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Measurement series of radionuclides in ground-level air in Braunschweig

History

For the first time, the general public became aware of the "radioactivity in air" after the nuclear weapons explosions over Hiroshima and Nagasaki. In the following time, the activity concentrations of artificial radionuclides in ground-level air constantly increased due to the arms race of the nations in possession of nuclear weapons at that time. In Germany, fresh fission products were for the first time deteced in 1953 on the top of "Mount Schauinsland" near Freiburg in the Breisgau by the former Institute for Atmospheric Radioactivity of the Federal Office for Civil Defense, which today is part of theOpens external link in new window Federal Office for Radiation Protection. In 1955, the Opens external link in new windowGerman Meteorological Service was legally authorized to regularly measure the Opens external link in new windowradioactivity in air and precipitation at the stations of its meteorological measurement network.

In 1963, PTB started regular measurements of the radioactiviy in ground-level air. For a long time, the measurements were performed within the scope of research projects which served to compare the activity concentrations in North Germany and Scandinavia. Already at the beginning of the weapon fallout, contamination of the other environment and the human nutritional chain resulting from the contamination of the air were measured in Germany by many federal authorities (today’s coordinating laboratories), authorities of the federal states, universities and research institutes. After the reactor incident in Chernobyl in 1986, the measurements of the environmental radioactivity were reorganized, and the different approaches for measurement and evaluation procedures harmonized, which clearly improved the comparability of the measurement results obtained. At that time, the measuring station of PTB in Braunschweig was integrated as one of the 14 German trace survey stations into the newly established Opens external link in new windowIMIS.

Since that time, trace measurements have been performed within the scope of IMIS with the aim of observing the presently existing activity concentrations and recognizing their tendencies. This routine task comes, however, rather under the scope of application of general environmental monotoring or radioecology. Opens external link in new windowRadiation protection precaution in the narrow sense comprises the two additional measurement objectives

  • to achieve a "possible advance warning time" to take preparatory measures in cases, in which an approaching strongly contaminated cloud is recognized very early with the aid of traces of characteristic artificial radionuclides and
  • holding in readiness of measurement values for "zero-level detection" which is required to estimate the consequences of a fresh contamination.

Since 2001, the European Union (EU) operates an EU-wide "sparse network" for sensitive radionuclide measurements in the environment. For this purpose, measurement values of a few selected measuring stations from the national measurement networks, which are representative of the respective regions of the member states, are made use of for the EU reports on environmental radioactivity. One of the four trace survey stations for air monitoring in Germany is the PTB station in Braunschweig (responsible for the "North region").

After the "Comprehensive Nuclear-Test-Ban Treaty" (CTBT) had been adopted in the UNO General Assembly on September 10, 1996, a world-wide measuring network was established to check compliance with this Treaty. This network is operated by the "Comprehensive Nuclear-Test-Ban Treaty Organization", Opens external link in new windowCTBTO, in Vienna and is based on globally distributed measuring stations for seismic, infrasound, hydroacoustics and artificial radionuclides in the air. The German contributions to this measuring network are described on the Opens external link in new windowHomepage of BfS under "Measuring Station "Schauinsland" and the Opens external link in new windowBundesanstalt für Geowissenschaften and Rohstoffe (BGR).

Radioecology and measurement results

The Initiates file downloadfigure "Radionuclides in ground-level air" shows which radionuclides exist in ground-level air and which range of activity concentrations is observed.

It should be noted that the unit "Bq/m3" has been selected here for the mean activity concentration. The unit normally used for trace measurement is, however, the "Microbequerel" per cubic metre (µBq/m3) which is therefore preferred for the representations of long-term measurement series. For illustration: The prefix "micro" means "millionth", i. e. if the recent activity concentration of a radionuclide amounts to 1 µBq/m3, only one atom per 1 million seconds (which corresponds to approx. 11.5 days) decays on average in one cubic metre of air.

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Short-term measurement series

After an increase in the activity concentration of 137Cs it is helpful to know whether there is any "fresh" 137Cs contained in the sample or whether only a variation of the "old" contamination has been observed. Therefore we compare the mean weekly activity concentration of 137Cs with that of 40K, assuming that in the first approach the alkaline elements behave similarly because they are either strongly attached to the ground-level aerosols or included in them.

By observation of the activity ratio A(137Cs)/A(40K) in the weekly aerosol ash samples and by relating it to the wind and weather conditions observed during the sampling period it should be possible to discriminate between the activity ratio contained in the local dust and that contained in dust originating from higher contaminated areas, e. g. in Eastern Europe by the Chernobyl accident. This influence should lead to an increase in the ratio. On the other hand it should also be possible to recognize any input of 40K which may be expected from fertilizers spread onto the nearby fields or of 40K from other sources.

Observations repeatedly made qualitatively show that typical ranges of the activity ratio A(137Cs)/A(40K) can be attributed to different sources. In order to know the local base-line activity ratio, a γ-ray spectrometric measurement was performed on the < 0.2 mm grain fraction of a top soil sample taken from a neighbouring field in summer 2002. The ratio determined was 0.029 ± 0.001. Today, the value is about 4 % lessdue to the radioactive decay of Cs-137. In periods in which weak winds allowed only a short-range transport of aerosols, the measured activity ratio in the air dust was in the range of 0.03 to 0.05. That value increased to a range from about 0.1 to 0.25 when stronger winds were blowing from the east in dry weather periods. Since 1998, the maximum value observed under such weather conditions was 0.41 and was reached in 2003 in week 38. The strongest annual deviation to the opposite direction was observed more or less regularly in dry New Years Eve’s nights mostly. The ratio decreased to 0.001 due to the immission of "additional" 40K contained in the oxidizing agents (e. g. KClO4) of the New Years Evexs fireworks. Up to now, the maximum activity concentration of 40K displayed in the Initiates file downloaddiagram was measured in sample from the New Years Eve’s week 2003/2004.

During the vegetation period, also some contributions from fertilisers spread on the neighbouring fields might be possible. In most weeks, the wind direction variation does not show a strong predominant wind direction over the longer part of the sampling period so that two or more sources could have contributed simultaneously.

The annual variation of the mean weekly activity concentrations of both 40K and 137Cs is shown for the year Initiates file download2003. The most probable sources are indicated by the frames.

The comparison of the activitc ratio A(137Cs)/A(40K) in the weeks 3 and 8 of the year Initiates file download2003 shows the influence of different wind conditions. Displayed are the hourly mean values of the wind direction recorded by DWD, Agrometeorological Research Centre in Braunschweig (ZAMF). The directions are given in degree: North = 360° (and 0°), East = 90°, South = 180° and West = 270°. The wind direction alone gives little information only about the route which the air mass had passed and it is possible to discriminate probable sources of radionuclides. Source discrimination can be oftenly achieved by use of meteorologic trajectory calculations or dispersion modelling which is performed at the Opens external link in new windowDWD headquarter in Offenbach.

The example shows in Initiates file downloadweek three the typical west wind situation with an negliglible amount of only about 0.6 % wind from east. In Initiates file downloadweek eight, this wind direction contributes to about 51 % when the wind predominantlycame out of the northeast sector.

Please note:
Normally, the current diagram is updated after the evaluation of the γ-ray spectrum is finished. The routine low-level measurement times are in the range of about one week, mainly in order to determine the radionuclide 22Na shown in the Initiates file downloadlong-term measurement series. Therefore, the recent results of the low-level measurements of the γ-ray emitting radionuclides appear in this diagram about 1.5 weeks after the end of aerosol sampling.

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Long-term measurement series

Generally, the activity concentrations of Initiates file downloadartificial radionuclides in ground-level air have decreased after the implementation of the Nuclear-Test-Ban Treaty. However, as not all states in possession of nuclear weapons have signed the treaty, nuclear weapons tests were still performed in the free atmosphere until 1980, as a consequence of which fresh fission products rose up to the stratosphere. From there, they increasingly returned to the earth surface late in spring/early in summer, as in this season the vertical exchange of air is increased due to warming-up of the atmosphere. This can clearly be seen in the Initiates file downloadfigure "7Be and 137Cs in the air in Braunschweig since 1963" of the long-term measurement series for the fission product 137Cs and for 7Be which was produced until 1986 due to the cosmic radiation.

However, as a result of the Chernobyl nuclear power station accident in April of that year, the amount of 137Cs which entered was so large that the effect can no longer be observed. Today, mainly 137Cs is contained in the air, which was raised ("resuspended") from the ground so that a possible effect by trace fractions of the weapons fallout still falling down from the stratosphere is no longer measurable.

After the reactor accident in Fukushima which was caused by an earth quake and a subsequent tsunami in March 2011, daily measurements were performed at PTB. After the contamination had decreased below the limit of detection achievable by daily measurements, the sampling periods were extended to two and three days. The results show, that the highest Initiates file downloaddaily values reached the activity concentration of the natural radionuclide 7Be when the first two contaminated clouds were passing and rapidly decreased then. The influence of the 137Cs released from Fukushima is clearly visible in the Initiates file downloadlong-term measurements series of the mean monthly activity concentrations. The highest activity concentrations observed were in between a thousandth and some tenth of a thousandth of the maximum values observed after the Chernobyl accident. An overview on the contamination measurements all over Europe after the Fukushima accident can be found in the literature.

The long-term measurement series available at PTB of the natural radioactive nuclides which are always contained in ground-level air, begin at different times. This can mainly be attributed to the low activity concentrations of some radionuclides and to the fact that analytical and metrological improvements could be achieved only gradually. An essential improvement was, for example, the development of high-resolution γ-spectrometry with germanium spectrometers which allowed many γ-ray emitting radionuclides to be measured directly in the sample, without the necessity of submitting them before the measurement to a radiochemical analysis, in which activity losses are mostly inevitable.

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Natural radioactive nuclides

The curves of the two (= "cosmogeneous") radionuclides 7Be and 22Na formed by the interaction of the cosmic radiation with the atoms of the atmosphere show a clear seasonal cycle in an approximately constant range. The maximum lies in late spring or early summer and the minimum in winter, as the permeability of the tropopause is different depending on the season. The curve is subject to a longer-periodic change due to the sunspot cyle of approximately eleven years, because the cosmic particle flow in the vicinity of the earth decreases in case of high sun activity which also causes a decrease in the formation rate of 7Be and 22Na.

210Pb is one of the natural radionuclides from the radioactive decay chain of uranium. As a progeny of 238U, it is contained in the mineral fraction of air dust. In addition, it is also formed in the decay chain as a daughter nuclide of the natural radioactive radon (222Rn) which is always contained in ground-level air. Under chemical aspects, radon is a noble gas which, due to its nonreactivity, forms only very few chemical compounds and, as an elementary one-atomic gas, therefore reaches the air easily not only from the soil but also from mineral construction materials. However, during the radioactive decay of 222Rn, chemically reactive atoms of the heavy metals polonium, bismuth and lead are formed again, which practically immediately attach themselves to any available air particle. From this moment on, they are "carried" by the dust particles, the solid aerosols. This is therefore also technically referred to as "air-borne radioactivity" or "aerosol-bound radionuclides". The activity concentration of 210Pb therefore strongly depends on the dust concentration of the air. It lies in an almost constant range. For a short time it may, however, vary very strongly when the wind and weather conditions change (e. g. in the case of starting rain or snow fall, frost, snow-covered ground or dry-wind weather conditions). But also human activities which cause a formation of dust such as, for example, road traffic or agricultural cultivation, exert a measurable influence on the activity concentration of 210Pb in ground-level air.

Like 210Pb, 40K is also always contained in air dust. As 238U or 232Th, 40K is a "primordial radionuclide" which has not yet decayed since its formation with the other chemical elements about 4 to 5 billion years ago - its half-life amounts to approximately 1.3 billion years. As an alkaline metal, 40K exists practically everywhere in the inanimated and animated nature.

A human being contains a specific activity of approximately 50 Bq 40K per kilogram of his body weight. Multiplied by the body weight of the reader, this mean value thus furnishes the number of 40K atoms decaying each second in the body of the reader. Here, it is obvious to compare this activity with the activity concentrations of the natural radionuclides in ground-level air. On the one hand, the sensitivity with which radionuclides in air can be measured is clearly shown, on the other hand, the activities naturally existing in the body of humans since mankind came into being can be seen.

The long-term measurement series shown illustrates that the monthly mean activity concentration of 40K has decreased since the end of the eighties, which can be explained by the general efforts made to prevent air pollution and by the modernization of industrial plants and heating installations.

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Artifical radioactive nuclides

Please note:
Normally, the current diagram is updated after the evaluation of the last γ-ray spectrum of the last weekly aerosol ash is finished. The routine radiochemical separation and purification procedures necessary for the determination of the α- or β-particle emitting radionuclides are performed on monthly instead on quarterly samples since January 2007. They are very time consuming and, e. g. for the determination of 238Pu, the following measurement requires counting times of up to six weeks per sample. Therefore, the recent results of the low-level environmental monitoring of strontium-, uranium- and plutonium-isotopes appear in this diagram with the corresponding delay after the end of aerosol sampling.

The Initiates file downloaddiagram shows the long-term measurement series of those artificial radionuclides which are regularly measured in Braunschweig within the scope of the Opens external link in new windowIntegrated Measurement and Information System IMIS. The measurement results from the time before 1987 stem from activities performed within the scope of research projects which are summarized in a PTB Report of 1992.

85Kr and 133Xe
For the determination of these two radioactive noble gases, PTB weekly performs sampling and sends the precleaned noble gas samples to the Bundesamt für Strahlenschutz, Freiburg, where the samples are further cleaned and, after gas-chromatographic separation of krypton and xenon, finally measured. The results shown for 133Xe in the curve lie in the order of the achievable detection limits of a few millibequerel per cubic meter (mBq/m3). Due to its short half-life of 5.3 days, 133Xe is an indicator of a release from a source in which nuclear fission occurs or had occurred shortly before release took place.

85Kr is a radionuclide which is formed both in nature and by human activities. It is formed naturally from cosmic radiation as well as from the spontaneous fission of uranium. Artificially, it is formed by nuclear fission in nuclear weapons or nuclear reactors. It is the only artificial radionuclide at present regularly measurable whose activity concentration did not decrease in air, but slowly increased due to its release during reprocessing of spent nuclear fuel. Its half-life amounts to 10.6 years. Its release rate exceeded its decay rate from the radioactive decay so that in Germany an annual increase of approx. 30 mBq/m3 was observed, which came up to the global tendency. In the period between 2002 and 20045, apparently a new stable level was reached again.

The sum of the contributions of the radioactive noble gases 85Kr and 133Xe contained in air to the local doserate is below 30 nano-sievert a year (nSv/a). This is negligible compared to the mean annual radiation burden to which a person from the general public is exposed. Expressed in the same unit, the mean annual radiation burden amounts to approx. 4100 nSv/a. The numbers of this example have been taken from the report "Environmental radioactivity in the Federal Republic of Germany 2014" and vary only insignificantly from year to year.

90Sr, 137Cs and Pu-Isotopes
These artifical radionuclides mainly stem from two sources. In the time before 1986, 90Sr, 137Cs and (239+240)Pu were (in addition to a few percent 238Pu) released by the nuclear weapons tests performed in the atmosphere. Other comparably small 238Pu quantities stem from satellite crashs. If the satellite was equipped with a 238Pu isotope battery for current supply, this battery was also destroyed.

The indication (239+240)Pu clearly shows that the measured activity is a mixture of the two plutonium isotopes 239Pu and 240Pu. When the most popular measurement method - i.e. alpha spectrometry - is applied, it is difficult or even impossible to distinguish the two isotopes. The energies of their alpha radiation are too similar. As a result, one only sees the sum of the two activities in the alpha spectrum. Modern programs for evaluation of alpha spectra meanwhile allow the activities to be separated by computation. This, however, requires that the energy resolution of the detector is sufficient and that the activities of the two isotopes of the counting source are sufficiently large. This last requirement is in most cases not met by environmental samples. Today, separate determination has become possible with modern mass-spectrometric measurement methods. It is, however, still relatively complex and expensive so that it is performed only at a few institutes as routine method.

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