783620H / 1013 6. ANALYSIS METHODS
3) To reduce the MDA correction to the value f
max
shown in Eq. 181, it is clear that the beta risk error must be increased to decrease the k
1-
β
value. The new beta risk error can be calculated from:
(182)
If the upper limit of R is set to 3, the corresponding maximum MDA correction factor is about
1.5. In GammaVision, R
Max
can be any value from 2 to 1000 (the “Maximum ISO NORM MDA
Ratio factor” in the
b30winds.ini
file). If you modify
b30winds.ini
to a value outside this range,
GammaVision uses the default value of 3.
Maximum MDA Report
If R
Max
is 3.0, the uncertainty to reach the maximum correction is about 35.1% with all parameters at default values. When the actual uncertainty is larger than this critical uncertainty, a new k
1-
β
is calculated from Eq. 181 so the MDA ratio R remains at R
Max
. If the uncertainty is extremely large, the asymptotic value of k
1-
β
is zero and the corresponding value for the beta risk factor is 50%.
When the maximum MDA ratio has been reached, an “I” flag is reported next to the isotope name as shown here:
AM-241 I 5.7471E+03 6.7293E-01% 6.0850E+01% 1.000E+4
At the end of the ISO NORM table, the flags are defined as:
I - ISO NORM MDA ratio at maximum for one or more peaks
Note that the “I” flag supercedes other nuclide flags (A, B, C, F, and H). If there is more than one peak defined in the library for an isotope, it is possible that the ISO NORM MDA for one peak is limited even though the MDA is not capped for other peaks of the same isotope. But the isotope would still have the “I” flag assigned. However, if the nuclide is not present and the
MDA is reported as the activity, the less-than flag may supercede all other flags, including the
“I” flag, since that flag is of higher importance to the users. At default values, the uncertainty corresponding to the maximum MDA ratio is about 35%. The best estimated uncertainty can be used as an indication of whether the maximum MDA ratio has reached.
321
