RE: Self-Absorption Effects on Air Filters

["Frame, Paul" <FRAMEP@ORAU.GOV>]

Elizabeth:

Since you are using a membrane filter (mixed esters of cellulose), I
would assume that there are no burial losses worth worrying about, no
matter what the flow rate, particle size or pore size of the filters.
NRC Reg Guide 8.21 comments "the radiation safety staff should include
appropriate corrections for alpha and beta absorption by the filter
media and material collected on the filter whenever these corrections
may change results by more than 10 percent." Certainly the burial losses
in the filter itself would be less than 10%. Still, the assumption that
membrane filters will not result in alpha burial losses might not always
be valid, at least according to Jonassenn and Hayes 1974 "Absorption of
5.47 alpha particles by membrane filter material" Health Physics 27:310.

The most extensive study that I know of dealing with alpha burial losses
in filters was by Higby in 1984 "Effects of particle size and velocity
on burial depth of airborne particles in glass fiber filters" Battelle
PNL-5278 UC-41 although it only dealt with glass fiber filters. He
quantified Pu-239 on the filters by counting the x-rays and compared
these analyses with the gross alpha analyses as a way to determine
burial losses.  He concluded that an assumption of a burial loss of 10 -
15 % would be conservative. 

Alpha burial losses in cellulose filters have been estimated at:

50% (Boback 1963 "Absorption of uranium alpha particles by Whatman 41
filter paper" Proceedings of the ninth annual conference on bioassay and
analytical chemistry TID-7696.)

50% ( Simons 1956 "Correction for the absorption of plutonium alpha
particles in filter paper used for dust sampling" United Kingdom Atomic
Energy Authority AERE HP/M87

30% (Alercio and Harley 1952. Evaluation of alpha particle absorption by
filter paper" Nucleonic 10(11):87)

40% (Denham 1969 "Effectiveness of filter media for surface collection
of airborne radioactive particulates" Proceedings of the 3rd Midyear
meeting of the Health Physics Society page 1093.) 

To measure it yourself, you would have to collect an alpha emitter on
the filter (e.g., Pu or U)perform a gross alpha count and then quantify
the alpha emitter by radiochemistry, gamma spec or some other means.  An
alternative, is to collect radon daughters on a type of filter which
would have no self absorption (e.g., a membrane filter), calculate the
beta to alpha ratio and then do the same for the filter in question. The
difference in the ratios could be used to estimate the burial losses.
Radon daughters are readily available and of the size that are most
likely to penetrate the filter i.e., the size that might be expected to
experience the greatest burial losses. The problem is that you are
interested in membrane filters. 


Paul Frame
Professional Training Programs
ORAU
framep@orau.gov
www.orau.gov/ptp/ptp.htm
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