Deposition samples collected on a continuous basis

Deposition samples are collected using a device placed outdoors, specifically designed for this purpose. Today, samples are being collected on a continuous basis in nine localities around Finland. In addition, there are deposition collection stations in the vicinity of nuclear power plants.

The collection area is 0.05 or 0.07 square metres, and wet and dry depositions are not separated. In Helsinki, there is an extra device with a larger collection area that separates wet and dry depositions. The devices used in the vicinity of nuclear power plants are also large in area.

Some of the deposition collection stations are operated by the Defence Forces or the Finnish Meteorological Institute, and two stations are operated by the Frontier Guard. STUK collects samples in Helsinki and Rovaniemi, the cities where it is present.

In this context, deposition refers to radioactive materials that have fallen to the ground or water from air. Deposition may come with rain (wet deposition) or as dry deposition. Deposition samples are collected on a continuous basis, and they are usually analyzed monthly. 

If analyses of air samples or external radiation indicate signs of fresh fallout, deposition samples are collected at shorter intervals. This ensures that information on the composition of the fallout and the amount of radioactive substances in different localities is obtained promptly. The results also provide the basis for studies investigating the migration of radioactive materials in earth and water environments.

Analysis of samples

Usually, the volume of deposition samples is reduced by evaporation prior to analysis. This helps to detect even the lowest concentrations of radioactive materials. The evaporation residue is placed in a small container and analyzed for gamma radiation-emitting radioactive substances with a gamma spectrometer. In addition, samples can be analyzed for beta or alpha emitters, such as strontium-90 and plutonium, that require radiochemical detection. Tritium concentrations in rain water are also measured. 

Annual deposition of cesium-137 in 2011

Annual deposition of cesium-137 in 2011 (becquerels per square metre) in different localities. The differences between localities are due to the uneven distribution of the Chernobyl fallout. Dust remigrates into the air from soil and falls down again, usually with rain, into the collector.

Measurement data

Measurement data are published once the analysis has been completed.

Tritium concentrations in rainwater samples in 2014

Helsinki 17.1.2014 1,4
Helsinki 17.2.2014 2,0
Helsinki 17.3.2014 1,5
Helsinki 18.4.2014 2,0
Helsinki 19.5.2014 1,7
Helsinki 17.6.2014 2,0
Helsinki 18.7.2014 2,0
Helsinki 18.8.2014 1,3
Helsinki 16.9.2014 1,7
Helsinki 19.10.2014 2,0
Helsinki  18.11.2014 1,6 
Helsinki 19.12.2014 1,5 
Rovaniemi 21.1.2014 <2
Rovaniemi 17.2.2014 <2
Rovaniemi 20.3.2014 <2
Rovaniemi 21.4.2014 1,1
Rovaniemi 19.5.2014 1,7
Rovaniemi  16.6.2014 1,8 
Rovaniemi 18.7.2014 1,7 
Rovaniemi 18.8.2014 1,4 
Rovaniemi 15.9.2014 <2 
Rovaniemi 17.10.2014 <2  
Rovaniemi 18.11.2014 <2  
Rovaniemi 19.12.2014 <2  


STUK monitors depositions of radioactive materials in nine localities

Depositions are monitored using deposition collectors specifically made for this purpose.

  137Cs 90Sr Koko vuosi
  1.1.-31.3.2014 1.4.-30.6.2014 1.7.-30.9.2014 1.10.-31.12.2014 1.1.-30.6.2014 1.7.-31.12.2014 137Cs 90Sr
Helsinki <0,1 0,1 <0,1 0,1 <0,03 <0,05    
Imatra <0,1 0,3 0,1 0,1 <0,04 <0,04    
Ivalo <0,1 0,1 0,1 <0,1 <0,05 0,1    
Kajaani 0,1 0,4 0,2 0,1 0,05 0,09    
Kotka 0,2 0,5 0,4 0,4 0,05 0,1    
Kuopio 0,1 0,5 0,2 0,1 <0,04 <0,04    
Rovaniemi <0,1 <0,1 0,2 <0,1 <0,04 <0,03    
Sodankylä <0,1 0,1 <0,1 <0,1 <0,05 <0,04    
Ylöjärvi 0,2 1,4 0,6 0,3 <0,04 <0,09

The data are presented in becquerels per square metre (Bq/m2).

The tables include quarterly and annual totals of cesium-137 depositions in the different localities. Since 1 July 2012, samples to be analyzed for strontium-90 are combined every six months for each locality. The data are presented in becquerels per square metre (Bq/m2).

Activity concentrations of iodine-131, cesium-134 and cesium-137 in deposition samples collected from Helsinki after the Fukushima nuclear power plant accident

The collection period for deposition samples is one month. After the Fukushima nuclear power plant accident, the collection period was made shorter in Helsinki. The collector at the Helsinki monitoring station collects wet and dry deposition separately.

At the moment, radioactivity concentrations in deposition are very low in Finland. Small amounts of iodine-131 and cesium-137 were detected in dry and wet deposition samples collected in March.  
After the Chernobyl accident in 1986, the highest cesium-137 concentration measured in a deposition sample was 32,000 becquerels per square metre and the lowest 140 becquerels per square metre. The highest iodine-131 concentration measured was 36,000 becquerels per square metre.

The tables show the iodine-131, cesium-134 and cesium-137 concentrations measured in wet and dry deposition samples in Helsinki after the Fukushima accident. The unit is becquerels per square metre. The table was updated approximately weekly as long as iodine-131 and cesium-134 were detectable. 

Wet deposition

Collecting period

Cesium-137 activity concentration Bq/m2 (uncertainty %)

Cesium-134 activity concentration Bq/m2 (uncertainty %)

Iodine-131 activity concentration Bq/m2 (uncertainty %)

1.1.-31.1.2011 0,03 (9)    


1.3.-21.3.2011 <0,015   <0,086
21.3.-30.3.2011 0,4 (16) 0,5 (15) 14,8 (5)
30.3.-6.4.2011 1,2 (7) 1,1 (8) 17,7 (4)
6.4.-13.4.2011 0,14 (22) 0,11 (19) 0,7 (12)
21.4 - 29.4.2011 <0,09 <0,07 <0,018


Cesium-137 and strontium-90 depositions 

Cs-137 and Sr-90 depositions in Helsinki

Cesium-137 and strontium-90 depositions in the Helsinki region from 1960 to 2012.  The peaks in the 1960s and 1980s are due to nuclear tests performed in the atmosphere and the fallout caused by the Chernobyl accident in 1986.

Cs-137 and Sr-90 depositions in Rovaniemi

Cesium-137 and strontium-90 depositions in Rovaniemi from 1972 to 2012.

Data published on a regular basis

STUK publishes data from deposition radioactivity monitoring on a regular basis:



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  • Kaisa Vaaramaa / Senior Inspector
    Tel. +358975988521