Measurement of the mobility of ionic clusters using the 20ms Isomer 24mNa as Tracer
Diploma Thesis of Bernhard Abmayr
Technische Universität München, Department for Physics
Institute E17, Prof. Dr. H. Morinaga
Garching b. München
A new tracer method was developed. With a very short half-life time of only 20ms it can be used to detect movements of Na+ ions in the velocity range of some m/s. A big advantage of this tracer is that the 24mNa nucleuses are not produced directly, but are generated by the β- decay of 24Ne with a half-life time of 3.38min. Therefore they are not neutral atoms, but positive charged 24mNa+ ions until recombining with an electron and they can be moved applying an electrical field. The velocity of the ions can be calculated from the change of the gamma decay rate of the isomeric state. With this method the mobility of sodium ions was measured in gaseous neon. It was found to depend heavily on the concentration of the water vapor in the gas mixture. Even if the part of water in the gas is less than one per thousand, the water molecules stick to the sodium ion to build a cluster so quickly after generation of the sodium ion, that the mobility of the ions is drastically smaller even during the first few milliseconds of the ions’ live.
Measurements of the sodium ion mobility in gas mixtures of neon with argon, helium und nitrogen showed that the influence of water vapor on the ion mobility depends on the type of gas. For argon and nitrogen the mobility was similar to the theoretical value without water, but for helium the mobility was much less than expected. While the heavy argon atoms and nitrogen molecules can destroy a cluster of a sodium ion with water molecules when hitting it, the helium atoms are too light to do this.
The ion mobility within a mixture of neon and carbon dioxide, however, was larger than expected, as the water molecules stick better to the sodium ions than the carbon dioxide molecules due to the higher dipole moment of water molecules. Such the water molecules inhibited building clusters with carbon dioxide molecules.
Measurements with various mixtures of neon and ethanol vapor showed, that the method is able to measure the change of the ion mobility’s dependence on the mixture ratio in agreement with theory.
A diffusion cloud chamber was used to test this tracer method for investigations of the growth of large clusters or even small drops in gas systems near to the condensation or critical point. Within the diffusion cloud chamber a dynamic equilibrium with oversaturated ethanol vapor was generated to measure the ion mobility in this zone. There the mobility decreased heavily and could be explained assuming that big clusters had grown in few milliseconds.
NOTE: This abstract is not part of the original diploma thesis. It was written years later for publishing it in the internet.