Re: Dust Loading on Air Samples for Alpha Activity

[Wes Van Pelt <VanPeltW@mail.IDT.net>]

Alden Tschaeche wrote:
> 
> It is well known that nonradioactive dust on air samples, if not
> corrected for, can cause underestimation of air concentrations of alpha
> emitters.  Does anyone know of published data that gives curves or
> equations for calculating correction factors for dust loading on alpha
> air samples?  Of course one must know by some measurement the areal
> density of the dust loading in terms of mg-cm-2 for a filter.  And, one
> would know the alpha energy.  I would like a curve that, for a given
> alpha energy and counting geometry, would tell me, for example, that a
> dust loading of 1mg-cm2 would require multiplying the measured activity
> by 2 to get the actual alpha activity on the filter paper.  Then there
> is the complication of deposition of alpha activity as a function of
> depth in the dust layer. 

Also, the alpha activity will distribute through the thickness of the
filter.
This effect is much less for a membrane (e.g., "Millipore") than for a
glass
fiber or paper filter.  You can get a pretty good idea of whether there
is
significant attenuation by looking at an alpha energy spectrum using a
semi-conductor alpha detector with the filter in a vacuum chamber. If
very
little attenuation, you will see a nice peak, otherwise the alpha
particle energy
distribution is schmeared (sp?) down to zero keV.

> One could always make the conservative
> assumption that all the alpha activity were under the entire thickness
> of dust.  But, maybe, if the contaminated aerosol were assumed to be in
> time uniformly deposited, the correction factor could take that into
> account.  Thanks in advance.  Al Tschaeche xat@inel.gov

Regards, Wes
-- 
Wesley R. Van Pelt, Ph.D., CIH, CHP                   KF2LG
President, Van Pelt Assoc., Inc.      vanpeltw@mail.idt.net
Consulting in radiological health and safety.
"TIME, its what keeps everything from happening at once."