.3 dac fraction

[wlrun3@aol.com]

   ...i was recently asked, given an air sample volume and the concept of minimum detectable activity, how a field check of an air sample filter patch with a standard frisker can determine less than .3 dac fraction...

   ...what is the methodology supporting the graphs used in making these air sample field check determinations...

[RRhoads]

So, i take it you never calc'd an air sample activity on the NUF exam??
The basics look something like this;

NCPM/vol(eff.)2.22 E6(1000 for liters or 28320 for CFM)
Seeing that a windows PC seems to to never be too far away anymore, using a PC calculator in the SCI mode, this can usuall be done in the field, least at the plant i work at.

 

[G-reg]

You're going to have to make an initial assumption regarding your most limiting isotope.  Since I am working at a power R_X, I'm going to assume that our limiting isotope here is ⁶⁰Co (based on its abundance in the plant, and its restrictive DAC value).

From 10CFR20 Appx B Table 1, ⁶⁰Co has a DAC value of 1E-8 mCi/ml.  (I'm using Class Y, which I base on an assumed chemical composition for our assumed ⁶⁰Co isotope.)  Bear with me, we're almost done with our assumptions.

Since we have determined that DAC = 1E-8 mCi/ml, then 0.3DAC =
[0.3][1E-8 mCi/ml] = 3E-9 mCi/ml

The next step is to convert mCi to dpm:
[3E-9 mCi][2.2E6 dpm] = 6.6E-3 dpm/ml
[   ml   ][  1 mCi  ]

Then, for use with our field instruments, we need to convert dpm into ccpm.  I'll use a detector efficiency of 10%, which is typical of E-140N and L-177 field instruments.
[6.6E-3 dpm][0.10 ccpm] = 6.6E-4 ccpm/ml
            [  1 dpm  ]

Most field air samplers measure sample volume in either cubic feet or liters.  Where I work, the 'standard' air sample volume is 18ft³.  So 0.3DAC on an 18ft³ air sample works out to be:
[6.6E-4 ccpm][28300 ml][18 ft³] = 336 ccpm
[     ml    ][   ft³  ]

Therefore, as a simplified rule of thumb:
If a field check on a standard (18ft³) PAS shows >300 ccpm, you're over 0.3 DAC.