultra-low-level measurements and in the course of this work I did a lot of

work to optimize the counting conditions. There are even a few publications

on that, though most such optimization work I did not regard worth

publishing, because it was just, what everybody performing measurements

should do: optimize measurement parameters like type of vial, type of

cocktail, mixing ratio of sample vs. cocktail, volume of sample etc. Those

parameters are often counteracting like: More aqueous sample - more quench -

less efficiency. It needs not hard, but time-consuming work to optimize the

conditions. The same is true if one works with alpha-beta discrimination,

one always has to chose compromises - a high efficiency associated with a

bad separation does not help. A high efficiency of an unquenched tritium

sample does not make sense if one has to measure tritium in water!

Especially in low-level counting an optimized window is essential and it is

in many circumstances crucial to cut the efficiency by a narrow window, thus

cutting background effectively and to avoid interferences from other

radionuclides present.



Some of your values I seriously doubt even for "unquenched" samples, but

alpha-emitters are correctly cited with 100% efficiency and this will be

true for a large range of quench.



Btw, I used since 1983 a total of five "Quantulus" ultra low-level liquid

scintillation spectrometers, whose software made optimization of counting

windows very easy by treating the spectra recorded after the measurement.



Do not hesitate to ask questions if you think I could be of help.



Best regards,



Franz