Gamma Spectrometry Forum

Detection of Chernobyl-Derived 137Cs in Forest Soils near Rosenheim

Objective:
The aim of this small experiment was to investigate the presence of radioactive caesium-137 originating from the Chernobyl fallout in undisturbed forest soil. The investigation was conducted by taking soil samples and performing gamma-spectroscopy using a KC761C detector and a self-built lead shield.

Methods:
-Sampling Site: An undisturbed forest floor in the region of Rosenheim, South Bavaria.
-Sampling Depth: Samples were taken from a depth of approximately 10 cm. This depth was chosen based on published research (e.g., Zibold et al., J. Environ. Radioact., 2009) indicating that Chernobyl-derived Cs is typically found in this horizon in such soils.
-Sample Preparation: Soil was collected, not dried, and placed in a suitable counting geometry. I did not dry the sample as a quantification was out of scope anyways.

Measurement Equipment:
-Detector: KC761C (CsI, 2.54cm^3)
-Shielding: A self-built lead castle to reduce background radiation.
-Setup: The soil sample was measured inside the lead castle. A background spectrum of the empty shielded castle was also acquired.

Acquisition Time: Both sample and background spectra were counted for 10 hours each.

Results and Analysis:
The spectra were analyzed by comparing the sample measurement (green line) with the background measurement (pink line), which was subtracted to reveal the net signal from the soil.

Key observations after background subtraction are:
1. Characteristic Gamma Peak at ~662 keV: A small but clear peak is observable in the sample spectrum at approximately 660 keV. This energy is the unmistakable gamma-ray signature of 137Cs. Its presence confirms that the soil contains measurable amounts of this artificial radionuclide.
2. Low-Energy X-Ray at ~32 keV: A distinct signal is also visible at approximately 32 keV. This corresponds to the characteristic X-rays emitted by the daughter product (137mBa) following the decay of 137Cs. The ability to detect this low-energy signal indicates okayish detector sensitivity and effective background reduction from the lead castle.

Next Steps:
To further refine this project, I would like to:
-Take a depth profile (e.g., samples at 0-5 cm, 5-10 cm, 10-15 cm, 20-30 cm) to see the vertical distribution.
-get me a much better detector (at the brink to buy the GS1515-CsI), and then repeat the exercise with that much more sophisticated equipment.
-taking samples from Berchtesgaden (closer at/in the Alps) where the Cs-contamination is even much higher (as the Alps are a normal physical barrier for fallout clouds.)

Congratulations, very cool experiment.

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Thank you Paulo.

There are for sure several shortcomings here, and mistakes I did. I have been roasted quite heavily on reddit with my post, as the argument was, that you find Cs137 almost anywhere on the planet, and that this finding is not correlated to Chernobyl at all.

I guess that argument came mainly from non-EU folks, who are not familiar with the Chernobyl fallout amount here in Western Europe, which is about 9 times higher than that of nuclear weapon tests from the 50s/60s.
Attached a hotspot map, there you can see how much affected southern Germany/Austria/Slovenia was....and still is: a little bit more than 1HL has passed.

Here a publication which goes into detail on the Chernobyl/Weapon test fallouts: you can distinguish between those two because of the Cs135/137 ratio.

https://doi.org/10.1021/acs.est.3c03565

The result from this study shows that the weapon test fallouts are already in the realm of 20-40cm depth in the soil, the one from Chernobyl is at the stage of entering that area. That's why you find wild boar contamination still to be quite high with weapon test fallout, and now increasing with Chernobyl fallout. Story behind is the truffle mushrooms, which are consumed by the wild boar. The truffles grow in 20-40cm depth.

Found this publication very interesting...

So next step would be to do a depth profile, of course we can't detect Cs135, but I just want to see, how this increases for Cs137 in the soil down to 50cm, maybe in 10cm steps.