Diploma thesis in Physics submitted by Florian Freundt born in ...

4 Measuring methods 4.2. On site measurement of O2, CO2, CH4 and Radon
infrared spectroscopy. The range of the CO2 and of the CH4 sensor is 0 – 100 Vol% with an
accuracy of ±0.5 Vol% at 0 – 5, ±1.0 Vol% at 5 – 15 and ±3.0 Vol% above 15 Vol%. Calibration
of the device was verified by using a certified test gas, confirming the provided accuracies but
showing a slight overestimation of O2, well within the given error range though (see Table B.2).
Atmospheric air was regularly sampled before and after soil air samplings (see Figure C.7), the
mean O2 value of (20.89±0.04) Vol% is in good agreement with the literature value of 20.9 Vol%
[Porcelli et al., 2002].
4.2.2 Durridge RAD7 Radon Detector
The Durridge RAD7 Radon Detector detects α-particles of characteristic energies created within
the decay chains of 222 Rn and 220 Rn, using a solid state detector. Solid state detectors consist of
semiconductor material, a PN junction diode in reverse bias. An α-particle hitting the depletion
layer creates electron-hole pairs that get separated by the electrical field spanning the junction,
leading to a measurable current. The energy resolution depends on the semiconductor’s bandgap,
allowing to differentiate between the different decaying Radon isotopes: The RAD7 detects
the 6.00 MeV α-decay of 218 Po, which itself is the product of 222 Rn decay, and the 6.78 MeV
α-decay of 216 Po, belonging to the 220 Rn decay chain. This allows for the parallel monitoring
of 222 Rn and 220 Rn, leading to a more detailed source of information: while the readings of the
long-lived 222 Rn (t 1/2 = 3.82 days) will usually increase in every one of the three 10 minute
cycles due to its long lifetime, readings of the short-lived 220 Rn (t 1/2 = 55.6 sec) should be
constant in all three cycles. A change in 220 Rn readings in one of the three cycles therefore
indicates an inflow of differently composed air causing dilution or enrichment depending on its
220 Rn content.
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