The limit of detection of a detector (LOD) describes the fewest number of counts which can be detected reliably by the system. This property is particularly relevant in the case of monitoring radionuclide activity at decommissioning sites as it describes the lowest activity which can be detected in a defined time frame. There are two aspects to consider when defining the detection limits of a detector, whether the sample is radioactive or not, and whether we can quantitatively measure its activity. This section shall consider the limit of detection for the sensor and the duration measurement required to achieve it.
In a simple case where a decision must be made whether a borehole contains radioactivity, the number of counts from the sampled water,
, and the number of counts from the background,
, are subtracted to produce a result,
, which can be compared with a critical level,
. This is used to determine the likelihood of measured counts representing true activity and not fluctuations in statistical noise. The minimum detectable activity,
, is calculated with the following equation, where
is defined as the minimum number of counts needed to ensure that the detector does not produce a false-negative rate that exceeds 5% (an industry standard value) [
16].
where
f is the radiation yield per disintegration,
e is the absolute detection efficiency, and
T is the time taken to count the sample. When this formula is applied to
Sr and
Y, the expected low detectable limits are 323 BqL
and 35 BqL
, respectively. The WHO guideline value for
Sr contamination in water is 10 BqL
. This suggests that, in its current state, the detector would be unsuited for monitoring radiation close to or at the guideline value. Performance could be improved by eliminating some of the background noise, which can be achieved by refining the electronic design and accounting for temperature control systems in the detector. The background noise for the detectors was collected while they were at room temperature, but the real world application will see them deployed in groundwater, which typically has a temperature of 7
C. A reduction in background noise of 50% would reduce the lower detectable limit by 29%, to 229 BqL
, leaving it still significantly short of the safe drinking water limit. Alternatively, the counting time for the detector could be increased. The results of this are documented in
Table 2. By increasing the measurement time to 24 h, the LOD would be significantly reduced, to 66 BqL
, while still obtaining results on a daily basis. The background noise in the detector would have to be significantly reduced, by up to 98%, for the detector performance to reach the safe water drinking limit. However, daily sampling is far from the norm in the industry, where monthly sampling is considered high frequency. Longer time frame measurements are, therefore, much more viable, and this would allow the detector to produce results closer to the guidance level.
An ideal detector with 100% efficiency would collect beta spectra with identical characteristics, no matter the activity of the water. The maximum recorded energy in the detector decreases as the activity decreases. As the activity goes from 100,000 BqL to 10,000 BqL, the maximum recorded energy moves from 0.514 MeV to 0.403 MeV, a decrease of 21.6%. This shift increases to 25.5% and 41.0% for 10,000 BqL and 1000 BqL, respectively. A similar shift in Y spectra is seen, although to a less significant extent. In that case, the maximum energy shifts by 7% from the maximum at 100,000 BqL to 22.8% at 1000 BqL. This is the result of the statistics at play. In a typical beta spectrum, few particles are emitted at the end point energy, and, in this scenario, fewer still are detected.
These results suggest that the detector is unlikely to compete with traditional methods for monitoring
Sr contamination in terms of precision. If truly instant results are required, the detector could be applied to monitor activity levels in highly concentrated areas above 800 BqL
. By increasing the counting time to 168 h, or more, the detector can assess lower activity levels which approach the guideline value for
Sr contamination in water. This would still produce results in quicker fashion than traditional techniques and would allow for rapid scanning of large areas to identify possible spikes in activities or new leaks. Operating over such a time period may present additional challenges, such as detector stability over time [
17], changes in temperature, and power requirements.