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APPENDIX C. FORMULAS page23
Model AM-255
C.4. RELIABLE DETECTABLE ACTIVITY (RDA) FORMULA
This formula calculates the minimum activity, in disintegrations per minute, that can be reliably
detected under a given set of operational conditions.
BKG = total background counts
per count time
CON = confidence sigma
CT = count time in seconds
E = system efficiency in percent
F = false alarm level in cps
FA = false alarm sigma
G = intermediate variance
RDA = reliable detectable activity
in DPM
C.5. SIGNAL TO BACKGROUND RATIO
The following formula is helpful is determining the optimum discriminator settings. Always
perform a variance test at the final setting of the lower-level discriminator to ensure that system
noise is not being introduced into the amplifier stage.
Q = Quality factor
S = Net signal (count with source - background)
B = Background count
Where:
B = Background count
Higher values of Q result in better sensitivity