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Re:MDA
The NUREG-CR-5849 formula is a field approximation of the MDA for Surface Contamination. It looks like the author was thinking of alpha surface contasmination. In the field, its nearly impossible to use the equipment described, NaI 2x2, to scan soils and get an actual activity measurement. We have tried to correlate NaI response with soil Ra-226 concentrations on a number of projects, and have not had great success. you are right in that geometry needs to be taken into account, also shielding from buried contaminants. I can't give you a formula for Minimum Detectable Activity for the surveying situation you are describing, but you might use the DOE concept of minimum detectable count rate. The formula for the MDR is
for the same counting time for sample and background:
MDR = [(4.65*SQRT(bkgd counts))/(count time)] + 2.71/count time
Reference DOE Radiochemistry methods for different bkgd and sample count times.
>
Note that for a survey meter, measuring cpm with a background of about 8000 cpm, this yields an MDR of about 420. The problem is that the variance in meter readings is often greater than plus or minus 500 cpm.
Bottom line is that we use the NaI 2x2 as a screening instrument, but rely on lab counted soil samples to get the actual concentration.
Without setting up a "field lab" with reproducable geometries, homogenized samples, etc. you can't give a very accurate cpm per pCi/gm.
Luke McCormick c0etxlim@mrd42.mrd.usace.army.mil
_______________________________________________________________________________
> T all fellow radsafers:
> I need some help on some technical issues. In NUREG/CR-5489 there is a
> formula for MDA for instruments using an integrated count for a preset time:
>
> 2.71 + 4.65 x sqrt of Background Count Rate x count time
> ___________________________________________________________
> count time x efficiency x active probe area (cm2)/100
>
>
> I have been told that this formula is incorrect. Is it?? If so then what is
> the correct formula??
>
> Also I am going to use a Ludlum Model-3 with an attached 2" x 2" NaI detector
> for release surveys of outside ground areas for a decommissioning project (meter
> reads in CPM). Main radionuclides Co-60 and Cs-137. The MDA formula for this
> type of instrument is a 25% or 50% increase in the audible count. Regulators
> have pointed out that this is a MDC calculation because probe area and
> efficiency are not used in the calculation. My question is can the MDA be
> calculated??? If so, how??? Again I am scanning outdoor ground surface area
> (some gamma logging too) they (the regulators) want to know how many CPM above
> background relates to pCi/gram. I think the geometry and type of survey make it
> a mute point and really make it a qualitative (go-no go) type of instrument.
> Any help in these matters would be greatly appreciated.
> reply to:
> radconpro@aol.com
> Dan
>
>