IV. The In situ gamma spectrometric systems
From Intamap
IV. The In situ gamma spectrometric systems
To determine the nuclide specific soil activity (Bq/m²) under routine conditions or after deposition of radionuclides, so called In situ measurements are used. The terrestrial component of the external radiation is determined by the geologic formation i.e. the composition of radionuclides on the ground. In order to quantify the contributions of different radionuclides to the dose rate at each measuring point of the GDR network additional in-situ-measurements are regularly performed using high resolution so called hyper-pure germanium (HP)Ge-detectors.
Strategy of In situ Measurements
In the German natwork 6 mobile systems are used to determine the nuclide specific contribution to soil contamination:
- HP-Ge detector with 10% rel. efficiency
- liquid nitrogen cooled
- battery operated mode with 4h operation time
- GPS based positioning system
- GSM-based data communication
- detection limit 200 Bq/m² for Cs-137 (30 min measurement)
The GM tubes are used to measure the total dose rate at a measuring point whereas In-situ systems are used to measure the dose rate specific to each nuclide. The nuclide specific dose rate can be calculated from the nuclide specific acitivity using conversion factors. Adding all nuclide specific contributions of the In situ measurement results in the total terrestrial dose rate which must be identical to the value measured using the GM tube. Therefore the comparison of the results of both measurements performed simultantously are used as quality assurance method. Measurements are updated in a 3-year-cycle at each site of the dose rate monitoring network. The comparison of the results of old and new measurements again are used as quality assurance method.
In an emergency the new contamination is calculated as the difference from (earlier) background measurement and the current measurement.
The nuclide specific distributions of Cs-137 and K-40 based on the measurements performed during the last years are displayed related to their geographic positions.
From figure 9 it can be seen, that due to wash-out effects after the reactor accident in Chernobyl the southern part of Germany has been affected. The measured data are consistent with the levels of Cs-173 deposition measured after the Chernobyl accident, shown in Figure 11.
To compare In situ and GDR data sets, self-effect and response to the secondary cosmic radiation must be compensated and the terrestial component measured using the GDR network rate must be separated and compared to the summed dose rate calculated from the nuclide specific In situ measurements. The comparison already done showed a good agreement between the two different data sets. The agreement presents an independ verification of the two complementary measuring techniques.
Deposition of radioactivity
In a nuclear accident, when a radioactive plume is released, two types of ground deposition occur: dry deposition and wet deposition. Concerning the amount on radioactive material deposited to the ground, wash-out effects due to precipitation are at least an order of magnitude more efficient than dry deposition. Therefore after the Chernobyl nuclear accident, locations at which plume passage and precipitation events coincided had much more total ground deposition than those over which the radioactive plume passed without rain or snowfall.
This relation between radioactive plume and precipitation helps to understand the very inhomogenuouse distribution of Caesium-137 in Germany after the Chernobyl reactor accident contamination. Due to this phenomenon the density of the GDR network has been choosen with a mean distance between neighbouring measuring posts of 15 km.
Back to WP5 discussion.
