Information on the interpretation of ODL measuring results

Humans are permanently exposed to natural, external radiation that consists of two components:

• The terrestrial radiation originates in the natural radioactive materials (radionuclides) occurring everywhere in the soil and in building materials, such as uranium, thorium or potassium (K-40).
• The cosmic radiation originates in space; when interacting with the atmosphere, it is significantly reduced, but a certain portion reaches the Earth's surface.

The intensity of the external radiation is indicated in microsievert per hour (μSv/h). The natural ambient dose rate in Germany is between 0.05 and 0.2 microsievert per hour, depending on the local conditions.

The radiation released by the Chornobyl (Russian: Chernobyl) nuclear accident in 1986 contributes only a small part to the gamma ambient dose rate (ODL) measured today.

Fig. 1: Ordinary time series at the Schleswig measuring station

Ordinary sequence of the time serie

The external radiation exposure at any given place remains largely constant. Figure 1 shows an ordinary sequence of the time series representing the ambient dose rate at measuring station 010590751 in Schleswig between 13 April 2011 and 15 April 2011.

This time series is free of temporary occurrences, such as rain, snow or technical failures.

Increase due to rain

The atmosphere always contains the naturally occurring radioactive gas radon. Since it is radioactive, it constantly decays into its so-called "decay products". Short-term increases of the ambient dose rate up to a factor of 2 occur where radioactive decay products of radon are washed out of the atmosphere by precipitation and are deposited on the ground.

Fig. 2: Time series including temporary occurrences of rain

It is typical for natural occurrences that the ambient dose rate increases for a short period of time and returns to its ordinary level within just a few hours. The decrease of the ambient dose rate is usually asymptotic (the curve approaches the original level slowly) with a typical half-life period of 30 minutes; it occurs usually more slowly than the increase. Figure 2 shows the occurrence of rain in the time series registered at a measuring station in Todendorf.

For a better understanding, Figure 2 does not only show the ambient dose rate (black) but also the time series of the weather radar (red) published by Germany's National Meteorological Service (DWD) that reflects the measured cloud density. A high cloud density means a strong probability of precipitation. During the occurrence of precipitation the ambient dose rate increases temporarily because radon decay products are washed out of the atmosphere and are deposited on the ground.

Trend of the time series in case of snowfall

Fig. 3: Example of a time series including snow cover

Snowfall may also lead to an increase of the ambient dose rate as in the case of rain, in particular if the ground was free of snow before. If the ground is now covered with snow, however, the ambient dose rate will remain on a lower level (see Figure 3). The snow shields the terrestrial radiation effectively. Once the snow begins to thaw, the ambient dose rate will return to its former level.

In addition to such short-term variations, the trend of the ambient dose rate is also subject to seasonal variations over longer periods of time that can amount to several 10 nSv/h (nanosievert per hour). The primary reason for this are constant changes in soil moisture, the geology of the ground, the surface properties of the ground and the microclimate on site.

Variations in ODL measurements

Radiological incidents or technical problems may cause even larger variations in the ambient dose rate measurements. The measuring stations detect such technical problems (such as battery mode for more than 5 hours, automatic shutdown due to flat batteries, battery errors, failures of the counter tubes) and radiological incidents that can be recognized when threshold values are exceeded and send automatically "instant messages" in order to alert the BfS staff.

The data is checked daily for plausibility. All measured values that have exceeded the thresholds will be checked in detail and failures and radiological incidents can be rapidly identified in this process. The counter tubes are very sensitive so that they are unfortunately prone to interference. In the case of long-term, continuous measurements, the trend of the ambient dose rate can also be influenced by a failure somewhere in the measurement chain (probe, transmitter, power supply) or by an atmospheric discharge (lightning).

Potential technical failures

The potential technical failures can have very different causes. In Figures 4 - 8 you will find a selection of frequently occurring failures.

Failure of the low-dose counter

Fig. 4: Failure of the low-dose counter (does not emit pulses)

Figure 4 shows a typical time series in the event of a failure of the low-dose counter. The time series starts with ordinary values but then the individual values vary considerably. The measurements show only certain numerical values but the ambient dose rate does not take any intermediate values.

If the pulses are also taken into account it becomes immediately obvious that there is a failure in the low-dose counter. The ambient dose rate is only calculated on the basis of the pulses from the high-dose counter. The stepped structure of the time series is due to the low sensitivity of this counter.

The transmitter detects this error, recognises the failure of a counter tube and sends an instant message to the relevant measuring node of the Federal Office for Radiation Protection.

Failure of both counter tubes

Fig. 5: No low-dose or high-dose pulses

If the transmitter does not receive any pulses from the low-dose counter nor from the high-dose counter (Figure 5), the ambient dose rate cannot be determined.

The most frequent cause for a simultaneous failure of both counters is, according to experience, a disruption of the probe cable. A failure of the probe electronics would be another typical cause.

Wind effects

At locations where the probe is exposed to wind, strong winds can cause vibrations in the entire probe including the standpipe. In the event of mechanical stimulation at the resonance frequency, the wire in the counter tube can start to vibrate as well, thus shortening the distance to the anode. This can trigger discharges between anode and cathode that the transmitter would interpret as gamma pulses.

Fig. 6: Time series including wind effects

If wind effects occur, no regular pattern can be recognized, neither in the amplitude nor in the sequence of the ambient dose rate values. The increases will stop after a few hours and might recur after a longer period of time, as can be seen in Figure 6.

In order to solve this problem, the probe could be replaced by a less sensitive probe. If the problem cannot be solved this way, a reinforced standpipe could be used at this site or the probe might be relocated to a more protected site.

Duplication error

If significant or even regular increases in the ambient dose rate are observed, this may be due to a duplication error. This error indicates a technical problem in the probe.

Fig. 7: Duplication error

Following an ordinary sequence, drastic increases in the measured values are observed that amount to twice the base level (Figure 7). After one or several hours, the values usually return to the base level.

The reason for such duplication is that a pulse measured by the counter tube triggers follow-up pulses that are not due to external radiation. This is often due to a temperature effect that indicates age-related problems of the counter tube. The follow-up pulses are counted as well, thus the ambient dose rate appears to be higher.

Single peaks

Within the ordinary sequence of the measured values, individual increases can be observed (Figure 8) that amount to several 0.1 μSv/h. The sequence of the increases does not show any regular pattern. This is mostly due to electromagnetic interference at the supply lines. Thanks to technical advancements in the probe design such peaks hardly occur any more today.

Fig. 8: Individual peaks

Conclusion

These cases only represent a small part of the potential failures. Probes that show these or other failures will be immediately replaced or repaired. Since the BfS staff verifies the measured data daily, defective systems are rapidly identified and are tagged as "defective" in the database operated by the measuring network office. The data transmitted by these probes will thus not be taken into account for early alert messages.

Please note, however, that the hourly measured values published on this website are unverified raw data that may not be free of technical interference.

Radiological incidents only give reason to worry if a significantly increased ambient dose rate is registered over a longer period of time (e.g. one day or longer) or if the increase is more than two-fold and there is no technical failure. Temporary increases of the ambient dose rate caused by radon decay products being washed out of the atmosphere, as described above, cannot be considered as abnormalities. Unusual values detected in the measuring system always have to be subject to an intensive plausibility check.

Measuring network

Fig. 9: ODL measuring probe operated by BfS

Six measuring nodes of the Federal Office for Radiation Protection, situated in Berlin, Bonn, Freiburg, Neuherberg near Munich, Rendsburg and Salzgitter, are responsible for service and maintenance work on the measuring network. In addition to the automatically running systems BfS also operates mobile systems that are used to perform nuclide-specific measurements in regular intervals.

Figure 9 shows a probe belonging to the measuring network. It contains two Geiger-Müller counters that measure the ambient dose rate. The data logger (transmitter) connected to the probe condenses the data registered by the probe into 1-minute and 10-minute values that are transmitted to the measuring nodes in regular intervals. In the event of a failure or increased measuring values, the probe automatically sends a message. In order to be able to compare the values measured by the probes distributed all over Germany, the selected sites must meet certain criteria. An optimal site for a probe would be a flat, trimmed meadow free of larger plants, sealed surfaces or buildings.