Analysing deposition samples provides information on the quantities of radioactive materials in different parts of the country and forms the basis for reports and studies on the migration of radioactive materials in the soil and water environment.

Collection of samples

Deposition samples are taken on a continuous basis in eight localities. The collection period for deposition samples is typically one month. Deposition samples are collected with a stainless-steel instrument with a collection area of 0.07 m². The instrument is not able to differentiate between wet deposition caused by rainfall and dry deposition, and measures them as a total. To facilitate samples during winter months, the instruments are fitted with heat resistance to keep the samples unfrozen and to ensure that all snow collected in the collection container is included in the sample.

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 analysed for gamma radiation-emitting radioactive substances, such as cesium-137, with a gamma spectrometer. In addition, samples can be analysed for beta or alpha emitters, such as strontium-90 and plutonium, that require radiochemical detection. Furthermore, rainwater samples taken from two localities are analysed for tritium concentrations.

Measurement data

STUK publishes the measurement data once the analysis has been completed.

Deposition of radioactive materials in eight localities in 2021

The tables shows quarterly totals of cesium-137 depositions in the different localities. For strontium-90, the samples are combined annually according to location.

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

Deposition of radioactive materials in eight localities in 2020

The tables shows quarterly totals of cesium-137 depositions in the different localities. For strontium-90, the samples are combined annually according to location.

The data is presented in becquerels per square metre (Bq/m²). *Collection period 6 months (Jan-Jun 2020)

Tritium concentrations in rainwater samples in 2020

The tritium concentrations in rainwater samples were small in 2020. The activity concentrations of tritium were between 1-2 Bq/l or below the detection limit.

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Cesium-137 and strontium-90 depositions

Radioactive materials identified in depositions preceding 1986 originate from the fallout resulting from nuclear weapon testing performed in the atmosphere. Measurements from this type of fallout typically vary between the different seasons of the years. The Cs-137 and Sr-90 detected from post-1986 depositions mainly originate from the explosion at the Chernobyl power plant. The radioactive materials carried by fallout migrate locally, with a small portion of them re-entering the atmosphere.

Cesium-137 and strontium-90 depositions in the Helsinki region from 1960 to 2019. The peaks in 1960s–1980s are due to nuclear tests performed in the atmosphere and the fallout caused by the Chernobyl accident in 1986.

Cesium-137 and strontium-90 depositions in Rovaniemi from 1972 to 2019.