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SARAD GmbH
Detection Limit
The term Detection Limit defines the smallest value of the Radon concentration which
delivers a non-zero reading of the instrument within a given integration interval (at least 1
decay per interval). Because of the statistical behaviour a related confidence interval has to
be stated.
Why is it necessary to know the Detection Limit? If the set integration interval is short and
the Radon concentration low, the expected "true" value of the number of detected decays
may be around or less than 1. Because of the statistical variations, intervals without any
detected decay will appear frequently. The most extreme situation would be a measurement
series with a lot of "zero" intervals and only one interval with one detected decay (because a
decay cannot be split).
When calculating the Radon concentration by the given formula, the concentration value for
the interval with the one count is much too high while all other values show zero. Then, all
intervals have to be averaged to get a usable result. This procedure is nothing else than to
create an integration interval long enough to meet the Detection limit for the applied Radon
concentration. To avoid zero readings, set the integration interval with respect to the lowest
expected concentration level during measurement.
The mean („true") value of the number of decays during an integration interval in case of a
Radon concentration in the surrounding of the detection limit is less than 16 and therefore
the statistical fluctuations have to be derived by the Poisson distribution. The stated
confidence interval gives the probability that the detected number of decays within the
interval is not zero.
Confidence Interval
63,2%
95%
99,75%
Example:
Determination of the detection limit of the Monitor using the „Fast-Mode" and an
integration interval of 60 Minutes. The confidence interval shall be 95% (that means in about
95 from 100 intervals a no zero reading should appear):
Required mean value (number of counts from the table): N = 3. Calculating the detection
limit by the formula:
C = N / (T * S) = 3 cts / (60 min * 8 cts/(min*kBq/m³)) = 0.00625 kBq/m³ = 6.25 Bq/m³ The
detection limit in this case is 6.25 Bq/m³.
User Manual RTM1688-2 06-02-2019.docx
Required Mean Value for N at the Detection Limit
1
3
6
16
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