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Re: Liquid Scintillation Counters

At 20:03 23.04.98 -0500, you wrote:
>We are presently making a decision concerning the purchase of a new LSC and
>would appreciate any suggestions, critiques, or anecdotes regarding your
>experiences with LS counters and their accompanying software.
>We are a research laboratory dealing with the usual isotopes found in a
>research/university setting (3H, 14C, 32P, 33P, 35S, 51Cr, 125I, 131I,
>etc.).  Primary uses would include laboratory swipe tests, waste sampling,
>and possibly sealed source leak testing (137Cs).
>Two of are main concerns are the ability to detect and identify small
>amounts of contamination and the accurate analysis of unknown, multiple
>isotope waste (will we be able to distinguish among isotopes to some degree
>of precision?).
>We known that Beckman, Packard, and Wallac all have comparable systems.  Are
>there others?  Which have you had experience with?  What do you like/dislike
>about your system?  Do you know of accompanying software to help with the
>isotope seperation problem?
>Any information would be appreciated.

We use LSC for a wide range of applications, among which there is also
swipe tests, excretion analysis for tritium, drinking water and waste
water. One of your criteria should be, how low you have to get with your
lower limits of detection - with other words, is it for instance thousands
of Bq of tritium or a few? Do you work mostly with single nuclides or do
you have mixtures? With single nuclides you do not have the problem of
distinguishing between different radionuclides.

We have four counters of the type "Quantulus" (Wallac Oy). There is a
logarithmic pulse height scale, which I much prefer over the linear one,
especially when measuring tritium or C-14. I know about some computer
programmes (as far as I know, they are not available commercially) for
deconvolution of spectra, but I personally think that the errors introduced
due to the propagation of errors soon become very big. If you know, what
radionuclides are present it might work a little better. By looking at the
spectra obtained you can in any case easily distinguish alpha emitters from
beta-emitters and the Quantulus has also a well working alpha-beta
separation, with which you can get the background in the alpha spectrum
down to practically zero. In the case of the waste water from nuclear
research reactor we can easily identify tritium because of its low energy.
Even C-14 has a very characteristic shape and you can check always with an
internal standard of C-14, whether the shape is identical. Sr-90 can be
easily identified because of the double "peak" with the high energetic Y-90
in equilibrium (we had such cases with waste water). For the high energetic
P-32 you can combine the spectrum shape with half life determination. And
so on.

We use the Quantulus, because we need really very low lower limits of
detection and until now nobody could show that any other instrument would
be comparable. It may be more expensive than other, but besides the fact of
the LLD we have instruments running since 1983, 1985, 1988 and 1990
practically day and night, weekdays and weekends and we had only a few
mechanical failures which were easily fixed. 

I have experience with only one instrument of a competitor, but it would be
unfair to compare.


Franz Schoenhofer
Federal Institute for Food Control and Research
Department for Radiochemistry and Radioactivity of Food
Kinderspitalg. 15
A-1095 Vienna
tel.: +43-1-40491-520
fax.: +43-1-40491-540