Aeroradiometry can be used to detect radioactivity on the ground from the air. Every year, the National Emergency Operations Centre (NEOC), with the support of PSI, carries out measuring flights to determine the radiological situation in Switzerland.
The helicopter’s rotor blades vibrate and lift the five-tonne behemoth into the air, seemingly without effort – the quiet roar and the distinctive smell inside the Super Puma could make you think you were inside a gigantic lawnmower. From the launch pad at Dübendorf airport, it races along at 100 kilometres per hour, high above the city of Zurich and along the Limmat towards the Wasserschloss, where the Limmat and Reuss rivers join the Aare. This is the actual area that is to be surveyed in order to determine the radiological situation of PSI, the Swiss interim storage facility for radioactive waste ZWILAG, and the two nuclear power plants in Leibstadt and Beznau.
To record the measured data as accurately as possible, the helicopter must be kept at a height of 90 metres – the cargo bay in its belly holds the measuring equipment used to detect the radioactivity of the earth’s surface. Maintaining a constant height above the ground is a challenge for both the pilots and the crew, because the ground level below is changing all the time, meaning that you feel every hillock in the landscape. And so the helicopter climbs steeply up the ridge of Villigen’s Geissberg – pressing the crew down into their seats – only to plunge just as steeply down the other side. It feels like riding a roller coaster – and you begin to understand why sick bags were handed out before take-off.
The helicopter follows precisely specified parallel paths. With a field of view that is 300 metres in diameter and a distance of 250 metres between adjacent paths, it’s possible to scan the entire ground. The route can also be followed live on the computer screens of the two operators in the helicopter. An initial evaluation takes place in real time in this flying laboratory. The coloured areas on the screen allow you to immediately identify the radiological background: blue and green correspond to about 40 to 100 nanosieverts per hour – a sievert is the physical unit for quantifying radiation exposure – everything’s in the green (or blue) range.
The problem with the rain
Three hours earlier, in an unostentatious conference room at the Dübendorf military airport: exercise leader Cristina Poretti presents today’s mission and shows the area to be surveyed, a map with parallel red lines drawn across it. The helicopter has to fly along these lines. “The weather looks bad, though. We’re expecting a rain front from the east towards evening.” Poretti, who is from Ticino, points to the tiny numbers from one to four above the red lines. “I’ve divided the area into four different priority zones: We should be able to manage Zones 1 and 2 today. We may have to postpone Zones 3 and 4 until the next measurement date.”
Her audience listens attentively: uniformed professional soldiers, militia members, civilian NEOC officials, and PSI physicist Alberto Stabilini. “Any comments from the pilots?” No comments. Cristina Poretti is in charge of the exercise – in her mind she goes through the process once again. In the event of a radiological emergency, she would be responsible for ensuring that everything runs smoothly.
That means: planning the mission, cooperating with partners such as the military and the cantons, localising the affected area, and planning and prioritising further measurements on the ground. All this would be handled by her department, which is why the annual measuring flights also serve as a training opportunity.
The Swiss Army provides the pilots and loadmasters, as well as the infrastructure at the Dübendorf military airport. The operators are military personnel with special training doing an extended term of service – militia members who are subordinate to the NEOC at the organisational level. “These people usually have a scientific background and act as specialists during the flight. Since everything takes place in real time, they can intervene directly in the measurement process and, if necessary, instruct the pilots, for example, to fly vertically over a certain point again.”
And what does the rain have to do with this? “The rain washes radon by-products out of the air,” explains Alberto Stabilini. Radon is a natural, radioactive noble gas that forms in the ground as part of the uranium decay chain. If radon gas enters the atmosphere, it decays further, forming bismuth and lead, among other elements. These so-called radon decay products are also radioactive and are suspended in the air. When it rains, they are washed out of the air and collect on the ground. “When analysing the readings, this leads to an overestimation of the uranium activity concentration in the ground.”
Rapid and extensive
In 1975, Swiss aeroradiometry was still purely military in nature, and the procedure was for a soldier to lean out of a helicopter with a Geiger counter to detect the radiation below. In 1986, the Swiss Geophysical Commission used the method for the first time, in a project to map the central massifs of the Aar and Gotthard. An algorithm for evaluating such data was then developed as part of two dissertations at ETH Zurich. From 1994 on, the NEOC took over the management of the project. For the first time, aeroradiometry was carried out aboard one of the Swiss Army’s Super Puma helicopters, which can be used for missions in difficult weather conditions and at night.
In 2003, the scientific expertise was transferred from ETH Zurich to PSI, in cooperation with ENSI. The new measuring system was finally put into operation in 2018. Since then, four highly sensitive detectors with associated electronics and software have been available at two locations in Switzerland. Such a system can be installed in the Super Puma within a few hours. All data from the helicopter, such as GPS, altitude, and speed, are included in the analysis. In effect, the helicopter merges with the detector to become a single measuring device; thanks to the simultaneous evaluation of the data, it also serves as a laboratory.
This makes it possible to measure an area of up to 100 square kilometres within three hours without a stopover and to determine the radiological situation of this area in real time. Together with a dense network of permanent measuring probes on the ground throughout Switzerland, as well as other measuring teams and vehicles, aeroradiometry makes an important contribution to civil defence.
Teasing out the details
During the flight, Alberto Stabilini remains on the ground, working in the background on his laptop. It’s his job to intervene if a measurement result is unclear and his expert opinion is sought. “At 90 metres above the ground, only a small amount of radiation reaches the detector,” the physicist explains. “So in some cases it may not be possible to assign a signal clearly to a source.”
For the NEOC, the data analysis must be instantaneous, since it is ultimately the basis for deciding whether a radiological source poses a threat to the population and if so, what steps need to be taken next. “The system is perfectly adequate for this” says Stabilini. “Still, sometimes you need to take a closer look.”
If the operators are unable to assign a specific anomaly to a source using their onboard computers, or if they need a second opinion, they send the data set to Stabilini, who checks it using special software. “This code was developed by my predecessor Gernot Butterweck,” Stabilini explains. “It combines his more than 25 years of experience with field measurements. Speed, altitude, radiation geometry, and also the terrain can influence the signal – with this software we manage to tease out the maximum amount of information from the raw data.”
Today’s flight, though, was free of inconsistencies. In three hours, the team has managed to map 115 square kilometres. “A successful day’s work,” says Stabilini happily.
