Re: LSC for Gross Alpha Counting

[Schoenhofer <schoenho@via.at>]

At 10:29 19.02.1997 -0600, you wrote:
>In his reply to Andrew Welsh, Franz Schoenhofer stated "alpha-beta-separation
>is highly dependend on the energy of both alphas and betas, of the cocktail
>quench etc. It can be adjusted anyway to reduce cross talk from beta to 
>to zero, though a little efficiency for alphas may be lost."
>
>I would urge caution to anyone using an LSC to count alpha nuclide in the
>presence of high activity beta nuclides.  The reason that high activity
>beta sources can contribute to the alpha count is in the way a LSC
>discriminates between alpha and beta signals.
>
>As an LSC event consists of a light pulse that has a prompt and slow 
>component. Each component is used by the LSC electronics to deterimine
>the pulse decay time.  Beta events typically exhibit a fast pulse decay
>while an alpha event produces a significantly slower pulse decay 
>relative to the beta pulse.  In the Packard unit, with which I am most
>familiar having worked for Canberra,  the LSC uses the difference in
>pulse duration to discrimate between beta and alpha pulses.  Therefore,
>unlike an gas proportional counter,  an LSC does not discriminate
>between pulses but makes a decision to allow a pulse through a counting
>gate bases on the decay time of the pulse.  Where this may present a 
>problem is in the presence of high beta activity where the beta 
>pulses are so numerous they get summed together so that the decay
>pulse is slow enough to get counted as an alpha.  The summing of
>events actually leads to a broadening of the beta event pulse.
>
>Because of the above, as well as the influences that sample chemistry,
>geometry, quenching and calibration (a pure alpha emitter and pure 
>beta emitter standards as close the composition of the material to be
>counted is required to be counted separately to determine the 
>optimum setting of the discriminator), I would think that a LSC
>manufacturer would be hard pressed to guarantee "zero" false positives
>alpha counts in the presence of high beta activity.
>
>Regards,
>Vince Chase
>772EAR@delphi.com
>Radiation Safety Officer
>Boehringer Ingelheim Pharmaceuticals
>The opinions expressed here are those of the author and do not represent
>the opinions, policies or practices of Boehringer Ingelheim.
>
>-----------------------------------------------------------

Vince,

I do not agree with your mail, but I do not disagree completely. The
question is of course the absolute value of the "high beta activity" and the
energy of the radionuclides present. I mentioned already that for instance
in the case of extremely high tritium activity you would not even need any
alpha-beta separation, how high ever the tritium activity would be, because
due to the extremely low energy and produced pulse height there will be no
interference. We have measured besides our usual ultra low-level samples
extremely high activity tritium samples, but I am sorry to tell you that we
have never observed any "pile up" pulses which would show up in the high
energy pulse height range. Even for high energetic beta-pulses I doubt that
any pile up process could be observed, since this would essentially depend
on the dead time of the instrument. Since alpha-beta discrimination can only
work with extremely fast electronics I cannot believe that the electronics
would be so slow to allow for pile up. At least the "Quantulus" from Wallac
Oy (Turku, Finland), which we use has such fast electronics and we have
never observed any pile up. By the way there are quite a few different
methods to decide whether a pulse is a "beta" or "alpha" pulse. As far as I
know, the methods depend on the companies providing instruments with
alpha-beta separation. 

If the measurements were done because of radiation protection
considerations, then I would estimate, that at really extremely high beta
activities a very small alpha activity would not matter.

I stop here with what is close to speculations, because the actual
conditions and requirements have unfortunately not been communicated. So I
would think that the ancient method of "try and error" might be the best
applicable.....

Franz

Schoenhofer
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