Re: LSC for Gross Alpha Counting

[772EAR@delphi.com]

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.