Hauptabteilung für die Sicherheit der Kernanlagen Division principale de la Sécurité des Installations Nucléaires Divisione principale della Sicurezza degli Impianti Nucleari Swiss Federal Nuclear Safety Inspectorate

Eidgenössiche Technische Hochschule   Ecole polytechnique fédérale de Zurich  Politechnico federale svizzero di Zurigo Swiss Federal Institute of Technology Zurich

Ladislaus Rybach, Georg F. Schwarz, Fausto Medici
Construction of radioelement and dose-rate baseline maps by combining ground and airborne radiometric data

Contents

Abstract
1 Introduction
2 Input Data
2.1 Airborne radiometric measurements
2.2 In situ gamma-ray spectrometric measurements
2.3 Dose rate measurements
2.4 Laboratory measurements
3 Calibration
4 Collation/interpolation/mapping
5 Results and conclusions
References

Abstract

For emergency situations like nuclear accidents, lost isotopic sources, debris of reactor-powered satellites etc. well-documented baseline information is indispensable. Maps of cosmic, terrestrial natural and artificial radiation can be constructed by assembling different datasets such as ground and airborne gamma spectrometry, direct dose rate measurements, and soil/rock samples. The in situ measurements were calibrated using the soil samples taken at/around the field measurement sites, the airborne measurements by a combination of in situ, and soil/rock sample data. The radioelement concentrations (Bq/kg) were in turn converted to dose-rate (nSv/h).

First, the cosmic radiation map was constructed from a digital terrain model, averaging topographic heights within cells of 2 km x 2 km size. For the terrestrial radiation a total of 1615 ground data points were available, in addition to the airborne data. The artificial radiation map (Chernobyl and earlier fallout) has the smallest data base (184 data points from airborne and ground measurements). The dose rate map was constructed by summing up the above-mentioned contributions. It relies on a data base which corresponds to a density of about 1 point per 25 km².

The cosmic radiation map shows elevated dose rates in the high parts of the Swiss Alps. The cosmic dose rate ranges from 40 to 190 nSv/h, depending on altitude. The terrestrial dose rate maps show general agreement with lithology: elevated dose rates (100 to 200 nSv/h) characterize the Central Massifs of the Alps where crystalline rocks give a maximum of 370 nSv/h, whereas the sedimentary northern Alpine Foreland (Jura, Molasse basin) shows consistently lower dose rates (40 - 100 nSv/h).

The artificial radiation map has its maximum value in the southern part of Switzerland (90 nSv/h). The map of total dose rate exhibits values from 55 to 570 nSv/h. These values are considerably higher than reported in the Radiation Atlas ("Natural Sources of Ionising Radiation in Europe") published by the Commission of the European Comminities. The most frequent radiation range in Switzerland is 85 - 115 nSv/h (total dose rate outdoors).

1 Introduction

Radioactivity is part of our physical environment. The largest contribution to the radiation field is of natural origin: it is due to cosmic rays, the natural radioactivity of the ground and the radioactive decay products of radon in the air. Artificial radioactivity is emitted by nuclear power plants, industrial plants and research facilities. These emissions are very small in normal operation, although large amounts of radioactivity can be released to the environment through accidents.

Maps of terrestrial gamma radiation, when converted to dose rate distribution, are indispensable for various purposes: they provide basic information for the effects of low doses and a well documented reference base for accident and emergency situations (release from nuclear power plants, lost radioactive sources, debris from fission-powered satellites). Furthermore, they can aid geological mapping and/or prospecting for raw materials (e.g. by locating potassium alteration). Gamma radiation maps are especially needed on the scale of whole countries. We summarize here the methodology followed to establish a set of ground gamma radiation maps for Switzerland [1].

2 Input data

No single nationwide dataset of radioactivity in Switzerland was available with sufficient areal density. Therefore four different datasets have been combined for map generation (Fig. 1):

The aim of this project was to compile and combine these datasets to create a radiometric map of Switzerland. In a somewhat similar effort, the usefulness of combining airborne gamma spectrometric survey data with radiogeological ground data in estimating gamma-ray dose rates was demostrated in California [2].

2.1 Airborne gamma-ray spectrometric measurements

The Swiss Geophysical Commission (SGPK) performed the mapping of selected regions with elevated natural radioactivity within the framework of the Swiss National Geophysical Survey [3]. Attention was principally given to the crystalline rocks of the Central Massif of the Swiss Alps because of their relatively high natural radioactivity. Selected areas with typical lithology (Mesozic limestones, Tertiary Molasse sediments and Quaternary deposits) have also been surveyed. The area covered by this survey is about 3000 km2. In addition, an area of about 50 km2 surrounding each of the four nuclear power plants (Beznau, Goesgen, Leibstadt and Muehleberg) and the Swiss nuclear research facility (Paul Scherrer Institute) have been surveyed annually for the past four years (financed by the Swiss Federal Nuclear Safety Inspectorate (HSK)). The measurements aim to monitor the dose-rate distribution and to provide a documented reference base [4].

The measuring system consists of a helicopter-borne gamma-ray spectrometer with data control and storage and flight positioning instrumentation. The spectrometer covers the gamma-ray energy range of 40 keV to 3000 keV with 256 channels. An additional channel is used for the registration of high energy cosmic radiation. The detector used for the survey consists of a package of four 4" x 4" x 16" prismatic, thallium-doped, sodium iodide crystals (total volume 16.8 L). A PC-based data acquisition system synchronizes and controls the measurements. The spectrometer data are collected every second together with radar altitude, time, barometric pressure, outside air temperature and aircraft attitude angles. Positioning is done with the satellite navigation system GPS [5].

Processing covers the common calibration and reduction procedures (cosmic and background correction, spectral stripping, altitude correction) extended with an additional topographical correction to account for the effects of the rugged topography in the Swiss Alps. The data processing procedures adopted in this project, in particular for reducing the effects of topography on airborne gamma-ray spectrometry measurements, are described in [3] and [6].

2.2 In situ gamma-ray spectrometric measurements

The in situ gamma spectrometric data were collected by the Branch for Radioactivity Surveillance of the Federal Office of Public Health (SUeR). A total of 166 points are monitored for environmental radioactivity, particularly in the neighbourhood of nuclear installations and factories handling, processing or producing radioactive material ([7] and [8]).

The measuring instrument consists of a portable germanium detector system with 4096 channels. In addition to the in situ measurement, which lasts for about two hours, soil samples were collected from different depths for subsequent laboratory analysis.

2.3 Dose rate measurements

At the time of the in situ spectrometric measurements, a dose rate measurement with a Reuter-Stokes ionization chamber (RSS 111) was taken at every location. Many more measurements were made subsequently (especially inside houses) to give a more detailed map [9].

Data from 282 sites reported from an older survey with a 25 L, air-filled ionisation chamber instrument were also included [10]. Recent emergency preparedness exercises of the Swiss National Emergency Operation Centre (NAZ) with Geiger-Mueller (GM) counters contributed 389 additional points. The instrument used was the Automess 6150 AD2, calibrated to measure the total dose rate, and the measuring time was one hour [11].

2.4 Laboratory measurements

Since the late 1960's, 612 surface rock and soil samples have been analysed for the natural radioelements, potassium, uranium and thorium, with a laboratory sodium iodide system at the Geophysics Institute, ETH. The aim of these measurements was mainly to determine heat production ([12], [13] and [14]). Since 1990 these measurements have been performed with a modern high purity germanium system from Intergamma (efficiency 26%) with 4096 channels.

3 Calibration

When combining these datasets, it is essential to account for the fact that they have been acquired with different methods. In particular, the representative volumes are different. An airborne measurement represents the topmost 50 cm in an area of about 300 m x 300 m. An in situ or dose rate measurement represents a much smaller area of 10 m x 10 m. The smallest volume is represented by the sample measurements which are normally carried out on 0.5 kg of material.

The in situ measurements were calibrated using the soil samples taken with every set of measurements. Experience showed that five to ten samples per site were enough for the calibration. Since the artificial isotopes are not distributed homogeneously in the soil a depth profile is taken to determine their distribution. The error of an in situ gamma spectrometric measurement can be estimated at about ± 20% for each isotope.

The airborne data were calibrated using a combination of in situ measurements and rock sample data. Since the airborne measurements cover a much larger sample area, an average of four in situ measurements and 50 rock samples in different lithological units were used for the calibration. Table 1 lists the calibration factors obtained.

The ground activities in Becquerels per kilogram have been converted to dose rates in nanoSieverts per hour using the following conversion factors (in nSv/h per Bq/kg): 0.05 for 40K, 0.51 for 214Bi, 0.71 for 208Tl and 0.17 for 137Cs [8]. Other isotopes (134Cs, 60Co, 7Be, etc...) occur only in traces and were neglected for this application. In the uranium and thorium decay chains, equilibrium up to the isotopes 226Ra, 228Ac, respectively, was assumed.

Table 1: Calibration factors for the airborne system (calculated from count rates measured with the airborne system at 120 m flight altitude and activity determinations in Bq/kg at ground).

4 Collation/interpolation/mapping

The radiation at a point is composed of three components:

The cosmic contribution of the radiation was calculated from a digital terrain model averaging topographic heights within a cell size of 2 km x 2 km using the relation:

Dcosmic [nSv/h] = 37.0 exp ( 0.38 altitude [km] ) [8]

which yields a map of the cosmic background in Switzerland (Fig. 2).

The map of the natural terrestrial radiation assembles the airborne gamma-ray spectrometry, in situ gamma-ray spectrometry and rock/soil sample data. In addition to the airborne data, a total of 1615 ground data points were available for the map production.

The variations in the natural dose rate (terrestrial and cosmic) due, for example, to radon washout, soil humidity, snow cover and sun activity are around 25%, and were neglected for this work. The artificial part of the radiation poses a specific problem. Since this contribution is not constant in time, the value depends on the time of the measurements.

In particular, data recorded in 1986 are strongly influenced by the fallout from Chernobyl. The map of the artificial radiation was therefore produced only using data from in situ and airborne spectrometric measurements taken after 1987, which yielded 167 data points. Laboratory measurements on rock samples could not be used for the map of artificial radiation, since they are usually taken from an unweathered (covered) part of a rock, and are therefore devoid of any artificial radioactive isotopes. This map (Fig. 4) has the smallest data base.

The total dose rate map of Switzerland was calculated by summing the previously described maps (cosmic, natural terrestrial and artificial dose rate).

For the interpolation of the maps, a simple inverse distance method with a search radius of 12 km and cell size of 2 km x 2 km was chosen. In areas with airborne data coverage the search radius was decreased to 2 km to give better spatial resolution.

Because of the generally large statistical errors in radiometric data, the sharp changes in radioactive field intensities and the inhomogeneous data point distribution the classical isoline representation was abandoned and the pixel representation was used (for details see [3] and [15]). The values on the maps are given in a 25-grade linear color scale (blue  cyan  green  yellow  red: increasing values). The color ranges are given on the respective figures, along with the maximum, minimum, and mean value with standard deviation.

5 Results and conclusions

By assembling different ground radiation datasets and after appropriately converting the measured values, a dose rate map of Switzerland could be constructed. It relies on a data base which corresponds to a density of about 1 point per 25 km2.

The map of the cosmic radiation (Fig. 2) shows elevated dose rates in the high parts of the Swiss Alps. The cosmic dose rate ranges from 40 to 190 nSv/h, according to the altitude.

The terrestrial natural dose rate map presented in Fig. 3 shows general agreement with lithology: elevated dose rates (100 to 200 nSv/h) characterize the Central Massifs of the Alps where crystalline rocks give a maximum dose rate of 370 nSv/h, whereas the sedimentary northern Alpine Foreland (Jura, Molasse Basin) shows consistently lower dose rates (40 - 100 nSv/h). The ground radiation can show short-term changes. After rainfall, the short-lived daughter products of atmospheric radon are deposited on the ground; they can increase the dose rate for about a day by several tens of nSv/h.

The artificial radiation map (Fig. 4) has its maximum value in the southern part of Switzerland (90 nSv/h). In this region it rained soon after the Chernobyl accident, so the airborne aerosols with radioactive isotopes were washed out and deposited.

And finally the map of the total dose rate of Switzerland is presented (Fig. 5), which corresponds roughly to the sum of the above three maps (with additional data points from the dose rate measurements). The values range from 55 to 570 nSv/h. The dose rate frequency distribution derived from the total dose rate map by counting the pixels in each interval is shown in the histogram of Fig. 6. The predominance of the dose rate range 85 - 115 nSv/h is evident.

These values are considerably higher that reported in the Radiation Atlas ("Natural Sources of Ionising Radiation in Europe") published by the European Communities
(60 - 70 nSv/h).

In summary, by properly treating the non-uniform data base available, a consistent ground radiation map of Switzerland has been established. The results indicate the dose rate varies over about one order of magnitude and the most frequent radiation level is in the range 85 - 115 nSv/h (total dose rate outdoors).

Acknowledgements

This mapping study was performed within the project framework of the Swiss Federal Nuclear Safety Inspectorate. We thank Dr. S. Prêtre and W. Jeschki (Würenlingen) for continuous support and valuable discussions.

Contribution no. 913, Institute of Geophysics ETH Zurich.

References