Gamma-spectrometry laboratory for trace analysis

• Accredited laboratory according to DIN EN ISO/IEC 17025:2018

Objectives

• environmental monitoring according to statutory obligations
• Detection of traces of radioactivity in airborne dust samples

Traces of radioactive substances in airborne dust are detected using gamma-spectrometry. The required samples are taken using high-volume air samplers, the sampling duration is usually one week. In case of an incident with enhanced emissions of radionuclides to the air daily samples can be taken.

The measurements are aimed at determining activities and activity concentrations of various gamma-emitting radionuclides collected from the air using high volume samplers.

In order to detect traces of radioactivity, dust samples are analysed in the gamma-spectrometry laboratory of the Freiburg offices. These samples are taken

• at the monitoring station on Mount Schauinsland and
• on the roof of the Freiburg offices.

High volume samplers draw air through large-surface aerosol filters at an airflow of 700 to 900 cubic meters per hour. The dust particles with the adhering radionuclides deposit on these filters.

Method

Exposed aerosol filter after sampling

The filters are pressed into pellets at the end of each sampling period (usually one week). In order to detect traces of radionuclides, the pellets are measured for several days using highly sensitive high-purity germanium detectors. Lead shielding is used to reduce the influence from background radiation, which is present naturally everywhere and can interfere with the measurement.

Typical limits of detection for the activity concentration of caesium-137 are at around 0.1 microbecquerel per cubic metre of air.

Not all radionuclides can be identified by their gamma-ray emissions. Radionuclides such as strontium-90 or plutonium have to be radiochemically separated and processed before measurement. This is routinely performed on monthly samples in the radiochemistry laboratory of the Freiburg office.

Filters pressed into pellet shape

Monitoring for traces of radioactivity in airborne dust is part of the measurement programmes given in the General Administrative Regulation on the Integrated Measurement and Information System for the monitoring of environmental radioactivity (AVV-IMIS) and of the EURATOM treaty.

Measurements beyond the scope of accreditation

Gaseous iodine

Gaseous iodine in the air is not collected on the filters. In order to detect gaseous iodine, it is adsorbed on the surface of a solid (activated carbon for example). The sample produced in this process is analysed using gamma-spectrometry.

Precipitation samples

If required (after the Fukushima accident for example), precipitation samples are also taken at the monitoring station on Mount Schauinsland and at the Freiburg office, and are analysed for radionuclides. These samples contain radionuclides that have been washed out from the air by precipitation.

Radioanalytical laboratory

Objectives:

• environmental monitoring according to statutory obligations

• Detecting radioactive elements in airborne dust samples:

  • strontium
  • uranium
  • plutonium
• Detecting clandestine nuclear activities

Airborne dust samples collected at the monitoring station Schauinsland and in Freiburg are initially analysed in the gamma spectrometry laboratory. Then, in the radioanalytical laboratory, they are processed using radioanalytical methods to separate strontium, uranium and plutonium.

Method

In order to achieve the lowest possible limit of detection, between four and five weekly samples are combined to a monthly sample and subsequently ashed. The activity concentrations of strontium, uranium and plutonium are determined from the ashes of these samples.

The sample ash is dissolved in acid and processed in a specially designed microwave oven. Then, the nuclides to be determined are separated using radioanalytical methods and are deposited on filters or stainless steel planchets.

The strontium isotopes are measured using a low-level alpha/beta counter. This system is used for detecting the lowest detectable activities of alpha and beta emitters.

After electrochemical deposition on stainless steel planchets, the uranium and plutonium isotopes are measured in an alpha spectrometer.

Filter samples are processed in the radiochemistry laboratory

Limits of detection

With the described method, the following detection limits are achieved:

• 1 microbecquerel per cubic metre of air for strontium-89,
• 0.03 microbecquerel per cubic metre of air for strontium-90 and
• 0.0005 microbecquerel per cubic metre of air for the isotopes uranium-234, uranium-235, uranium-238, plutonium-238, plutonium-239 und plutonium-240.