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Tracing: Measurement of radionuclides in ground-level air

Tracing: Measurement of radionuclides in ground-level air

"Tracing" generally means the measurement of traces of radioactive substances. In practical application, this often means the measurement of an activity of a specific radionuclide which lies only a little above the sensitivity limit ("detection limit") of today’s technical-analytical possibilities.

"Trace survey" (or "trace analysis") of radioactivity in ground-level air in terms of the Integrated Measurement and Information system (IMIS) for monitoring of environmental radioactivity means the very sensitive measurement of smallest activities to be able to detect, for example, an immission of radionuclides from accidents or impermissible releases still at very large distances. The trace survey station of PTB has been in use since October 1963 and, since 1989, has been one of a total of 14 German trace survey stations for the monitoring of radioactive substances in ground-level air which are operated within the scope of Opens external link in new windowIntegrated Measurement and Information system (IMIS).

In October 2013, the trace survey station of PTB could celebrate 50 years of regular measurements of radioactive traces in air. This gave cause to publish a principal topic („Schwerpunktthema“) in the PTB-Mitteilungen which is available only in German.

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Analytical procedures

Picture 1: Air dust samplers

With an air dust samplers (figure on the left side) reaching a volume flow rates of  1000 m3/h, the dust contained in the air is collected on special filter materials. Optimal comparability of the measurement results from different sampling places would require ideal conditions at all collecting places. In most cases, these conditions are not, however, given in reality. Ideal conditions would guarantee a free approaching flow of air from all directions, no forests or buildings in the vicinity, a sampling height of aerosols approx. 1.5 m to 1.8 m above ground.

In view of the required energy supply, operation by expert staff, protection against unauthorized access or even vandalism, the aerosol samplers stand, however, on fenced-in premises of institutes or authorities. Moreover, the technical design of all aerosol samplers must be absolutely identical and include collection properties for the air dust. Last but not least, the collection properties also depend on the sample itself, as the flow velocity of the air through the filter decreases with increasing dust quantity in the course of the sampling process. Despite the restrictions mentioned, comparability of the measurement results is more than sufficient for the purposes of IMIS. For more detailed scientific or radio-ecological investigations, site-specific differences should, however, possibly be taken into account.

The photograph shows the location of the aerosol samplers at the border of the PTB premises on a semi-circular clearance which can be reached by the approaching air from the eastern direction via free fields. In the west, the meadow is surrounded by a low forest which must not be further cleared for reasons of environmental protection.

IMIS-consistently, the aerosol samples are collected from Monday to Monday as weekly composite. The pre-treatment of air dust samples before the measurement is performed differs depending on the metrological specifications or the analytical procedures applied by the individual German trace survey stations.

Picture 2: The location of the aerosol samplers at the border of the PTB premises

At PTB, first a gamma-spectrometric measurement is performed to check for the presence of iodine-isotopes which are hightly volatile in heat (the "key nuclide" is 131I). After that, the aerosol samples are ashed at approximately 400°C to 450°C, by which losses of the other radionuclides are prevented. The ash obtained is compressed to tablets which are subsequently measured by gamma spectrometry in a well-type high-purity germanium spectrometer. After measurement times between approximately three days and one week, detection limits of less than 5•10-8 Bq/m3 are achieved with this method for most gamma-ray emitting radionuclides.

To determine the pure beta-praticle emitting radionuclide strontium-90 (90Sr) or alpha-particle emitting radionucles (uranium and plutonium isotopes) in the air dust, the weekly air dust ashes are combined at PTB to a monthly sample and subjected to complex radiochemical separation and purification procedures for the respective radionuclides. By alpha-spectrometric measurement, detection limits of less than 5•10-10 Bq/m3 can be achieved in monthly samples for both uranium and plutonium isotopes. This corresponds to one decay per second in an air volume of 6.6 cubic kilometres!

In the following, two examples of typical measurement series are given, current measurement series can be found here.

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Example 1: Short-term measurement series

The short-term measurement series of the activity concentration of the natural radionuclide potassium-40 (40K) as well as of the artificial caesium-137 (137Cs) in air in Braunschweig (from Initiates file download2004) is shown as a typical result of the regular weekly measurements. 40K is a "primordial radionuclide", i. e. it has not yet decayed since its formation (together with the other chemical elements) about 4 to 5 billion years ago, because its half-life (amounts to approx. 1.3 billion years. As an alkali metal element, 40K is contained everywhere in the inanimated and animated nature. 137Cs, which can still be measured regularly in the air today, is a fission product and stems from the nuclear weapon fallout in the middle of last century and from the reactor accident in Chernobyl (April 1986). The contribution originating from the reactor accident in Fukushima, Japan, in March 2011 is of minor importance.

The shape of the two curves is characteristic of the annual changes in both the activity concentration of the single radionuclides and the activity ratio of 137Cs to 40K. The radionuclides bound to the air dust are transported together with the dust. This is why the measured activity concentrations depend on the constantly changing wind and weather conditions (origin of the dust, wash-out by rain) and on the season (summer half-year: agricultural activities, winter half-year: heaters in operation). The comparison between the activity concentrations of 137Cs and 40K allows conclusions to be drawn of whether the sample contains "additional" 137Cs, e.g. when a greater amount of soil dust, which is more strongly contaminated with 137Cs from the reactor accident in Chernobyl of April 1986, is brought in from Eastern Europe in case of dry weather conditions and east wind. On the other hand, the air dust sample which contains the dust of New Year’s Eve shows, depending on the weather conditions, a more or less clear increase in the activity concentration of 40K. This increase, which was particularly distinctive on New Year’s Eve 2003/2004, is to be attributed to the potassium chemicals contained in the propelling charges of the rockets which inevitably contain this radionuclide.

The corresponding diagrams from 1998 to 2016 can be found in the report of the trace survey station from these years, which is available in printed form.

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Example 2: Long-term measurement series

As a typical result of the regular measurements of many years, the figure shows the temporal variation of the activity concentration of the natural radionuclide beryllium-7 (7Be) and of the artificial caesium-137 (137Cs) in air in Braunschweig from the long-term measurement series of PTB. 7Be is produced in the stratosphere due to the reaction of the cosmic radiation with the gas atoms of the Earthxs atmosphere. 137Cs, which is today still measurable in ground-level air, is a fission product and stems from the nuclear weapons fallout in the middle of last century and from the reactor accident in Chernobyl (April 1986). The left hand arrow marks the influence of the last atmospheric nuclear weapons test which was carried out in China on 1980-10-16 and became measurable in Braunschweig in summer 1981. The influence of the 137Cs released after the reactor accident in Fukushima is indicated by the right hand arrow.

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