Luminescence and LSCs

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

Franz,

        As I understand/remember it (Liquid Scintillation Counters aren't my
expertise), when an energetic particle (let's use a beta as an example)
interacts with a solvent/fluor molecule, light is emitted from the L.S.
sample vial in all directions and is "directed" into two photomultiplier
tubes which convert the light into a measurable electrical pulse.  As you
noted, an energetic Beta particle like P-32 can give rise to more than 300
photons.

        Luminescence from chemiluminescence or photoluminescence is commonly
referred to as "single events" since each reaction results in the emission
of a "single" photon. The coincidence Gate Circuit was introduced to reduce
noise level in L.S. counters.  Up to a certain number of single events, the
coincidence circuit can discriminate between single events and true
gamma/x-ray photons or beta particles.  
        Except in "rare" cases, luminescence is not a problem with "state of
the art" LSC's because they "tell you" if you have a problem!  I have a
LS-6000 series Beckman and I feel "very" comfortable with the data received.
I don't want to start a technology (or any other type of) war here because I
respect your opinion, but in my "personal" opinion, Quantulus' are re-warmed
50's technology.  Having said that, however, you can count a dirty soil
sample a LOT easier than I can...(grin).

        Joel
 

At 03:00 PM 8/20/97 -0500, you wrote:
>Schoenhofer
>Habichergasse 31/7
>A-1160 Wien
>AUSTRIA
>Tel./Fax: +43-1-4955308
>Mobiltel.: +43-664-3380333
>e-mail: schoenho@via.at
>
>----------
>> Von: William Lorenzen <LORENZEN_W@A1.TCH.HARVARD.EDU>
>> An: Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
>> Betreff: Re: Fe-55
>> Datum: Mittwoch, 20. August 1997 16:06
>> 
>> Franz,
>> 
>> Are you indicating that chemiluminesence is more prevelent with lsc
>counting od 
>> Fe-55 or just a general statement about LSC counting?
>> 
>> Regards,
>> 
>> William Lorenzen
>> Children's Hospital
>> Boston, MA
>> 
>> lorenzen_w@a1.tch.harvard.edu
>> -----------------------------------------------------------------
>
>William,
>
>I wrote this warning, because in the course of measuring smearsamples for
>Fe-55 we found out only after carefully looking at the results and the
>chemiluminescense spectra that what we in the beginning mistook for Fe-55
>was in fact chemiluminescense. This occured though the samples were kept in
>the dark for hours and our Quantulus counters are cooled down to about 14
>degree Celsius. The smear samples had been taken from the vicinity of a
>Fe-55 source at a machine. We still do not know, what caused the
>chemiluminescence, it might have been some dirt or oil on the surface where
>the smear samples were taken. Something similar might occur when taking
>smear samples in a laboratory. We had a similar experience when we tried to
>measure distilled spirits directly for a rough estimation of C-14, but in
>this case the chemiluminescence could be easily distinguished from the C-14
>spectrum. 
>It cannot be distinguished so easily from the much lower energetic tritium
>betas. An indication of chemiluminescence would be the occurrance of
>significantly lower values when remeasuring the same sample after some time
>(hours or days). 
>
>Modern liquid scintillation cocktails are relatively insensitive to
>chemiluminescence, however I wanted to point out that one should be
>cautious. Whenever one measures by LSC a radionuclide with higher energetic
>betas like C-14, Sr-90 or alphas from radon or radium there is no problem
>with chemiluminescence, because even if it would occur the spectral window
>can be set easily to cut off the chemiluminescense.
>
>To all LSC-users: I wish you happy counting without chemiluminescence and
>quench!
>
>Franz
>
>
Joel T. Baumbaugh (baumbaug@nosc.mil)
Naval Research and Development (NRaD)
San Diego, CA., U.S.A.

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