Equation for MDA

["Lavera, Ron" <RLavera@ENTERGY.COM>]

We have a digital radiation monitoring system installed.  A question has

arisen as to what is the most correct method of determining the Minimum

Detectable Activity ( MDA ).



The vendor manual list the MDA = (a * ( B/(2*T) )^0.5 ) * C

      a = confidence intervals

      T = instrument response time - because this is a digital instrument,

this varies with the detector count rate

                T = 10 if B < 100 cpm

                T = 1000/B for   100 < B < 100,000 cpm

                T = 0.01 * B      B > 100,000 cpm

      B = Background Count Rate

      C = uCi/cc per cpm conversion factor



This appears to be based on the treatment of the system as a Count Rate

Meter ( digital Frisker ! ) and determining the standard deviation as you

would for that type of instrument.  This appears to be an Lc value converted

to Activity.



Currie (NUREG/CR-4007) on page 91, appears to indicate a different method of

calculating MDA that is essentially :

    

          MDA = ( ( 2.71 + 3.29 * (Sb)^0.5) * C

     Sb = Standard deviation of the background

     C  = uCi/cc per cpm conversion factor

this assumes that the Background and sample counting times are the same.

                    

The questions are :

   

   1 -  What is the correct method for determining the MDA for this type of

instrument.

   2 -  Which value should be used for setting the "alarm setpoint".

   3 - does anyone know of a reference that describes the derivation of the

first equation 



Thank you all for your time and consideration.



Ron LaVera 



RLaVera@entergy.com

914-736-8433

914-736-8419 FAX

Entergy Nuclear Northeast

Entergy Nuclear Operations, Inc.

Indian Point 3 NPP

P.O. Box 308 

Buchanan, NY 10511

 



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