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

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3.2. Sampling methods 3 Sampling sites and methods Over 350 noble gas samples were taken during the sampling period, 118 of which were successfully evaluated in the mass spectrometer. The remaining samples either proved leaky or were not analyzed in the mass spectrometer due to time constraints. 3.2.2 O2, CO2 and CH4 O2, CO2 and CH4 were measured on site using a Geotech BM2000 Biogas Monitor with a built-in pump. Unfortunately the device was available only as of February 2011, leaving noble gas samples A01 – A10, B01 – B09 and C01 – C05 without corresponding O2, CO2 and CH4 measurements. The BM2000 was connected to the sampling tube parallel to the RAD7, measurements were taken during the first two minutes as well as during the last ten minutes of the 30 minute pumping period at each depth. Since variations between the two measurements never exceeded the nominal accuracy of the sensors, the data from the last 10 minutes of pumping was used for its closer proximity to the noble gas sample. 3.2.3 Radon A Durridge RAD7 Radon Detector was used as the primary sampling pump as well as for measuring 222 Rn and 220 Rn concentrations, given in Bq/m 3 . The RAD7 was set up behind the noble gas sampling tubes and shielded from humidity by a Drierite desiccant tube. The RAD7 was set for three 10 minute cycles, only 222 Rn values from the last cycle were used. 3.2.4 Temperatures Atmospheric ambient temperatures at 2 m height above ground were provided by a weather station run by meteomedia close to Site A. The sampling sites were equipped each with LogTag TREX-8 and TRIX-8 data loggers (see Figure C.5), the latter featuring build-in resistive temperature sensors, the former attached to external resistive temperature sensors. The loggers were set to record the temperature at an hourly interval. Accuracy of the sensors between −10 ◦ C and +40 ◦ C is according to the specifications (see Datasheet D.2) ±0.5 ◦ C. While the TRIX-8 was placed inside the instrumentation box shielded from moisture (but also from fast temperature changes due to the insulating air), none of the external sensors attached to the TREX-8 s were rated for deployment in moist to wet surroundings. They were therefore insulated using shrink-sleeves with glue and tested in water beforehand. Testing showed that moisture intrusion leads to temporary offsets in temperature readings rather than complete failure. 42
Chapter 4 Measuring methods 4.1 Mass spectrometry of He, Ne, Ar, Kr and Xe Measurements of He, Ne, Ar, Kr and Xe were conducted using a GV Instruments MM5400 mass spectrometer and a preparation line built and set up by Friedrich [2007]. The samples were measured in four distinct runs, the first three during September to November 2010 and the last one in May 2011. The first three runs utilized the same setup and sample size, while some changes were made for the fourth run, including a slightly different inlet setup, different sample size and modified measuring processes for He, Ar, Kr and Xe. 4.1.1 Sample preparation and measuring procedure The copper sample tubes were installed in an inlet volume of known size which was evacuated to less than 5.0 × 10 −6 mbar. The process of installing the sample tubes inside the inlet was responsible for most sample losses since mechanical stress had to be exerted on the fragile sealing before the surrounding volume was evacuated. The sample was vented into the evacuated inlet volume by applying pressure perpendicular to one of the pinch points using a modified valve, thereby cracking the sealing and releasing the gas into the inlet volume almost instantaneously. In rare cases the cracking was so minute that the venting happened slowly over a timespan of 3 to 10 minutes. After taking a pressure reading of the gas inside the inlet volume, the gas was transferred to the preparation line where gas separation was done: The different noble gases cannot be measured side by side in a single step but have to be separated. This is due to the differences in concentration magnitudes as well as to prevent interference effects: The mass spectrometer discriminates mass-to-charge ratios, therefore a twice ionized 40 Ar atom will provide the same result as a single-ionized 20 Ne atom. The gas separation was realized by an adsorption refrigeration system, adsorbing the noble gases Ar, Kr and Xe, as well as all other permanent gases, at 25 K in a permanent gas trap while He and Ne were adsorbed at temperatures below 12 K in an activated carbon reservoir. The gases were then released separately through incremental heating 43
Page 1: Faculty of Physics and Astronomy He
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Page 8 and 9: CONTENTS CONTENTS 3.1 Setup of the
Page 11 and 12: Chapter 1 Introduction Understandin
Page 13: 1 Introduction Molins and Mayer [20
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Page 54 and 55: 5.4. Borehole sealing 5 Results 5.4
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Page 58 and 59: 5.5. Noble gases 5 Results 8 days,
Page 60 and 61: 5.5. Noble gases 5 Results 5.5.3 So
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Page 76 and 77: A.1. Gas sample size A Calculations
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Page 80 and 81: Table B.3: Noble gas volume mixing
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C Additional plots and figures �
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C Additional plots and figures Figu
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C Additional plots and figures O 2
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C Additional plots and figures 4 He
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C Additional plots and figures 20 N
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C Additional plots and figures Figu
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C Additional plots and figures 10 1
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Appendix D Datasheets 107
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D Datasheets LogTag TREX-8 Technica
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Bibliography [Aeschbach-Hertig 1994
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BIBLIOGRAPHY BIBLIOGRAPHY [Friedric
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BIBLIOGRAPHY BIBLIOGRAPHY [Riveros-
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Acknowledegment At the end of this
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