RE: alpha and beta efficiencies for plastic scintillators

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

The standard thinking is as follows: The release criteria in dpm/100 cm^2

are 4 pi values, i.e., total activity/concentration.

Ignoring the correction for probe area (to keep it simpler), the 4 pi

activity on a surface could be calculated using the 4 pi efficiency: to get

the 4 pi activity, the net count rate is divided by the 4 pi efficiency.  To

determine the 4 pi counting efficiency, the net count rate obtained from a

calibration standard source is divided by the 4 pi activity of the standard

source.



There are a couple of problems with this approach: First, the manufacturer

of the standard source really can't certify the 4 pi activity because it is

a calculated value based on the 2 pi activity.  They can certify the 2 pi

activity because it is measured directly (e.g., in a windowless proportional

counter). The manufacturer's reported 4 pi activity is essentially an

estimate that assumes a certain amount of backscatter. Second, the real

world efficiencies that exist when counting surfaces such as concrete or

drywall are lower than determined by counting the calibration source

(because alphas or betas emitted by the calibration source are subject to

little or no attenuation and because there is usually more backscatter

associated with the calibration source).  The result is that the normal way

of doing things results in an underestimate of surface activity.



A solution of sorts is provided by ISO 7503-1. This method is also

recommended and described in MARSSIM (NUREG 1575) and NUREG 1507.  It is the

method preferred by the NRC for evaluating residual contamination for the

purpose of demonstrating compliance with the release criteria.



In this approach the 4 pi activity on the contaminated surface is calculated

by taking the net count rate and dividing it by two (not one) efficiencies:

the "instrument efficiency" (i.e., the 2 pi counting efficiency determined

with the 2 pi activity of the calibration source) and the "source

efficiency."  The latter is the fraction of decays on the surface that

result in a particle leaving the surface. The ISO standard recommends

default values for the surface efficiency of 0.5 for beta emitters with

maximum beta energies above 400 kev, 0.25 for maximum beta energies between

150 and 400 keV and 0.25 for alpha emitters. 



Obviously the ISO approach is a crude and imperfect solution but it can be

considered better than what most of us have been doing. NUREG 1507 provides

all sorts of great info about this subject. 



Finally, if the source certificate really only reports the Sr-90 activity, I

would expect that your measured 4 pi efficiency should be 60%+. If it

includes the Y-90 betas, the efficiency will be much lower. I would expect

that it is the total Sr and Y activity being reported because that is what

the source manufacturer measures. But maybe not.



In any event, it is crucial that the efficiency you use be consistent with

the derived concentration guideline level (DCGL) used to demonstrate

compliance.  As an example, the NRC default Sr-90 DCGL (screening level) of

8700 dpm/cm^2 is strictly Sr-90. It would equate to a total Sr-90 and Y-90

activity of 17,400 dpm/cm^2.  The 4 pi efficiency you use to calculate Sr-90

surface contamination should be for Sr-90 alone i.e., cpm/dpm Sr-90. This

value would be close to 60%.  



Paul Frame

Professional Training Programs

http://www.orau.com/ptp/ptp.htm

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